Research ArticleExperimental Study on Static Load of Large-Diameter Piles inNonuniform Gravel Soil

Baoyun Zhao 123 Xiaoping Wang12 Mijia Yang4 Dongyan Liu12 DongSheng Liu5

and Shuguo Sun5

1School of Civil Engineering and Architecture Chongqing University of Science and Technology Chongqing 401331 China2Chongqing Key Laboratory of Energy Engineering Mechanics amp Disaster Prevention and Mitigation Chongqing 401331 China3Key Laboratory of Well Stability and Fluid amp Rock Mechanics in Oil and Gas Reservoir of Shaanxi ProvinceXirsquoan Shiyou University Xirsquoan 710065 China4Department of Civil and Environmental Engineering North Dakota State University Fargo 58108-6050 ND USA5Chongqing Bureau of Geology and Minerals Exploration Chongqing 401121 China

Correspondence should be addressed to Baoyun Zhao baoyun666163com

Received 7 December 2019 Revised 28 May 2020 Accepted 9 June 2020 Published 11 July 2020

Academic Editor Pier Paolo Rossi

Copyright copy 2020 Baoyun Zhao et al +is is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

With the development of tourism the number of multistorey buildings in mountain areas is increasing gradually and the re-quirements of the form and bearing capacity of foundation in landslide areas are gettingmore demanding than ever In-situ testing ofrock and soil mass in slope area has important practical significance for improving the stability of building foundation Taking aproject in Baishi Mountain located in southwest of China as an example firstly the geological structure andmechanical properties ofsoil are analyzed +en two types of pile foundations ie empty-bottom pile foundations and solid-bottom pile foundations aredesigned based on the characteristics of the geological structure for carrying out the static load test on pile foundation+e test resultsare as follows (a) the load settlement curve (Q-S) of the empty-bottom test pile shows a steep drop while the Q-S curve of the solid-bottom test pile shows a gradual change showing that the end-bearing friction pilersquos property and the ultimate bearing capacity of thesolid-bottom pile are higher than those of the empty-bottom pile (b) +e maximum lateral friction of the four test piles is139158 kPa 148015 kPa 150828 kPa and 154956 kPa respectively (c)+e shaft skin resistance under ultimate load is coming closeto the maximum value and the maximum values are 9792mm 7939mm 9881mm and 1497mm respectively Research resultscan serve as design bases for the pile foundation of multistorey buildings located in landslide areas of Baishi Mountain in thesouthwest of China and also as references for the engineering application of pile foundation in similar geological fracture areas

1 Introduction

Due to its high bearing capacity small settlement goodstability and other characteristics pile foundation has beenextensively studied by many scholars around the world in thepast few decades In the early research of pile foundation theintegrity of pile body and the bearing capacity of single pile aremainly tested by low strain integrity testing [1 2] acoustictransmission method [3] penetration test (CPT) [4] andother methods but the bearing capacity of pile foundationobtained by the above methods is only estimated value

+e field static load test method is the most reliablemethod to determine the bearing capacity In recent years

according to the field static load test research of bored pileswith different length-diameter ratios it has been confirmedthat the bearing performance of the piles is related to thelength-diameter ratio of the piles and the constructiontechnology of the pile foundation itself increasing the bottomof pile and no sediment at the bottom will improve thebearing capacity of pile and the larger the length-diameterratio the lower the bearing capacity [5ndash7] In addition to thespecific length-diameter ratio the rock-soil properties [8]where the pile foundation is located also affect the bearingcapacity +erefore in the foundation design the effectivestress distribution of the soil around and below the foun-dation unit must be determined first because it is the basis for

HindawiAdvances in Civil EngineeringVolume 2020 Article ID 6291826 15 pageshttpsdoiorg10115520206291826

design analysis [9] Until now scholars around the world havealso carried out extensive load test research on the bearingcapacity of pile foundation in different rock and soil massincluding laboratory tests [10 11] and field static load tests ofpile foundation in different geologic structure in differentregions Guo and Ghee [12] studied the influence of lateralmovement of upper soil mass on pile bending moment andsoil reaction force through model test and found that whenthere is vertical load on the pile top the maximum defor-mation of the pile body the maximum bending moment andthe soil reaction caused by the movement of the upper soilmass are obviously smaller than those caused by no verticalload on the pile top According to Zhou et al [13] takingWuding Expressway project in loess region as an example theshaft skin resistance of six test piles is studied through staticload testing and multiparameter statistical analysis +emultiparameter statistical analysis method is compared withthe static load test results and the error is controlled within20 Hai and Fellenius [14] conducted static loading testsusing single-level O-cells at Ho Chi Minh City Vietnam andshowed the shaft resistance to be postpeak softening and thepile toe stress-movement responses were essentially linear andalmost identical for the two piles With the development ofthe computer numerical simulation technology such as finiteelement method and finite difference method the numericalsimulation method is gradually applied to the prediction andevaluation of the bearing capacity of pile foundation [15 16]

+e numerical simulation calculation results and theexperimental results are easily affected by soil propertiesand there are few researches on the bearing capacity of pilefoundation in the landslide area +erefore in this paper westudy the field in which the vertical static load test of largediameter manually bored cast-in-place pile was carried outin the Baishi Mountain located in southwest of China tostudy the bearing performance and load transfer law of pilefoundation in the slope area

2 Soil Conditions

+e test is carried out in the slope area of Baishi Mountain+e rocks in the test site are in different sizes and shapes andthe geologic structure is uneven Before the static load test ofpile foundation multichannel transient surface wave testand superheavy (N120) cone dynamic sounding test areconducted on the test site area in order to determine thephysical and mechanical properties of gravel soil

In the multichannel transient surface wave test DZQ24-2A series high-resolution seismograph and 24-poundhammering source are used +ere are 10 observation sys-tems with a distance of 2m between each two+eminimumoffset distance is 10 meters the sampling interval is 512 microsand the total number of sampling points is 1024 In view ofthe difference in frequency and propagation speed betweensurface wave and other seismic wave fields and the dis-persion characteristic of surface wave f-k analysis method[17] was used in data processing and the shear wave velocityprofile was generated after Kriging meshing and Gaussiansmoothing As shown in Figure 1 the abscissa is the sectionlength L (m) and the ordinate is the formation depth Z (m)

According to the profile of shear wave velocity in Figure 1the site can be divided into four layers +e first layer iscultivated soil with a depth of about 1 meter and an averageshear wave velocity is less than 150ms +e second layer issandy soil a depth about 1 to 3 meters and an average shearwave velocity of 150 to 200ms +e third layer is silty claywith a relatively a small amount of gravel soil content a depthabout 3 to 6 meters and an average shear wave velocity of 200to 250ms+e fourth layer is gravel soil with a depth of morethan 6m and a wave velocity greater than 250ms In thelongitudinal direction the wave velocities of surface soil anddeep soil are quite different In the transverse direction thedistribution of chromatic aberration in shear wave velocityimage is chaotic According to the above findings it can beconcluded that the shear wave velocity of rock and soil masson the site changes greatly and the testing site is composed ofmany kinds of rock and soil with different sizes and shapesand shows the characteristics of strong nonuniformity

Cone dynamic sounding test is a commonly used in-situtest method to evaluate the mechanical properties ofgranular soil In this test 4 locations are selected as testpoints and superheavy 120 kg dynamic penetrometer wasused+e statistical results of penetration hammering countsare shown in Table 1 +e relationship between hammeringcounts and penetration depth is shown in Figure 2

According to the standard values of the superheavy N120dynamic sounding hammering count of the four test points andChina Code for Investigation of Geotechnical Engineering(GB50021 2016) [18] it can be concluded that the density ofgravel soil in the site is of medium density From the single-holehammering count obtained from the test it can be seen thathigh hammering counts and low hammering counts are in-terwoven indicating that the particle size of rock and soil masson the site is complicated and the uniformity is poor In order toinvestigate the grain size relationship of soil and rock of eachtest pile the field stratified sampling and screening test wascarried out on the gravel soil Due to the crushing of machinesand tools the topsoil (about 1m thick) was removed before thetest Samples were taken from the point of 1m and thensampled at intervals of 3m in sections with each sectionsampling about 200 kg After sampling the soil was dried in anoven (105sim110degC) and the block stones with a short side di-ameter ofmore than 60mmwere weighed+en the remaining

0 4 8 12 16 20 24 28 32 36 40 44 48(m)

(m)

(m)

0

3

6

9

12

15

0

3

6

9

12

15

No1 No4

No2No3

150172197226256296339388455

(ms)

Figure 1 Profile of shear wave velocity on site area

2 Advances in Civil Engineering

soil was placed on the standard screen selected in batches of10 kg each time for vibration screening for 10 to 15 minutesFinally the mass of samples on each screen was weighed andthe soil gradation was calculated +e statistical analysis of testdata is shown in Table 2 And the grain size distribution curvesat the site of A B C and D are shown in Figure 3

During on-site screening at positions A B and C with adepth from 1 to 4m the particle sizes of rock and soil bodiesdiffer greatly with missing intermediate particles faultsnonuniform soil particles and easy compaction At Dscreening positions the particles with a depth of less than4m are relatively uniform and easy to compact According tostatistical data the nonuniformity coefficient Cu of gravelsoil is 4042 to 35641 the average value is 9461 +e cur-vature coefficient Cc is 245 to 4359 with an average value of3227 According to the grain size distribution curves (asseen in Figure 3) it can be seen that on the site there is alarge difference in particle size soil particles are nonuni-form and particles are loose +e site is a poorly graded siteand is not conducive for stabilizing the foundation

According to the results of the above-mentioned mul-tichannel transient surface wave test superheavy (N120) conedynamic sounding test and gravel soil particle grading testit is concluded that the geological structure of the site iscomplex the content of gravel soil is high and the non-uniformity between particles is high Based on the prelim-inary engineering survey report and the above-mentionedin-situ test results detailed physical parameters of the rockand earth mass on the site are obtained as shown in Table 3

3 Pile Test Program

31 Pile Foundation Construction According to the above-mentioned in-situ test results in order to study the internalforce characteristics of pile shaft axial force shaft skin re-sistance ultimate tip resistance and other internal force

Table 1 Statistical table of test indexes of superheavy dynamic sounding test (N120)

Test point Scope of statistics (m) Range value Average hammering counts forsingle-hole correction N120

Standard value Variable coefficient

No 1 03～100 096sim19552 755 460 0609No 2 03～100 182sim19552 882 436 049No 3 03～100 188sim17766 856 386 0450No 4 03～150 1sim20032 788 398 0505

0 2 4 6 8 10 12 14 16 18 20 22

3m10

m3m

No 1No 2

No 3No 4

SPT N

Average value of SPT N

Cultivatedsoil

Muddysilty clay

Gravelsoil

16

14

12

10

8

6

4

2

0

Dep

th Z

m

Figure 2 SPT N120 values with depth

Table 2 Statistics and analysis of gravel soil screening test andconventional physical parameters

Specimen number Sample depth (m) Cu CcA 1sim4 15643 4357A 4sim7 4042 2494A 7sim10 6228 3232B 1sim4 17627 3303B 4sim7 5297 3175B 7sim10 466 2763C 1sim4 35641 4359C 4sim7 7738 3806C 7sim10 5333 291D 1sim4 9133 2737D 4sim7 467 2747D 7sim10 6853 4123D 10sim13 548 272D 13sim15 4105 245Statistics 14 14Average value 9461 3227Maximum value 35641 4359Minimum value 4042 245

Advances in Civil Engineering 3

characteristics of pile foundation in gravel soil bearing stratumof different depths static load tests are carried out in the areawhere shear wave velocity is relatively uniform ie within 30 to38m along the length direction of wave velocity profile+e testpile is a C35 manually excavated cast-in-place pile with a di-ameter of 09m A total of 4 piles were arranged See Figures 1and 4 for the plan layout and pile numbers +e characteristicparameters of each tested pile are shown in Table 4

+e No 1 pile is empty-bottom pile (as shown inFigure 5(a)) and Nos 2 3 and 4 piles are solid-bottom piles(as shown in Figures 5(b) and 5(c) resp)+e empty-bottompile is welded to a 20mm thick circular steel plate at thebottom of the steel cage the diameter of the steel plate is(850mm) slightly smaller than the diameter of the pile holeWhen pouring concrete the steel cage is suspended to makethe bottom of the steel cage suspended 1m

+ere are 12Φ 14 HRB 400 reinforcements used as mainreinforcements of all test piles +e inner stirrup adoptsHPB235 reinforcement with a diameter of 16mm and itstarts from 2m below the top of the longitudinal rein-forcement and is evenly arranged each 2m+e outer stirrupadopts HPB235 reinforcement with a diameter of 8mm andit is arranged from below the longitudinal rib top +espacing between the outer stirrup and the longitudinal ribtop is 100mm within the range of 3m and the rest of thespace is 200mm At the same time 6 anchor piles with alength of 15m are arranged in 4 of them the diameter is1300mm and the othersrsquo diameter is 1100mm +e

longitudinal bar of anchor piles is composed of 16 and 20finished reinforcing bars with the diameter of 28mm +econcrete reinforcement layout and strain gauges layout ofthe test pile are seen in Figure 5

32 Measuring Instrument Resistive strain gauges aresymmetrically arranged on the main reinforcement (asshown in Figure 5) to measure the vertical strain of rein-forcement and pile body +e DM-YB1840 dynamic andstatic strain testing system automatically collects strain dataand calculates axial force on pile body and shaft skin

100 10 1 01 001

AB

CD

Grain size (dmm)

Pass

rate

(P

)

0

20

40

60

80

100

Figure 3 +e grain size distribution curves at the site of A B C and D

Table 3 Physical and mechanical properties of soil

Soil layer Weight density c (kNm3) Internal friction angle φ (deg) Cohesion C (kPa)Cultivated soil 18 18 15Sandy soil 19 21 mdashSilty clay 19 13 23Gravel soil 20 25 5

30003000

No 4

No 2

3000

No 3

No 1

2500

2500

Figure 4 Layout of pile position

4 Advances in Civil Engineering

Table 4 +e characteristic parameters of each tested pile

Pilesrsquo no Pilesrsquo length L (m) Pilesrsquo diameter D (mm) Concrete strength FormNo 1 10 900 C35 Empty-bottomNo 2 10 900 C35 Solid-bottomNo 3 10 900 C35 Solid-bottomNo 4 15 900 C35 Solid-bottom

1700

1700

1700

1700

1700

300

1000

0

Ground

400

300

1000

0

900

Within 3mbelow the pile

top 100

3m belowthe pile top

200

Under thepile top 2m 2000

Strain gauges

Ground

Steel platewith D 850

20 mm thick

Empty-bottom

12 14

(a)

1600

1600

1600

1600

1700

1700

300

1000

0

400

300

1000

0

900

Ground Ground

Under thepile top 2m 2000

Within 3 mbelow the pile

top 100

3m belowthe pile top

200

Strain gauges

12 14

(b)

1600

1600

1600

1600

1800

1800

1600

1600

1600

300

1500

0

400

300

900

1500

0

GroundGround

Under thepile top 2m 2000

Within 3 mbelow the pile

top 100

3m belowthe pile top

200

Strain gauges

12 14

(c)

900

The stirrup of outside

50mm thickconcrete protective layer

Main reinforcement 120iexclatilde

The stirrup of insideStrain gauges

(d)

Figure 5 +e reinforcement layout and strain gauges layout of the test pile (a) Empty-bottom pile No 1 (b) Solid-bottom pile No 2 andNo 3 (c) Solid-bottom pile No 4 (d) Plan of layout of pile

Advances in Civil Engineering 5

resistance +e embedment of strain gauges shall be carriedout strictly in accordance with the operating instructionsand maintenance work shall be done to improve the successrate for the on-site embedding photo of strain gauges seeFigure 6

33 Static LoadMethod of Pile Foundation In the YD10000-200 hydraulic jack applying load the load reaction force isprovided by anchor pile Four FP-50 displacement sensorswere used for settlement which were installed on the ref-erence beam at the top of the test pile +e support of thesensor was fixed on the jack with magnetic bearing and thereference beam was fixed on the steel beam in an inde-pendent position unaffected by deformation calibrating thedisplacement sensor with the displacement sensor calibra-tion table before the test and using a force transducer tocalibrate the accuracy of the load on the jack +e Y-LINKstatic load test system automatically collects and stores testdata+e vertical compressive static load device of single pileis shown in Figure 7(a) and the photo of field static load testis shown in Figure 7(b)

+e test is carried out according to China Technical Codefor Testing of Building Foundation Piles (JGJ106-2014) [19]the specific steps are as follows

(1) In the test the step loading method was adopted (2)measuring the settlement every 5 minutes 10 minutes and15 minutes after each step of loading was applied every 15minutes after that and every 30 minutes after a total of 1hour and then loading stopped (3) +e settlement per hourcould not exceed 01mm When it occurred twice in suc-cession it was considered to be relatively stable and the nextstep of loading could be applied (4) When the settlement ofthe pile was 5 times of the settlement under the previous loadand the total settlement at the top of the pile exceeded40mm or the settlement of the pile was greater than 2 timesof the settlement under the previous load and had notreached a stable state after 24 hours loading stopped

34 Test Results of Pile Foundation +e line type of load-displacement (Q-S) curve in static load test can macro-scopically reflect pile bodyrsquos failure mechanism and failuremode of soil around pile [20] When the load is small thedisplacement curve tends to develop linearly and is in theelastic stage With the increasing load the pile graduallychanges from elastic deformation to elastic-plastic defor-mation As the load continues to increase the displacementincreases sharply and enters the plastic stage and the pilereaches the failure state +e Q-S curves of piles Nos 1 2 3and 4 are shown in Figure 8

It can be seen from Figure 8 that the Q-S curve of No 1pile has obvious inflection point indicating that it is a steepdrop curve +e pile top load corresponding to the inflectionpoint of Q-S curve is taken as the ultimate bearing capacityof the test pile [21] +e Q-S curves of the other three pileshave similar trends and there is no obvious steep dropsection indicating that they are slowly changing curves sothe ultimate bearing capacity is confirmed according to 442in =e Technical Specification of China Building Foundation

Piles +e bearing capacity under ultimate load of No 1 pileis 3600 kN and the corresponding pile top settlement is1289mm the maximum load is 4000 kN and the maximumsettlement is 4068mm similarly the maximum loadingload of Nos 2 3 and 4 piles and their corresponding pile topdisplacement values bearing capacity under ultimate loadand corresponding pile top displacement and settlementvalues are shown in Table 5

Figure 9 shows the S-lgt curve of settlement of each pilewith time which can further explain the value of ultimatebearing capacity At the last step of loading an obviousdownward bending is shown in the displacement of the fourpiles the displacement exceeded 40mm and the pile reachesthe failure state [22]

4 Discussion

41 =e Characteristics of Pile Shaft Force Transfer +e re-inforcement strain εi(microε) at the corresponding depth is di-rectly measured by the strain gauges meter embedded andthen the reinforcement stress σi is calculated according to

σsi K middot ε0 minus εi( 1113857 (1)

where σi is the stress on the reinforcement K is calibrationcoefficient indicated by the manufacturer and its valueranges from 06 to 08 εi is average strain measured by straingauges on the same section i is the serial numbers of theburied sections of the strain gauges i 1 2 L n are arrangedfrom top to bottom

According to Hookersquos law and the stress on the rein-forcement the real strain value of the strain gauges string iscalculated

εsi σsi

Es

(2)

where εsi is the real strain of the strain gauges string Es is theelastic modulus of reinforcement with a value of204 times 104 MPa σi and i are the same as in formula (1)

Assuming that the compression amount of concrete isequal to the real strain value of the strain gauges string andthe cross-sectional area of the pile is unchanged the concretestress can be obtained according to

σci εsi middot Ec (3)

where σci is the concrete stress value εsi is the real strain ofstrain gauges string Ec is the modulus of elasticity ofconcrete with a value of 104MPa i is the same as in formula(1)

Finally the respective internal forces are obtained bymultiplying stress values by the respective cross-sectionalareas and then the pile axial force of this section is obtainedby the stress of reinforcements plus the stress of concreteand the axial force of other cross-sectional pile bodies can beobtained similarly the calculation formula is as follows

Qi σciAci + σsiAsi (4)

where Qi is the axial force of tested section σci and σsi are thesame as in formulas (3) and (1) respectively Aci is the

6 Advances in Civil Engineering

concrete area of pile body 063585m2 Asi is the area ofreinforcement in pile body 184632 times 10minus3 m2

Figures 9(a)ndash9(d) show the axial force distribution of pilesNos 1 2 3 and 4 with depth under different loading steps

respectively With the increase of loading steps axial force isgradually generated at the lower part of each solid-bottompile and pile tip resistance is gradually mobilized [23] Whenreaching the ultimate load pile tip resistance of piles Nos 2 3and 4 is 2100 kN 1418 kN and 3463 kN respectively+e firstpile is an empty-bottom pile with the increase of load axialforce is gradually generated at the lower part of the pile bodyAfter reaching the ultimate bearing capacity the load willcontinue to be applied and the displacement of pile top willrapidly increase from the original 1289mm to 4068mm+is is because the first pile is an empty-bottom pile and thereis no soil at the bottom to generate tip resistance When theshaft skin resistance reaches the maximum value and cannotresist the axial force in the vertical direction if the loadcontinues to be applied after reaching the ultimate bearingcapacity sudden settlement of the pile body occurs and thesettlement exceeds the limit specified in the specificationindicating that pile failure has occurred

It can be seen from the stress conditions of the solid-bottom pile the empty-bottom pile the 10-meter-long pilebody and the 15-meter-long pile body in Figure 10 that theaxial force of the empty-bottom pile and the solid-bottompile is relatively small at the initial loading step With theincrease of loading step the axial force gradually develops

(a) (b)

Figure 6 Site photo of strain gauges embedding (a) Part photo (b) +e overall photo

Displacementmeter

Hydraulicjack

Test beam(s)

Test pile

Steel testplate

Load transfercolumn

Anchor pileMinimum clear

distance is 25m

(a) (b)

Figure 7 Schematic diagram and field photo of static load test (a) Schematic diagram (b) Photograph

0 1200 2400 3600 4800 6000 7200 8400 9600

Disp

lace

men

t (m

m)

Load at pile top (kN)

No 1No 2

No 3No 4

45

40

35

30

25

20

15

10

5

0

Figure 8 Load-displacement curve of static load test

Advances in Civil Engineering 7

Table 5 Results of static load test

Pile number Maximum load (kN) Maximum load settlement (mm) Bearing capacity underultimate load (kN)

Settlement underultimate load (mm) Rebound rate ()

No 1 4000 4068 3600 1289 1006No 2 5400 401 4800 3001 1236No 3 4800 4051 4200 2892 1684No 4 8800 4083 8000 2676 1894

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

lgt (min)

S (m

m)

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

(a)

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

lgt (min)

S (m

m)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

(b)

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

lgt (min)

S (m

m)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

(c)

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

S (m

m)

lgt (min)

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

(d)

Figure 9 S-lgt curve of each pile (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

8 Advances in Civil Engineering

and transmits to the pile bottom and decreases with theincrease of the depth Under different loading steps theattenuation rate of axial force curve is different +e at-tenuation rate of axial force curve reflects the development ofshaft skin resistance In low loading steps the attenuationrate of axial force curve is not fast and relatively uniformWith the increase of loading steps the attenuation rate ofaxial force curve at the upper part of pile body is not fastwhile the attenuation rate of axial force curve at the lowerpart of pile body is obvious which shows that no matter it isan empty-bottom pile or solid-bottom pile the force

transmission characteristics are similar With the increase ofloading steps the axial force gradually transmits to the lowerpart of the pile body If load continues to be applied afterreaching the ultimate load the empty-bottom pile will besuddenly destroyed while the solid-bottom pile will be able tobear due to soil in the lower part and will show a slow change

42 Analysis of Characteristics of Shaft Skin Resistance of PileBody Based on the principle of static balance regardless ofthe influence of pile body weight [24] the pile shaft skin

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

0 400 800 1200 1600 2000 2400 2800 3200 3600 4000 4400Axial force (kN)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

(a)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 600 1200 1800 2400 3000 3600 4200 4800 5400 6000Axial force (kN)

(b)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 600 1200 1800 2400 3000 3600 4200 4800 5400Axial force (kN)

(c)

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

16151413121110

9876543210

Dep

th Z

m

0 800 1600 2400 3200 4000 4800 5600 6400 7200 8000 8800Axial force (kN)

(d)

Figure 10 Distribution curve of axial force of each pile (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Advances in Civil Engineering 9

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

(a)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

(b)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

(c)

16151413121110

9876543210

Dep

th Z

m

0 20 40 60 80 100 120 140 160 180Skin friction of test pile (kPa)

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

(d)

Figure 11 Distribution of shaft skin resistance on each pile (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Table 6 Value of shaft skin resistance of piles at different depths when reaching ultimate bearing capacity

(a) No 1 pileDepth Zm 17 34 51 68 85Skin frictionkPa 60177 112974 133037 139158 135235

(b) No 2 pileDepth Zm 16 32 48 64 81 98Skin frictionkPa 43899 105055 128283 1391 146983 148015

(c) No 3 pileDepth Zm 16 32 48 64 81 98Skin frictionkPa 46948 106907 130723 145894 147938 150828

(d) No 4 pileDepth Zm 16 32 48 64 8 96 112 13 148Skin frictionkPa 63055 93009 117592 129804 141642 144953 145865 151216 154956

10 Advances in Civil Engineering

resistance value can be calculated by the following formulaaccording to the change value of the axial force of the pilebody

qsi Qi minus Qi+1

u middot li (5)

where qsi is the resistance between the i section and the i + 1section i is the same as in formula (1) u is the perimeter ofpile body li is the pile length between the i section and thei + 1 section

Figures 10(a)ndash10(d) show the distribution of shaft skinresistance of piles Nos 1 2 3 and 4 with depth underdifferent steps of loading

As can be seen from Figure 11 in lower loading step theshaft skin resistance of each pile is small with the increase ofloading steps the shaft skin resistance of each pile increasesOn the whole the shaft skin resistance of the pile is alsogradually transmitted from the upper part of the pile body tothe lower part of the pile body but compared with themiddle part of the pile body the shaft skin resistance of thelower part of the first pile body with empty-bottom shows a

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

10

8

6

4

2

0

Dep

th Z

m

(a)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

10

8

6

4

2

0

Dep

th Z

m

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

(b)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

10

8

6

4

2

0

Dep

th Z

m

(c)

Dep

th Z

m

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

151413121110

9876543210

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

(d)

Figure 12 Pile-soil relative displacement curve (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Advances in Civil Engineering 11

decreasing trend because there is no soil bearing the cor-responding load at the pile tip however for the remainingthree solid-bottom piles when the tip resistance is mobilizedto a certain extent the friction resistance in the middle partof the pile body is smaller than that in the lower part whichindicates that the soil at the pile tip can reinforce the shaftskin resistance of the pile [25] When the ultimate bearingcapacity is reached the shaft skin resistance values of thecorresponding piles at various depths are shown in Table 6

43 Relationship between Shaft Skin Resistance and Pile-SoilRelative Displacement +e pile-soil relative displacementobjectively reflects the mobilization of shaft skin resistance

According to the calculation method in [26] the pile-soilrelative displacement curves under different loading stepsare calculated and drawn +e curves of shaft skin resistanceand pile-soil relative displacement are shown inFigures 12(a)ndash12(d) respectively

In the calculation of the pile-soil relative displacement itis assumed that the soil around the pile will be not displacedAs can be seen from Figure 12 the pile-soil relative dis-placement at the same position of the pile body increases withthe increase of loading steps on the pile tipWhen the first twosteps of loading are applied the pile-soil relative displacementof the four piles increases obviously due to the compactioneffect of the first two steps of loading on the gravel soil aroundthe pile As the load level continues to increase the pile-soil

Skin

fric

tion

of te

st pi

le (k

Pa)

5 10 15 20 25 300Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

1-11-21-3

1-41-5

(a)Sk

in fr

ictio

n of

test

pile

(kPa

)

0 10 15 20 25 30 35 405Pile-soil relative displacement (mm)

2-12-22-3

2-42-52-6

0

20

40

60

80

100

120

140

160

(b)

Skin

fric

tion

of te

st pi

le (k

Pa)

5 10 15 20 25 30 35 400Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

3-13-23-3

3-43-53-6

(c)

Skin

fric

tion

of te

st pi

le (k

Pa)

0 10 15 20 25 30 35 405Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

4-14-24-3

4-44-54-6

4-74-84-9

(d)

Figure 13 Distribution curves of shaft skin resistances versus relative displacements (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4pile

12 Advances in Civil Engineering

relative displacement increment is relatively small because theshaft skin resistance gradually mobilizes its function toprevent the pile from moving With the increase of loadingwhen the shaft skin resistance cannot resist the load at theloading sections 4800 kNndash5400 kN 4200 kNndash4800 kN8000 kNndash8800 kN and 3600 kNndash4000 kN the maximumdisplacements of pile tips of Nos 2 3 and 4 solid-bottom pileand the empty-bottom pile increase by 9792mm 7939mm9881mm and 1497mm respectively

+e shaft skin resistance increases with the increase ofpile-soil relative displacement Although continuous loadinghas led to foundation soil destruction it can be concludedfrom Figure 13 that the lateral resistance in the middle andlower parts is not fully mobilized and a certain pile-soilrelative displacement is needed to promote the full mobi-lization of the shaft skin resistance However according tothe development trend that the slope of the shaft skin re-sistance gradually decreases it can be inferred that the shaftskin resistance will reach the maximum value At the firstsection position the ultimate shaft skin resistance of pilesNos 1 2 3 and 4 are 4695 kPa 4390 kPa 6018 kPa and6305 kPa respectively and the corresponding pile-soil rel-ative displacements are 3419mm 3442mm 2633mm and3279mm In the second section the shaft skin resistance ofthe four piles increases greatly so does the pile-soil relativedisplacement Since the soil above the first section position isbasically cultivated soil and it can resist small shaft skinresistance while the content of gravel soil of the secondsection is large and with the increase of depth the sur-rounding pressure of soil around the pile increases thereforethe ability of resisting pile foundation sliding and dis-placement increases+erefore between the first section andthe second section the shaft skin resistance increases greatlywhile the pile-soil relative displacement increases a little Inthe second section and beyond the soil is gravel soil and theshaft skin resistance increases slightly with the increase ofpile depth and the relative displacement decreases gradually

44 Pile Tip Resistance Varies with Load and DisplacementDuring the test the section where the largest embeddeddepth of strain gauges is very close to the bottom of the pilewas shown so the resistance at the pile tip is approximatelyequal to the axial force measured at this section namelyQb Q0 Under the action of all loading steps the resistancevariation curves at the lower tips of the Nos 2 3 and 4 pilesare shown in Figure 14

+e No 2 and No 3 piles in the first two loading stepsand No 4 pile in the first loading step have smaller resistanceat the pile tip +e resistance at the pile tip gradually in-creases until the third and fourth loading steps Under theaction of ultimate load the ratio of pile tip resistance to piletop load of Nos 2 3 and 4 is 377 3008 and 3992respectively all higher 15 which shows that pile tip re-sistance has been brought into play to a certain extent +epile tip resistance is shared by shaft skin resistance and piletip resistance and the pile is of tip bearing friction pile

According to the relationship curve between pile tipresistance and pile tip displacement shown in Figure 15 it

can be seen that the pile tip resistance and pile tip dis-placement present a double fold line model of work hard-ening +e Nos 2 3 and 4 piles all show a slow deformationfailure and under the action of the first loading step theslope of the curve is larger and the corresponding dis-placement is relatively small in latter steps of loading thecurvature curve becomes smoother the displacement of thepile tips of adjacent loads increases and the soil at the piletips will gradually be compacted

Pile

tip

load

(kN

)

1000 2000 3000 4000 5000 6000 7000 8000 90000Load at pile top (kN)

0250500750

100012501500175020002250250027503000325035003750

No 2No 3No 4

Figure 14 Variation curve of pile tip resistance under differentloading steps (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4pile

Pile

tip

load

(kN

)

5 10 15 20 25 30 35 40 450Pile tip displacement (mm)

0

1000

2000

3000

4000

5000

6000

No 2No 3No 4

Figure 15 Relationship curve between pile tip resistance and piletip displacement (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No4 pile

Advances in Civil Engineering 13

5 Conclusions

Analyses are carried out on data of the in-situ test of unevengravel soil in Baishi Mountain and the following conclu-sions are drawn

(1) Based on the analysis of the results of multichanneltransient surface wave test superheavy (N120) conedynamic sounding test and on-site screening testthe geological environment conditions of the unevengravel soil on the site are further explored +e stratawithin the range from 0 to 15m in Baizhi Mountaincan be divided into four layers ie cultivated soilsandy soil silty clay and gravel soil with depths of1m 2m 3m and 9m respectively

(2) +e axial force and shaft skin resistance of pile bodyin gravel soil are the same as those in other geologicalstructures At the initial loading step the axial forceand shaft skin resistance of pile body play a smallerrole With the increase of loading steps the axialforce and shaft skin resistance of pile body increasecorrespondingly Under the same loading step theaxial force of the pile body and the shaft skin re-sistance do not mobilize in the same time andgradually transfer from the upper part to the lowerpart of the pile body and the upper part of the pilebody reaches the limit value before the middle andlower parts As for the solid-bottom pile the soil atthe pile tip will strengthen the shaft skin resistance ofthe pile

(3) According to this experimental study it is suggestedthat similar projects should adopt concrete manuallybored cast-in-place piles with a diameter of 900mmand a length of 15m the wall-protecting thicknessshall not be less than 175mm and the grade ofconcrete cannot be lower than C35 +e pile end canbe widened or not and the pile bottom shall be freeof sediment

Data Availability

All the data in this paper are obtained from tests in thisstudy and no other data were used to support this study

Conflicts of Interest

+e authors declare that with regard to the publication ofthis paper and the funding for publishing it there are noconflicts of interest

Acknowledgments

+is study was partially supported by the National NaturalScience Foundation of China (Grant no 41302223) ChongqingNo 3 colleges and universities youth backbone teachers fundingplans Chongqing Research Program of Basic Research andFrontier Technology (Grant no cstc2016jcyjA0074cstc2016jcyjA0933 and cstc2015jcyjA90012) Scientific andTechnological Research Program of Chongqing Municipal

Education Commission (Grant no KJ1713327 and KJ1600532)Key Laboratory ofWell Stability and Fluid and RockMechanicsin Oil and Gas Reservoir of Shaanxi Province Xirsquoan ShiyouUniversity (Grant no FRM20190201002) Chongqing Post-graduate Research Innovation Project (Grant no CYS19353)and Chongqing University of Science and Technology GraduateStudentsrsquo Science and Technology Innovation Program (Grantno YKJCX1720601 and YKJCX1920613)

References

[1] F Kirsch and B Plabmann ldquoDynamic methods in pile testingdevelopments in measurement and analysisrdquo in Proceedings ofthe International Deep Foundations Congress 2002 pp 868ndash882 ASCE Orlando FL USA February 2002

[2] S H Ni L Lehmann J J Charng et al ldquoLow-strain integritytesting of drilled piles with high slenderness ratiordquo Computersand Geotechnics vol 33 no 6-7 pp 283ndash293 2006

[3] K Mori A Spagnoli Y Murakami G Kondo and I TorigoeldquoA new non-contacting non-destructive testing method fordefect detection in concreterdquo NDT amp E International vol 35no 6 pp 399ndash406 2002

[4] G Cai S Liu L Tong et al ldquoAssessment of direct CPT andCPTUmethods for predicting the ultimate bearing capacity ofsingle pilesrdquo Engineering Geology vol 104 no 3-4pp 211ndash222 2009

[5] X Y Bai M Y Zhang L Zhu et al ldquoIn-situ test and finiteelement analysis of bearing characteristics of rock-socketedshort piles in strongly weathered graniterdquo Journal of CentralSouth University Natural Science Edition vol 48 no 2pp 512ndash524 2017 in Chinese

[6] E C Leong and M F Randolph ldquoFinite element modelling ofrock-socketed pilesrdquo International Journal for Numerical andAnalytical Methods in Geomechanics vol 18 no 1 pp 25ndash471994

[7] G Gao M Gao Q Chen and J Yang ldquoField load testingstudy of vertical bearing behavior of a large diameter belledcast-in-place pilerdquo KSCE Journal of Civil Engineering vol 23no 5 pp 2009ndash2016 2019

[8] R Berardi and R Bovolenta ldquoPile-settlement evaluation usingfield stiffness non-linearityrdquo Proceedings of the Institution ofCivil Engineers - Geotechnical Engineering vol 158 no 1pp 35ndash44 2005

[9] B H Fellenius Basics of Foundation Design Lulu PressMorrisville NC USA 2016

[10] C Lam and S A Jefferis ldquoCritical assessment of pile modulusdetermination methodsrdquo Canadian Geotechnical Journalvol 48 no 10 pp 1433ndash1448 2011

[11] A Mohammadi T Ebadi and M R Boroomand ldquoPhysicalmodelling of axial compressive bearing capacity of instru-mented piles in oil-contaminated sandy soilrdquo Iranian Journalof Science and Technology Transactions of Civil Engineeringvol 44 no 2 pp 695ndash714 2019

[12] W D Guo and E H Ghee ldquoBehavior of axially loaded pilegroups subjected to lateral soil movementrdquo in Proceedings ofthe GeoShanghai International Conference 2006 pp 174ndash181ASCE Shanghai China June 2006

[13] Z Zhou Y Dong P Jiang D Han and T Liu ldquoCalculation ofpile side friction by multiparameter statistical analysisrdquo Ad-vances in Civil Engineering vol 2019 Article ID 263852012 pages 2019

14 Advances in Civil Engineering

[14] N M Hai and B H Fellenius ldquoO-Cell tests on two 70 m longbored piles in Vietnamrdquo in Proceedings of the Geo-Congress2014 pp 482ndash496 ASCE Atlanta Georgia February 2014

[15] S Gong G Cai S Liu et al ldquoNumerical Simulation of BearingCapacity and Consolidation Characteristics of PHC PileFoundationrdquo GeoShanghai International Conferencepp 178ndash185 Springer Singapore 2018

[16] M A Bashir H Furuuchi T Ueda and M Nauman BashirldquoNumerical simulation of axial anchorage capacity of con-crete-filled steel box footingrdquo Iranian Journal of Science andTechnology Transactions of Civil Engineering vol 40 no 3pp 257ndash262 2016

[17] N A Haskell ldquo+e dispersion of surface waves on multi-layered mediardquo Bulletin of the Seismological Society ofAmerica vol 43 no 1 pp 17ndash34 1953

[18] China Architecture amp Building PressNational Standard of thePeoplersquos Republic of China Code for Investigation of Geo-technical Engineering (GB50021 2016) China Architecture ampBuilding Press Beijing China 2016 in Chinese

[19] China Architecture amp Building Press Industry Standard of thePeoplersquos Republic of China Technical Code for Testing ofBuilding Foundation Piles (JGJ106-2014) China Architectureamp Building Press Beijing China 2014 in Chinese

[20] N Guo Z H Chen X F Huang et al ldquoResearch on the anti-pulling test of large-diameter cloth bag pile in soft rockfoundationrdquo Rock and Soil Mechanics vol 36 no 2pp 603ndash609 2015 in Chinese

[21] Y H Li X Zhu and T H Zhou ldquoField comparative ex-perimental study on the influence of post-tip grouting onlarge-diameter cast-in-situ pilesrdquo Rock and Soil Mechanicsvol 37 no 2 pp 388ndash396 2016 in Chinese

[22] G F Xin Z M Zhang T Xia et al ldquoExperimental study onbearing behavior of ultra-long pile under high loadrdquo ChineseJournal of Rock Mechanics and Engineering vol 13pp 2397ndash2402 2005 in Chinese

[23] Z Ling W Wang J Wu et al ldquoFull-scale loading test on pre-bored precast pile with enlarged base in Shanghairdquo in Pro-ceedings of the GeoShanghai 2018 International ConferenceAdvances in Soil Dynamics and Foundation Engineeringpp 637ndash645 Springer Singapore 2018

[24] J R Dong S T Lin and Y M Dai ldquo+e load tranfer behaviorof large diameter cast-in-situ pile in crushed pebble stratumrdquoChinese Journal of Geotechnical Engineering vol 16 no 6pp 123ndash131 1994

[25] X F Huang J H Zhang L G Ma et al ldquoExperimental studyon reinforcement effect of lateral resistance of collapsible loessrock-socketed perfusion pilerdquo Architectural Structure vol 41no 2 pp 351ndash355 2011 in Chinese

[26] J-J Zhou K-H Wang X-N Gong and R-H ZhangldquoBearing capacity and load transfer mechanism of a static drillrooted nodular pile in soft soil areasrdquo Journal of ZhejiangUniversity Science A vol 14 no 10 pp 705ndash719 2013

Advances in Civil Engineering 15

soil was placed on the standard screen selected in batches of10 kg each time for vibration screening for 10 to 15 minutesFinally the mass of samples on each screen was weighed andthe soil gradation was calculated +e statistical analysis of testdata is shown in Table 2 And the grain size distribution curvesat the site of A B C and D are shown in Figure 3

During on-site screening at positions A B and C with adepth from 1 to 4m the particle sizes of rock and soil bodiesdiffer greatly with missing intermediate particles faultsnonuniform soil particles and easy compaction At Dscreening positions the particles with a depth of less than4m are relatively uniform and easy to compact According tostatistical data the nonuniformity coefficient Cu of gravelsoil is 4042 to 35641 the average value is 9461 +e cur-vature coefficient Cc is 245 to 4359 with an average value of3227 According to the grain size distribution curves (asseen in Figure 3) it can be seen that on the site there is alarge difference in particle size soil particles are nonuni-form and particles are loose +e site is a poorly graded siteand is not conducive for stabilizing the foundation

According to the results of the above-mentioned mul-tichannel transient surface wave test superheavy (N120) conedynamic sounding test and gravel soil particle grading testit is concluded that the geological structure of the site iscomplex the content of gravel soil is high and the non-uniformity between particles is high Based on the prelim-inary engineering survey report and the above-mentionedin-situ test results detailed physical parameters of the rockand earth mass on the site are obtained as shown in Table 3

3 Pile Test Program

31 Pile Foundation Construction According to the above-mentioned in-situ test results in order to study the internalforce characteristics of pile shaft axial force shaft skin re-sistance ultimate tip resistance and other internal force

Table 1 Statistical table of test indexes of superheavy dynamic sounding test (N120)

Test point Scope of statistics (m) Range value Average hammering counts forsingle-hole correction N120

Standard value Variable coefficient

No 1 03～100 096sim19552 755 460 0609No 2 03～100 182sim19552 882 436 049No 3 03～100 188sim17766 856 386 0450No 4 03～150 1sim20032 788 398 0505

0 2 4 6 8 10 12 14 16 18 20 22

3m10

m3m

No 1No 2

No 3No 4

SPT N

Average value of SPT N

Cultivatedsoil

Muddysilty clay

Gravelsoil

16

14

12

10

8

6

4

2

0

Dep

th Z

m

Figure 2 SPT N120 values with depth

Table 2 Statistics and analysis of gravel soil screening test andconventional physical parameters

Specimen number Sample depth (m) Cu CcA 1sim4 15643 4357A 4sim7 4042 2494A 7sim10 6228 3232B 1sim4 17627 3303B 4sim7 5297 3175B 7sim10 466 2763C 1sim4 35641 4359C 4sim7 7738 3806C 7sim10 5333 291D 1sim4 9133 2737D 4sim7 467 2747D 7sim10 6853 4123D 10sim13 548 272D 13sim15 4105 245Statistics 14 14Average value 9461 3227Maximum value 35641 4359Minimum value 4042 245

Advances in Civil Engineering 3

characteristics of pile foundation in gravel soil bearing stratumof different depths static load tests are carried out in the areawhere shear wave velocity is relatively uniform ie within 30 to38m along the length direction of wave velocity profile+e testpile is a C35 manually excavated cast-in-place pile with a di-ameter of 09m A total of 4 piles were arranged See Figures 1and 4 for the plan layout and pile numbers +e characteristicparameters of each tested pile are shown in Table 4

+e No 1 pile is empty-bottom pile (as shown inFigure 5(a)) and Nos 2 3 and 4 piles are solid-bottom piles(as shown in Figures 5(b) and 5(c) resp)+e empty-bottompile is welded to a 20mm thick circular steel plate at thebottom of the steel cage the diameter of the steel plate is(850mm) slightly smaller than the diameter of the pile holeWhen pouring concrete the steel cage is suspended to makethe bottom of the steel cage suspended 1m

+ere are 12Φ 14 HRB 400 reinforcements used as mainreinforcements of all test piles +e inner stirrup adoptsHPB235 reinforcement with a diameter of 16mm and itstarts from 2m below the top of the longitudinal rein-forcement and is evenly arranged each 2m+e outer stirrupadopts HPB235 reinforcement with a diameter of 8mm andit is arranged from below the longitudinal rib top +espacing between the outer stirrup and the longitudinal ribtop is 100mm within the range of 3m and the rest of thespace is 200mm At the same time 6 anchor piles with alength of 15m are arranged in 4 of them the diameter is1300mm and the othersrsquo diameter is 1100mm +e

longitudinal bar of anchor piles is composed of 16 and 20finished reinforcing bars with the diameter of 28mm +econcrete reinforcement layout and strain gauges layout ofthe test pile are seen in Figure 5

32 Measuring Instrument Resistive strain gauges aresymmetrically arranged on the main reinforcement (asshown in Figure 5) to measure the vertical strain of rein-forcement and pile body +e DM-YB1840 dynamic andstatic strain testing system automatically collects strain dataand calculates axial force on pile body and shaft skin

100 10 1 01 001

AB

CD

Grain size (dmm)

Pass

rate

(P

)

0

20

40

60

80

100

Figure 3 +e grain size distribution curves at the site of A B C and D

Table 3 Physical and mechanical properties of soil

Soil layer Weight density c (kNm3) Internal friction angle φ (deg) Cohesion C (kPa)Cultivated soil 18 18 15Sandy soil 19 21 mdashSilty clay 19 13 23Gravel soil 20 25 5

30003000

No 4

No 2

3000

No 3

No 1

2500

2500

Figure 4 Layout of pile position

4 Advances in Civil Engineering

Table 4 +e characteristic parameters of each tested pile

Pilesrsquo no Pilesrsquo length L (m) Pilesrsquo diameter D (mm) Concrete strength FormNo 1 10 900 C35 Empty-bottomNo 2 10 900 C35 Solid-bottomNo 3 10 900 C35 Solid-bottomNo 4 15 900 C35 Solid-bottom

1700

1700

1700

1700

1700

300

1000

0

Ground

400

300

1000

0

900

Within 3mbelow the pile

top 100

3m belowthe pile top

200

Under thepile top 2m 2000

Strain gauges

Ground

Steel platewith D 850

20 mm thick

Empty-bottom

12 14

(a)

1600

1600

1600

1600

1700

1700

300

1000

0

400

300

1000

0

900

Ground Ground

Under thepile top 2m 2000

Within 3 mbelow the pile

top 100

3m belowthe pile top

200

Strain gauges

12 14

(b)

1600

1600

1600

1600

1800

1800

1600

1600

1600

300

1500

0

400

300

900

1500

0

GroundGround

Under thepile top 2m 2000

Within 3 mbelow the pile

top 100

3m belowthe pile top

200

Strain gauges

12 14

(c)

900

The stirrup of outside

50mm thickconcrete protective layer

Main reinforcement 120iexclatilde

The stirrup of insideStrain gauges

(d)

Figure 5 +e reinforcement layout and strain gauges layout of the test pile (a) Empty-bottom pile No 1 (b) Solid-bottom pile No 2 andNo 3 (c) Solid-bottom pile No 4 (d) Plan of layout of pile

Advances in Civil Engineering 5

resistance +e embedment of strain gauges shall be carriedout strictly in accordance with the operating instructionsand maintenance work shall be done to improve the successrate for the on-site embedding photo of strain gauges seeFigure 6

33 Static LoadMethod of Pile Foundation In the YD10000-200 hydraulic jack applying load the load reaction force isprovided by anchor pile Four FP-50 displacement sensorswere used for settlement which were installed on the ref-erence beam at the top of the test pile +e support of thesensor was fixed on the jack with magnetic bearing and thereference beam was fixed on the steel beam in an inde-pendent position unaffected by deformation calibrating thedisplacement sensor with the displacement sensor calibra-tion table before the test and using a force transducer tocalibrate the accuracy of the load on the jack +e Y-LINKstatic load test system automatically collects and stores testdata+e vertical compressive static load device of single pileis shown in Figure 7(a) and the photo of field static load testis shown in Figure 7(b)

+e test is carried out according to China Technical Codefor Testing of Building Foundation Piles (JGJ106-2014) [19]the specific steps are as follows

(1) In the test the step loading method was adopted (2)measuring the settlement every 5 minutes 10 minutes and15 minutes after each step of loading was applied every 15minutes after that and every 30 minutes after a total of 1hour and then loading stopped (3) +e settlement per hourcould not exceed 01mm When it occurred twice in suc-cession it was considered to be relatively stable and the nextstep of loading could be applied (4) When the settlement ofthe pile was 5 times of the settlement under the previous loadand the total settlement at the top of the pile exceeded40mm or the settlement of the pile was greater than 2 timesof the settlement under the previous load and had notreached a stable state after 24 hours loading stopped

34 Test Results of Pile Foundation +e line type of load-displacement (Q-S) curve in static load test can macro-scopically reflect pile bodyrsquos failure mechanism and failuremode of soil around pile [20] When the load is small thedisplacement curve tends to develop linearly and is in theelastic stage With the increasing load the pile graduallychanges from elastic deformation to elastic-plastic defor-mation As the load continues to increase the displacementincreases sharply and enters the plastic stage and the pilereaches the failure state +e Q-S curves of piles Nos 1 2 3and 4 are shown in Figure 8

It can be seen from Figure 8 that the Q-S curve of No 1pile has obvious inflection point indicating that it is a steepdrop curve +e pile top load corresponding to the inflectionpoint of Q-S curve is taken as the ultimate bearing capacityof the test pile [21] +e Q-S curves of the other three pileshave similar trends and there is no obvious steep dropsection indicating that they are slowly changing curves sothe ultimate bearing capacity is confirmed according to 442in =e Technical Specification of China Building Foundation

Piles +e bearing capacity under ultimate load of No 1 pileis 3600 kN and the corresponding pile top settlement is1289mm the maximum load is 4000 kN and the maximumsettlement is 4068mm similarly the maximum loadingload of Nos 2 3 and 4 piles and their corresponding pile topdisplacement values bearing capacity under ultimate loadand corresponding pile top displacement and settlementvalues are shown in Table 5

Figure 9 shows the S-lgt curve of settlement of each pilewith time which can further explain the value of ultimatebearing capacity At the last step of loading an obviousdownward bending is shown in the displacement of the fourpiles the displacement exceeded 40mm and the pile reachesthe failure state [22]

4 Discussion

41 =e Characteristics of Pile Shaft Force Transfer +e re-inforcement strain εi(microε) at the corresponding depth is di-rectly measured by the strain gauges meter embedded andthen the reinforcement stress σi is calculated according to

σsi K middot ε0 minus εi( 1113857 (1)

where σi is the stress on the reinforcement K is calibrationcoefficient indicated by the manufacturer and its valueranges from 06 to 08 εi is average strain measured by straingauges on the same section i is the serial numbers of theburied sections of the strain gauges i 1 2 L n are arrangedfrom top to bottom

According to Hookersquos law and the stress on the rein-forcement the real strain value of the strain gauges string iscalculated

εsi σsi

Es

(2)

where εsi is the real strain of the strain gauges string Es is theelastic modulus of reinforcement with a value of204 times 104 MPa σi and i are the same as in formula (1)

Assuming that the compression amount of concrete isequal to the real strain value of the strain gauges string andthe cross-sectional area of the pile is unchanged the concretestress can be obtained according to

σci εsi middot Ec (3)

where σci is the concrete stress value εsi is the real strain ofstrain gauges string Ec is the modulus of elasticity ofconcrete with a value of 104MPa i is the same as in formula(1)

Finally the respective internal forces are obtained bymultiplying stress values by the respective cross-sectionalareas and then the pile axial force of this section is obtainedby the stress of reinforcements plus the stress of concreteand the axial force of other cross-sectional pile bodies can beobtained similarly the calculation formula is as follows

Qi σciAci + σsiAsi (4)

where Qi is the axial force of tested section σci and σsi are thesame as in formulas (3) and (1) respectively Aci is the

6 Advances in Civil Engineering

concrete area of pile body 063585m2 Asi is the area ofreinforcement in pile body 184632 times 10minus3 m2

Figures 9(a)ndash9(d) show the axial force distribution of pilesNos 1 2 3 and 4 with depth under different loading steps

respectively With the increase of loading steps axial force isgradually generated at the lower part of each solid-bottompile and pile tip resistance is gradually mobilized [23] Whenreaching the ultimate load pile tip resistance of piles Nos 2 3and 4 is 2100 kN 1418 kN and 3463 kN respectively+e firstpile is an empty-bottom pile with the increase of load axialforce is gradually generated at the lower part of the pile bodyAfter reaching the ultimate bearing capacity the load willcontinue to be applied and the displacement of pile top willrapidly increase from the original 1289mm to 4068mm+is is because the first pile is an empty-bottom pile and thereis no soil at the bottom to generate tip resistance When theshaft skin resistance reaches the maximum value and cannotresist the axial force in the vertical direction if the loadcontinues to be applied after reaching the ultimate bearingcapacity sudden settlement of the pile body occurs and thesettlement exceeds the limit specified in the specificationindicating that pile failure has occurred

It can be seen from the stress conditions of the solid-bottom pile the empty-bottom pile the 10-meter-long pilebody and the 15-meter-long pile body in Figure 10 that theaxial force of the empty-bottom pile and the solid-bottompile is relatively small at the initial loading step With theincrease of loading step the axial force gradually develops

(a) (b)

Figure 6 Site photo of strain gauges embedding (a) Part photo (b) +e overall photo

Displacementmeter

Hydraulicjack

Test beam(s)

Test pile

Steel testplate

Load transfercolumn

Anchor pileMinimum clear

distance is 25m

(a) (b)

Figure 7 Schematic diagram and field photo of static load test (a) Schematic diagram (b) Photograph

0 1200 2400 3600 4800 6000 7200 8400 9600

Disp

lace

men

t (m

m)

Load at pile top (kN)

No 1No 2

No 3No 4

45

40

35

30

25

20

15

10

5

0

Figure 8 Load-displacement curve of static load test

Advances in Civil Engineering 7

Table 5 Results of static load test

Pile number Maximum load (kN) Maximum load settlement (mm) Bearing capacity underultimate load (kN)

Settlement underultimate load (mm) Rebound rate ()

No 1 4000 4068 3600 1289 1006No 2 5400 401 4800 3001 1236No 3 4800 4051 4200 2892 1684No 4 8800 4083 8000 2676 1894

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

lgt (min)

S (m

m)

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

(a)

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

lgt (min)

S (m

m)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

(b)

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

lgt (min)

S (m

m)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

(c)

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

S (m

m)

lgt (min)

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

(d)

Figure 9 S-lgt curve of each pile (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

8 Advances in Civil Engineering

and transmits to the pile bottom and decreases with theincrease of the depth Under different loading steps theattenuation rate of axial force curve is different +e at-tenuation rate of axial force curve reflects the development ofshaft skin resistance In low loading steps the attenuationrate of axial force curve is not fast and relatively uniformWith the increase of loading steps the attenuation rate ofaxial force curve at the upper part of pile body is not fastwhile the attenuation rate of axial force curve at the lowerpart of pile body is obvious which shows that no matter it isan empty-bottom pile or solid-bottom pile the force

transmission characteristics are similar With the increase ofloading steps the axial force gradually transmits to the lowerpart of the pile body If load continues to be applied afterreaching the ultimate load the empty-bottom pile will besuddenly destroyed while the solid-bottom pile will be able tobear due to soil in the lower part and will show a slow change

42 Analysis of Characteristics of Shaft Skin Resistance of PileBody Based on the principle of static balance regardless ofthe influence of pile body weight [24] the pile shaft skin

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

0 400 800 1200 1600 2000 2400 2800 3200 3600 4000 4400Axial force (kN)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

(a)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 600 1200 1800 2400 3000 3600 4200 4800 5400 6000Axial force (kN)

(b)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 600 1200 1800 2400 3000 3600 4200 4800 5400Axial force (kN)

(c)

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

16151413121110

9876543210

Dep

th Z

m

0 800 1600 2400 3200 4000 4800 5600 6400 7200 8000 8800Axial force (kN)

(d)

Figure 10 Distribution curve of axial force of each pile (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Advances in Civil Engineering 9

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

(a)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

(b)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

(c)

16151413121110

9876543210

Dep

th Z

m

0 20 40 60 80 100 120 140 160 180Skin friction of test pile (kPa)

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

(d)

Figure 11 Distribution of shaft skin resistance on each pile (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Table 6 Value of shaft skin resistance of piles at different depths when reaching ultimate bearing capacity

(a) No 1 pileDepth Zm 17 34 51 68 85Skin frictionkPa 60177 112974 133037 139158 135235

(b) No 2 pileDepth Zm 16 32 48 64 81 98Skin frictionkPa 43899 105055 128283 1391 146983 148015

(c) No 3 pileDepth Zm 16 32 48 64 81 98Skin frictionkPa 46948 106907 130723 145894 147938 150828

(d) No 4 pileDepth Zm 16 32 48 64 8 96 112 13 148Skin frictionkPa 63055 93009 117592 129804 141642 144953 145865 151216 154956

10 Advances in Civil Engineering

resistance value can be calculated by the following formulaaccording to the change value of the axial force of the pilebody

qsi Qi minus Qi+1

u middot li (5)

where qsi is the resistance between the i section and the i + 1section i is the same as in formula (1) u is the perimeter ofpile body li is the pile length between the i section and thei + 1 section

Figures 10(a)ndash10(d) show the distribution of shaft skinresistance of piles Nos 1 2 3 and 4 with depth underdifferent steps of loading

As can be seen from Figure 11 in lower loading step theshaft skin resistance of each pile is small with the increase ofloading steps the shaft skin resistance of each pile increasesOn the whole the shaft skin resistance of the pile is alsogradually transmitted from the upper part of the pile body tothe lower part of the pile body but compared with themiddle part of the pile body the shaft skin resistance of thelower part of the first pile body with empty-bottom shows a

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

10

8

6

4

2

0

Dep

th Z

m

(a)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

10

8

6

4

2

0

Dep

th Z

m

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

(b)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

10

8

6

4

2

0

Dep

th Z

m

(c)

Dep

th Z

m

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

151413121110

9876543210

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

(d)

Figure 12 Pile-soil relative displacement curve (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Advances in Civil Engineering 11

decreasing trend because there is no soil bearing the cor-responding load at the pile tip however for the remainingthree solid-bottom piles when the tip resistance is mobilizedto a certain extent the friction resistance in the middle partof the pile body is smaller than that in the lower part whichindicates that the soil at the pile tip can reinforce the shaftskin resistance of the pile [25] When the ultimate bearingcapacity is reached the shaft skin resistance values of thecorresponding piles at various depths are shown in Table 6

43 Relationship between Shaft Skin Resistance and Pile-SoilRelative Displacement +e pile-soil relative displacementobjectively reflects the mobilization of shaft skin resistance

According to the calculation method in [26] the pile-soilrelative displacement curves under different loading stepsare calculated and drawn +e curves of shaft skin resistanceand pile-soil relative displacement are shown inFigures 12(a)ndash12(d) respectively

In the calculation of the pile-soil relative displacement itis assumed that the soil around the pile will be not displacedAs can be seen from Figure 12 the pile-soil relative dis-placement at the same position of the pile body increases withthe increase of loading steps on the pile tipWhen the first twosteps of loading are applied the pile-soil relative displacementof the four piles increases obviously due to the compactioneffect of the first two steps of loading on the gravel soil aroundthe pile As the load level continues to increase the pile-soil

Skin

fric

tion

of te

st pi

le (k

Pa)

5 10 15 20 25 300Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

1-11-21-3

1-41-5

(a)Sk

in fr

ictio

n of

test

pile

(kPa

)

0 10 15 20 25 30 35 405Pile-soil relative displacement (mm)

2-12-22-3

2-42-52-6

0

20

40

60

80

100

120

140

160

(b)

Skin

fric

tion

of te

st pi

le (k

Pa)

5 10 15 20 25 30 35 400Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

3-13-23-3

3-43-53-6

(c)

Skin

fric

tion

of te

st pi

le (k

Pa)

0 10 15 20 25 30 35 405Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

4-14-24-3

4-44-54-6

4-74-84-9

(d)

Figure 13 Distribution curves of shaft skin resistances versus relative displacements (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4pile

12 Advances in Civil Engineering

relative displacement increment is relatively small because theshaft skin resistance gradually mobilizes its function toprevent the pile from moving With the increase of loadingwhen the shaft skin resistance cannot resist the load at theloading sections 4800 kNndash5400 kN 4200 kNndash4800 kN8000 kNndash8800 kN and 3600 kNndash4000 kN the maximumdisplacements of pile tips of Nos 2 3 and 4 solid-bottom pileand the empty-bottom pile increase by 9792mm 7939mm9881mm and 1497mm respectively

+e shaft skin resistance increases with the increase ofpile-soil relative displacement Although continuous loadinghas led to foundation soil destruction it can be concludedfrom Figure 13 that the lateral resistance in the middle andlower parts is not fully mobilized and a certain pile-soilrelative displacement is needed to promote the full mobi-lization of the shaft skin resistance However according tothe development trend that the slope of the shaft skin re-sistance gradually decreases it can be inferred that the shaftskin resistance will reach the maximum value At the firstsection position the ultimate shaft skin resistance of pilesNos 1 2 3 and 4 are 4695 kPa 4390 kPa 6018 kPa and6305 kPa respectively and the corresponding pile-soil rel-ative displacements are 3419mm 3442mm 2633mm and3279mm In the second section the shaft skin resistance ofthe four piles increases greatly so does the pile-soil relativedisplacement Since the soil above the first section position isbasically cultivated soil and it can resist small shaft skinresistance while the content of gravel soil of the secondsection is large and with the increase of depth the sur-rounding pressure of soil around the pile increases thereforethe ability of resisting pile foundation sliding and dis-placement increases+erefore between the first section andthe second section the shaft skin resistance increases greatlywhile the pile-soil relative displacement increases a little Inthe second section and beyond the soil is gravel soil and theshaft skin resistance increases slightly with the increase ofpile depth and the relative displacement decreases gradually

44 Pile Tip Resistance Varies with Load and DisplacementDuring the test the section where the largest embeddeddepth of strain gauges is very close to the bottom of the pilewas shown so the resistance at the pile tip is approximatelyequal to the axial force measured at this section namelyQb Q0 Under the action of all loading steps the resistancevariation curves at the lower tips of the Nos 2 3 and 4 pilesare shown in Figure 14

+e No 2 and No 3 piles in the first two loading stepsand No 4 pile in the first loading step have smaller resistanceat the pile tip +e resistance at the pile tip gradually in-creases until the third and fourth loading steps Under theaction of ultimate load the ratio of pile tip resistance to piletop load of Nos 2 3 and 4 is 377 3008 and 3992respectively all higher 15 which shows that pile tip re-sistance has been brought into play to a certain extent +epile tip resistance is shared by shaft skin resistance and piletip resistance and the pile is of tip bearing friction pile

According to the relationship curve between pile tipresistance and pile tip displacement shown in Figure 15 it

can be seen that the pile tip resistance and pile tip dis-placement present a double fold line model of work hard-ening +e Nos 2 3 and 4 piles all show a slow deformationfailure and under the action of the first loading step theslope of the curve is larger and the corresponding dis-placement is relatively small in latter steps of loading thecurvature curve becomes smoother the displacement of thepile tips of adjacent loads increases and the soil at the piletips will gradually be compacted

Pile

tip

load

(kN

)

1000 2000 3000 4000 5000 6000 7000 8000 90000Load at pile top (kN)

0250500750

100012501500175020002250250027503000325035003750

No 2No 3No 4

Figure 14 Variation curve of pile tip resistance under differentloading steps (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4pile

Pile

tip

load

(kN

)

5 10 15 20 25 30 35 40 450Pile tip displacement (mm)

0

1000

2000

3000

4000

5000

6000

No 2No 3No 4

Figure 15 Relationship curve between pile tip resistance and piletip displacement (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No4 pile

Advances in Civil Engineering 13

5 Conclusions

Analyses are carried out on data of the in-situ test of unevengravel soil in Baishi Mountain and the following conclu-sions are drawn

(1) Based on the analysis of the results of multichanneltransient surface wave test superheavy (N120) conedynamic sounding test and on-site screening testthe geological environment conditions of the unevengravel soil on the site are further explored +e stratawithin the range from 0 to 15m in Baizhi Mountaincan be divided into four layers ie cultivated soilsandy soil silty clay and gravel soil with depths of1m 2m 3m and 9m respectively

(2) +e axial force and shaft skin resistance of pile bodyin gravel soil are the same as those in other geologicalstructures At the initial loading step the axial forceand shaft skin resistance of pile body play a smallerrole With the increase of loading steps the axialforce and shaft skin resistance of pile body increasecorrespondingly Under the same loading step theaxial force of the pile body and the shaft skin re-sistance do not mobilize in the same time andgradually transfer from the upper part to the lowerpart of the pile body and the upper part of the pilebody reaches the limit value before the middle andlower parts As for the solid-bottom pile the soil atthe pile tip will strengthen the shaft skin resistance ofthe pile

(3) According to this experimental study it is suggestedthat similar projects should adopt concrete manuallybored cast-in-place piles with a diameter of 900mmand a length of 15m the wall-protecting thicknessshall not be less than 175mm and the grade ofconcrete cannot be lower than C35 +e pile end canbe widened or not and the pile bottom shall be freeof sediment

Data Availability

All the data in this paper are obtained from tests in thisstudy and no other data were used to support this study

Conflicts of Interest

+e authors declare that with regard to the publication ofthis paper and the funding for publishing it there are noconflicts of interest

Acknowledgments

+is study was partially supported by the National NaturalScience Foundation of China (Grant no 41302223) ChongqingNo 3 colleges and universities youth backbone teachers fundingplans Chongqing Research Program of Basic Research andFrontier Technology (Grant no cstc2016jcyjA0074cstc2016jcyjA0933 and cstc2015jcyjA90012) Scientific andTechnological Research Program of Chongqing Municipal

Education Commission (Grant no KJ1713327 and KJ1600532)Key Laboratory ofWell Stability and Fluid and RockMechanicsin Oil and Gas Reservoir of Shaanxi Province Xirsquoan ShiyouUniversity (Grant no FRM20190201002) Chongqing Post-graduate Research Innovation Project (Grant no CYS19353)and Chongqing University of Science and Technology GraduateStudentsrsquo Science and Technology Innovation Program (Grantno YKJCX1720601 and YKJCX1920613)

References

[1] F Kirsch and B Plabmann ldquoDynamic methods in pile testingdevelopments in measurement and analysisrdquo in Proceedings ofthe International Deep Foundations Congress 2002 pp 868ndash882 ASCE Orlando FL USA February 2002

[2] S H Ni L Lehmann J J Charng et al ldquoLow-strain integritytesting of drilled piles with high slenderness ratiordquo Computersand Geotechnics vol 33 no 6-7 pp 283ndash293 2006

[3] K Mori A Spagnoli Y Murakami G Kondo and I TorigoeldquoA new non-contacting non-destructive testing method fordefect detection in concreterdquo NDT amp E International vol 35no 6 pp 399ndash406 2002

[4] G Cai S Liu L Tong et al ldquoAssessment of direct CPT andCPTUmethods for predicting the ultimate bearing capacity ofsingle pilesrdquo Engineering Geology vol 104 no 3-4pp 211ndash222 2009

[5] X Y Bai M Y Zhang L Zhu et al ldquoIn-situ test and finiteelement analysis of bearing characteristics of rock-socketedshort piles in strongly weathered graniterdquo Journal of CentralSouth University Natural Science Edition vol 48 no 2pp 512ndash524 2017 in Chinese

[6] E C Leong and M F Randolph ldquoFinite element modelling ofrock-socketed pilesrdquo International Journal for Numerical andAnalytical Methods in Geomechanics vol 18 no 1 pp 25ndash471994

[7] G Gao M Gao Q Chen and J Yang ldquoField load testingstudy of vertical bearing behavior of a large diameter belledcast-in-place pilerdquo KSCE Journal of Civil Engineering vol 23no 5 pp 2009ndash2016 2019

[8] R Berardi and R Bovolenta ldquoPile-settlement evaluation usingfield stiffness non-linearityrdquo Proceedings of the Institution ofCivil Engineers - Geotechnical Engineering vol 158 no 1pp 35ndash44 2005

[9] B H Fellenius Basics of Foundation Design Lulu PressMorrisville NC USA 2016

[10] C Lam and S A Jefferis ldquoCritical assessment of pile modulusdetermination methodsrdquo Canadian Geotechnical Journalvol 48 no 10 pp 1433ndash1448 2011

[11] A Mohammadi T Ebadi and M R Boroomand ldquoPhysicalmodelling of axial compressive bearing capacity of instru-mented piles in oil-contaminated sandy soilrdquo Iranian Journalof Science and Technology Transactions of Civil Engineeringvol 44 no 2 pp 695ndash714 2019

[12] W D Guo and E H Ghee ldquoBehavior of axially loaded pilegroups subjected to lateral soil movementrdquo in Proceedings ofthe GeoShanghai International Conference 2006 pp 174ndash181ASCE Shanghai China June 2006

[13] Z Zhou Y Dong P Jiang D Han and T Liu ldquoCalculation ofpile side friction by multiparameter statistical analysisrdquo Ad-vances in Civil Engineering vol 2019 Article ID 263852012 pages 2019

14 Advances in Civil Engineering

[14] N M Hai and B H Fellenius ldquoO-Cell tests on two 70 m longbored piles in Vietnamrdquo in Proceedings of the Geo-Congress2014 pp 482ndash496 ASCE Atlanta Georgia February 2014

[15] S Gong G Cai S Liu et al ldquoNumerical Simulation of BearingCapacity and Consolidation Characteristics of PHC PileFoundationrdquo GeoShanghai International Conferencepp 178ndash185 Springer Singapore 2018

[16] M A Bashir H Furuuchi T Ueda and M Nauman BashirldquoNumerical simulation of axial anchorage capacity of con-crete-filled steel box footingrdquo Iranian Journal of Science andTechnology Transactions of Civil Engineering vol 40 no 3pp 257ndash262 2016

[17] N A Haskell ldquo+e dispersion of surface waves on multi-layered mediardquo Bulletin of the Seismological Society ofAmerica vol 43 no 1 pp 17ndash34 1953

[18] China Architecture amp Building PressNational Standard of thePeoplersquos Republic of China Code for Investigation of Geo-technical Engineering (GB50021 2016) China Architecture ampBuilding Press Beijing China 2016 in Chinese

[19] China Architecture amp Building Press Industry Standard of thePeoplersquos Republic of China Technical Code for Testing ofBuilding Foundation Piles (JGJ106-2014) China Architectureamp Building Press Beijing China 2014 in Chinese

[20] N Guo Z H Chen X F Huang et al ldquoResearch on the anti-pulling test of large-diameter cloth bag pile in soft rockfoundationrdquo Rock and Soil Mechanics vol 36 no 2pp 603ndash609 2015 in Chinese

[21] Y H Li X Zhu and T H Zhou ldquoField comparative ex-perimental study on the influence of post-tip grouting onlarge-diameter cast-in-situ pilesrdquo Rock and Soil Mechanicsvol 37 no 2 pp 388ndash396 2016 in Chinese

[22] G F Xin Z M Zhang T Xia et al ldquoExperimental study onbearing behavior of ultra-long pile under high loadrdquo ChineseJournal of Rock Mechanics and Engineering vol 13pp 2397ndash2402 2005 in Chinese

[23] Z Ling W Wang J Wu et al ldquoFull-scale loading test on pre-bored precast pile with enlarged base in Shanghairdquo in Pro-ceedings of the GeoShanghai 2018 International ConferenceAdvances in Soil Dynamics and Foundation Engineeringpp 637ndash645 Springer Singapore 2018

[24] J R Dong S T Lin and Y M Dai ldquo+e load tranfer behaviorof large diameter cast-in-situ pile in crushed pebble stratumrdquoChinese Journal of Geotechnical Engineering vol 16 no 6pp 123ndash131 1994

[25] X F Huang J H Zhang L G Ma et al ldquoExperimental studyon reinforcement effect of lateral resistance of collapsible loessrock-socketed perfusion pilerdquo Architectural Structure vol 41no 2 pp 351ndash355 2011 in Chinese

[26] J-J Zhou K-H Wang X-N Gong and R-H ZhangldquoBearing capacity and load transfer mechanism of a static drillrooted nodular pile in soft soil areasrdquo Journal of ZhejiangUniversity Science A vol 14 no 10 pp 705ndash719 2013

Advances in Civil Engineering 15

characteristics of pile foundation in gravel soil bearing stratumof different depths static load tests are carried out in the areawhere shear wave velocity is relatively uniform ie within 30 to38m along the length direction of wave velocity profile+e testpile is a C35 manually excavated cast-in-place pile with a di-ameter of 09m A total of 4 piles were arranged See Figures 1and 4 for the plan layout and pile numbers +e characteristicparameters of each tested pile are shown in Table 4

+e No 1 pile is empty-bottom pile (as shown inFigure 5(a)) and Nos 2 3 and 4 piles are solid-bottom piles(as shown in Figures 5(b) and 5(c) resp)+e empty-bottompile is welded to a 20mm thick circular steel plate at thebottom of the steel cage the diameter of the steel plate is(850mm) slightly smaller than the diameter of the pile holeWhen pouring concrete the steel cage is suspended to makethe bottom of the steel cage suspended 1m

+ere are 12Φ 14 HRB 400 reinforcements used as mainreinforcements of all test piles +e inner stirrup adoptsHPB235 reinforcement with a diameter of 16mm and itstarts from 2m below the top of the longitudinal rein-forcement and is evenly arranged each 2m+e outer stirrupadopts HPB235 reinforcement with a diameter of 8mm andit is arranged from below the longitudinal rib top +espacing between the outer stirrup and the longitudinal ribtop is 100mm within the range of 3m and the rest of thespace is 200mm At the same time 6 anchor piles with alength of 15m are arranged in 4 of them the diameter is1300mm and the othersrsquo diameter is 1100mm +e

longitudinal bar of anchor piles is composed of 16 and 20finished reinforcing bars with the diameter of 28mm +econcrete reinforcement layout and strain gauges layout ofthe test pile are seen in Figure 5

32 Measuring Instrument Resistive strain gauges aresymmetrically arranged on the main reinforcement (asshown in Figure 5) to measure the vertical strain of rein-forcement and pile body +e DM-YB1840 dynamic andstatic strain testing system automatically collects strain dataand calculates axial force on pile body and shaft skin

100 10 1 01 001

AB

CD

Grain size (dmm)

Pass

rate

(P

)

0

20

40

60

80

100

Figure 3 +e grain size distribution curves at the site of A B C and D

Table 3 Physical and mechanical properties of soil

Soil layer Weight density c (kNm3) Internal friction angle φ (deg) Cohesion C (kPa)Cultivated soil 18 18 15Sandy soil 19 21 mdashSilty clay 19 13 23Gravel soil 20 25 5

30003000

No 4

No 2

3000

No 3

No 1

2500

2500

Figure 4 Layout of pile position

4 Advances in Civil Engineering

Table 4 +e characteristic parameters of each tested pile

Pilesrsquo no Pilesrsquo length L (m) Pilesrsquo diameter D (mm) Concrete strength FormNo 1 10 900 C35 Empty-bottomNo 2 10 900 C35 Solid-bottomNo 3 10 900 C35 Solid-bottomNo 4 15 900 C35 Solid-bottom

1700

1700

1700

1700

1700

300

1000

0

Ground

400

300

1000

0

900

Within 3mbelow the pile

top 100

3m belowthe pile top

200

Under thepile top 2m 2000

Strain gauges

Ground

Steel platewith D 850

20 mm thick

Empty-bottom

12 14

(a)

1600

1600

1600

1600

1700

1700

300

1000

0

400

300

1000

0

900

Ground Ground

Under thepile top 2m 2000

Within 3 mbelow the pile

top 100

3m belowthe pile top

200

Strain gauges

12 14

(b)

1600

1600

1600

1600

1800

1800

1600

1600

1600

300

1500

0

400

300

900

1500

0

GroundGround

Under thepile top 2m 2000

Within 3 mbelow the pile

top 100

3m belowthe pile top

200

Strain gauges

12 14

(c)

900

The stirrup of outside

50mm thickconcrete protective layer

Main reinforcement 120iexclatilde

The stirrup of insideStrain gauges

(d)

Figure 5 +e reinforcement layout and strain gauges layout of the test pile (a) Empty-bottom pile No 1 (b) Solid-bottom pile No 2 andNo 3 (c) Solid-bottom pile No 4 (d) Plan of layout of pile

Advances in Civil Engineering 5

resistance +e embedment of strain gauges shall be carriedout strictly in accordance with the operating instructionsand maintenance work shall be done to improve the successrate for the on-site embedding photo of strain gauges seeFigure 6

33 Static LoadMethod of Pile Foundation In the YD10000-200 hydraulic jack applying load the load reaction force isprovided by anchor pile Four FP-50 displacement sensorswere used for settlement which were installed on the ref-erence beam at the top of the test pile +e support of thesensor was fixed on the jack with magnetic bearing and thereference beam was fixed on the steel beam in an inde-pendent position unaffected by deformation calibrating thedisplacement sensor with the displacement sensor calibra-tion table before the test and using a force transducer tocalibrate the accuracy of the load on the jack +e Y-LINKstatic load test system automatically collects and stores testdata+e vertical compressive static load device of single pileis shown in Figure 7(a) and the photo of field static load testis shown in Figure 7(b)

+e test is carried out according to China Technical Codefor Testing of Building Foundation Piles (JGJ106-2014) [19]the specific steps are as follows

(1) In the test the step loading method was adopted (2)measuring the settlement every 5 minutes 10 minutes and15 minutes after each step of loading was applied every 15minutes after that and every 30 minutes after a total of 1hour and then loading stopped (3) +e settlement per hourcould not exceed 01mm When it occurred twice in suc-cession it was considered to be relatively stable and the nextstep of loading could be applied (4) When the settlement ofthe pile was 5 times of the settlement under the previous loadand the total settlement at the top of the pile exceeded40mm or the settlement of the pile was greater than 2 timesof the settlement under the previous load and had notreached a stable state after 24 hours loading stopped

34 Test Results of Pile Foundation +e line type of load-displacement (Q-S) curve in static load test can macro-scopically reflect pile bodyrsquos failure mechanism and failuremode of soil around pile [20] When the load is small thedisplacement curve tends to develop linearly and is in theelastic stage With the increasing load the pile graduallychanges from elastic deformation to elastic-plastic defor-mation As the load continues to increase the displacementincreases sharply and enters the plastic stage and the pilereaches the failure state +e Q-S curves of piles Nos 1 2 3and 4 are shown in Figure 8

It can be seen from Figure 8 that the Q-S curve of No 1pile has obvious inflection point indicating that it is a steepdrop curve +e pile top load corresponding to the inflectionpoint of Q-S curve is taken as the ultimate bearing capacityof the test pile [21] +e Q-S curves of the other three pileshave similar trends and there is no obvious steep dropsection indicating that they are slowly changing curves sothe ultimate bearing capacity is confirmed according to 442in =e Technical Specification of China Building Foundation

Piles +e bearing capacity under ultimate load of No 1 pileis 3600 kN and the corresponding pile top settlement is1289mm the maximum load is 4000 kN and the maximumsettlement is 4068mm similarly the maximum loadingload of Nos 2 3 and 4 piles and their corresponding pile topdisplacement values bearing capacity under ultimate loadand corresponding pile top displacement and settlementvalues are shown in Table 5

Figure 9 shows the S-lgt curve of settlement of each pilewith time which can further explain the value of ultimatebearing capacity At the last step of loading an obviousdownward bending is shown in the displacement of the fourpiles the displacement exceeded 40mm and the pile reachesthe failure state [22]

4 Discussion

41 =e Characteristics of Pile Shaft Force Transfer +e re-inforcement strain εi(microε) at the corresponding depth is di-rectly measured by the strain gauges meter embedded andthen the reinforcement stress σi is calculated according to

σsi K middot ε0 minus εi( 1113857 (1)

where σi is the stress on the reinforcement K is calibrationcoefficient indicated by the manufacturer and its valueranges from 06 to 08 εi is average strain measured by straingauges on the same section i is the serial numbers of theburied sections of the strain gauges i 1 2 L n are arrangedfrom top to bottom

According to Hookersquos law and the stress on the rein-forcement the real strain value of the strain gauges string iscalculated

εsi σsi

Es

(2)

where εsi is the real strain of the strain gauges string Es is theelastic modulus of reinforcement with a value of204 times 104 MPa σi and i are the same as in formula (1)

Assuming that the compression amount of concrete isequal to the real strain value of the strain gauges string andthe cross-sectional area of the pile is unchanged the concretestress can be obtained according to

σci εsi middot Ec (3)

where σci is the concrete stress value εsi is the real strain ofstrain gauges string Ec is the modulus of elasticity ofconcrete with a value of 104MPa i is the same as in formula(1)

Finally the respective internal forces are obtained bymultiplying stress values by the respective cross-sectionalareas and then the pile axial force of this section is obtainedby the stress of reinforcements plus the stress of concreteand the axial force of other cross-sectional pile bodies can beobtained similarly the calculation formula is as follows

Qi σciAci + σsiAsi (4)

where Qi is the axial force of tested section σci and σsi are thesame as in formulas (3) and (1) respectively Aci is the

6 Advances in Civil Engineering

concrete area of pile body 063585m2 Asi is the area ofreinforcement in pile body 184632 times 10minus3 m2

Figures 9(a)ndash9(d) show the axial force distribution of pilesNos 1 2 3 and 4 with depth under different loading steps

respectively With the increase of loading steps axial force isgradually generated at the lower part of each solid-bottompile and pile tip resistance is gradually mobilized [23] Whenreaching the ultimate load pile tip resistance of piles Nos 2 3and 4 is 2100 kN 1418 kN and 3463 kN respectively+e firstpile is an empty-bottom pile with the increase of load axialforce is gradually generated at the lower part of the pile bodyAfter reaching the ultimate bearing capacity the load willcontinue to be applied and the displacement of pile top willrapidly increase from the original 1289mm to 4068mm+is is because the first pile is an empty-bottom pile and thereis no soil at the bottom to generate tip resistance When theshaft skin resistance reaches the maximum value and cannotresist the axial force in the vertical direction if the loadcontinues to be applied after reaching the ultimate bearingcapacity sudden settlement of the pile body occurs and thesettlement exceeds the limit specified in the specificationindicating that pile failure has occurred

It can be seen from the stress conditions of the solid-bottom pile the empty-bottom pile the 10-meter-long pilebody and the 15-meter-long pile body in Figure 10 that theaxial force of the empty-bottom pile and the solid-bottompile is relatively small at the initial loading step With theincrease of loading step the axial force gradually develops

(a) (b)

Figure 6 Site photo of strain gauges embedding (a) Part photo (b) +e overall photo

Displacementmeter

Hydraulicjack

Test beam(s)

Test pile

Steel testplate

Load transfercolumn

Anchor pileMinimum clear

distance is 25m

(a) (b)

Figure 7 Schematic diagram and field photo of static load test (a) Schematic diagram (b) Photograph

0 1200 2400 3600 4800 6000 7200 8400 9600

Disp

lace

men

t (m

m)

Load at pile top (kN)

No 1No 2

No 3No 4

45

40

35

30

25

20

15

10

5

0

Figure 8 Load-displacement curve of static load test

Advances in Civil Engineering 7

Table 5 Results of static load test

Pile number Maximum load (kN) Maximum load settlement (mm) Bearing capacity underultimate load (kN)

Settlement underultimate load (mm) Rebound rate ()

No 1 4000 4068 3600 1289 1006No 2 5400 401 4800 3001 1236No 3 4800 4051 4200 2892 1684No 4 8800 4083 8000 2676 1894

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

lgt (min)

S (m

m)

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

(a)

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

lgt (min)

S (m

m)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

(b)

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

lgt (min)

S (m

m)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

(c)

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

S (m

m)

lgt (min)

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

(d)

Figure 9 S-lgt curve of each pile (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

8 Advances in Civil Engineering

and transmits to the pile bottom and decreases with theincrease of the depth Under different loading steps theattenuation rate of axial force curve is different +e at-tenuation rate of axial force curve reflects the development ofshaft skin resistance In low loading steps the attenuationrate of axial force curve is not fast and relatively uniformWith the increase of loading steps the attenuation rate ofaxial force curve at the upper part of pile body is not fastwhile the attenuation rate of axial force curve at the lowerpart of pile body is obvious which shows that no matter it isan empty-bottom pile or solid-bottom pile the force

transmission characteristics are similar With the increase ofloading steps the axial force gradually transmits to the lowerpart of the pile body If load continues to be applied afterreaching the ultimate load the empty-bottom pile will besuddenly destroyed while the solid-bottom pile will be able tobear due to soil in the lower part and will show a slow change

42 Analysis of Characteristics of Shaft Skin Resistance of PileBody Based on the principle of static balance regardless ofthe influence of pile body weight [24] the pile shaft skin

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

0 400 800 1200 1600 2000 2400 2800 3200 3600 4000 4400Axial force (kN)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

(a)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 600 1200 1800 2400 3000 3600 4200 4800 5400 6000Axial force (kN)

(b)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 600 1200 1800 2400 3000 3600 4200 4800 5400Axial force (kN)

(c)

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

16151413121110

9876543210

Dep

th Z

m

0 800 1600 2400 3200 4000 4800 5600 6400 7200 8000 8800Axial force (kN)

(d)

Figure 10 Distribution curve of axial force of each pile (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Advances in Civil Engineering 9

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

(a)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

(b)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

(c)

16151413121110

9876543210

Dep

th Z

m

0 20 40 60 80 100 120 140 160 180Skin friction of test pile (kPa)

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

(d)

Figure 11 Distribution of shaft skin resistance on each pile (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Table 6 Value of shaft skin resistance of piles at different depths when reaching ultimate bearing capacity

(a) No 1 pileDepth Zm 17 34 51 68 85Skin frictionkPa 60177 112974 133037 139158 135235

(b) No 2 pileDepth Zm 16 32 48 64 81 98Skin frictionkPa 43899 105055 128283 1391 146983 148015

(c) No 3 pileDepth Zm 16 32 48 64 81 98Skin frictionkPa 46948 106907 130723 145894 147938 150828

(d) No 4 pileDepth Zm 16 32 48 64 8 96 112 13 148Skin frictionkPa 63055 93009 117592 129804 141642 144953 145865 151216 154956

10 Advances in Civil Engineering

resistance value can be calculated by the following formulaaccording to the change value of the axial force of the pilebody

qsi Qi minus Qi+1

u middot li (5)

where qsi is the resistance between the i section and the i + 1section i is the same as in formula (1) u is the perimeter ofpile body li is the pile length between the i section and thei + 1 section

Figures 10(a)ndash10(d) show the distribution of shaft skinresistance of piles Nos 1 2 3 and 4 with depth underdifferent steps of loading

As can be seen from Figure 11 in lower loading step theshaft skin resistance of each pile is small with the increase ofloading steps the shaft skin resistance of each pile increasesOn the whole the shaft skin resistance of the pile is alsogradually transmitted from the upper part of the pile body tothe lower part of the pile body but compared with themiddle part of the pile body the shaft skin resistance of thelower part of the first pile body with empty-bottom shows a

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

10

8

6

4

2

0

Dep

th Z

m

(a)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

10

8

6

4

2

0

Dep

th Z

m

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

(b)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

10

8

6

4

2

0

Dep

th Z

m

(c)

Dep

th Z

m

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

151413121110

9876543210

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

(d)

Figure 12 Pile-soil relative displacement curve (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Advances in Civil Engineering 11

decreasing trend because there is no soil bearing the cor-responding load at the pile tip however for the remainingthree solid-bottom piles when the tip resistance is mobilizedto a certain extent the friction resistance in the middle partof the pile body is smaller than that in the lower part whichindicates that the soil at the pile tip can reinforce the shaftskin resistance of the pile [25] When the ultimate bearingcapacity is reached the shaft skin resistance values of thecorresponding piles at various depths are shown in Table 6

43 Relationship between Shaft Skin Resistance and Pile-SoilRelative Displacement +e pile-soil relative displacementobjectively reflects the mobilization of shaft skin resistance

According to the calculation method in [26] the pile-soilrelative displacement curves under different loading stepsare calculated and drawn +e curves of shaft skin resistanceand pile-soil relative displacement are shown inFigures 12(a)ndash12(d) respectively

In the calculation of the pile-soil relative displacement itis assumed that the soil around the pile will be not displacedAs can be seen from Figure 12 the pile-soil relative dis-placement at the same position of the pile body increases withthe increase of loading steps on the pile tipWhen the first twosteps of loading are applied the pile-soil relative displacementof the four piles increases obviously due to the compactioneffect of the first two steps of loading on the gravel soil aroundthe pile As the load level continues to increase the pile-soil

Skin

fric

tion

of te

st pi

le (k

Pa)

5 10 15 20 25 300Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

1-11-21-3

1-41-5

(a)Sk

in fr

ictio

n of

test

pile

(kPa

)

0 10 15 20 25 30 35 405Pile-soil relative displacement (mm)

2-12-22-3

2-42-52-6

0

20

40

60

80

100

120

140

160

(b)

Skin

fric

tion

of te

st pi

le (k

Pa)

5 10 15 20 25 30 35 400Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

3-13-23-3

3-43-53-6

(c)

Skin

fric

tion

of te

st pi

le (k

Pa)

0 10 15 20 25 30 35 405Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

4-14-24-3

4-44-54-6

4-74-84-9

(d)

Figure 13 Distribution curves of shaft skin resistances versus relative displacements (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4pile

12 Advances in Civil Engineering

relative displacement increment is relatively small because theshaft skin resistance gradually mobilizes its function toprevent the pile from moving With the increase of loadingwhen the shaft skin resistance cannot resist the load at theloading sections 4800 kNndash5400 kN 4200 kNndash4800 kN8000 kNndash8800 kN and 3600 kNndash4000 kN the maximumdisplacements of pile tips of Nos 2 3 and 4 solid-bottom pileand the empty-bottom pile increase by 9792mm 7939mm9881mm and 1497mm respectively

+e shaft skin resistance increases with the increase ofpile-soil relative displacement Although continuous loadinghas led to foundation soil destruction it can be concludedfrom Figure 13 that the lateral resistance in the middle andlower parts is not fully mobilized and a certain pile-soilrelative displacement is needed to promote the full mobi-lization of the shaft skin resistance However according tothe development trend that the slope of the shaft skin re-sistance gradually decreases it can be inferred that the shaftskin resistance will reach the maximum value At the firstsection position the ultimate shaft skin resistance of pilesNos 1 2 3 and 4 are 4695 kPa 4390 kPa 6018 kPa and6305 kPa respectively and the corresponding pile-soil rel-ative displacements are 3419mm 3442mm 2633mm and3279mm In the second section the shaft skin resistance ofthe four piles increases greatly so does the pile-soil relativedisplacement Since the soil above the first section position isbasically cultivated soil and it can resist small shaft skinresistance while the content of gravel soil of the secondsection is large and with the increase of depth the sur-rounding pressure of soil around the pile increases thereforethe ability of resisting pile foundation sliding and dis-placement increases+erefore between the first section andthe second section the shaft skin resistance increases greatlywhile the pile-soil relative displacement increases a little Inthe second section and beyond the soil is gravel soil and theshaft skin resistance increases slightly with the increase ofpile depth and the relative displacement decreases gradually

44 Pile Tip Resistance Varies with Load and DisplacementDuring the test the section where the largest embeddeddepth of strain gauges is very close to the bottom of the pilewas shown so the resistance at the pile tip is approximatelyequal to the axial force measured at this section namelyQb Q0 Under the action of all loading steps the resistancevariation curves at the lower tips of the Nos 2 3 and 4 pilesare shown in Figure 14

+e No 2 and No 3 piles in the first two loading stepsand No 4 pile in the first loading step have smaller resistanceat the pile tip +e resistance at the pile tip gradually in-creases until the third and fourth loading steps Under theaction of ultimate load the ratio of pile tip resistance to piletop load of Nos 2 3 and 4 is 377 3008 and 3992respectively all higher 15 which shows that pile tip re-sistance has been brought into play to a certain extent +epile tip resistance is shared by shaft skin resistance and piletip resistance and the pile is of tip bearing friction pile

According to the relationship curve between pile tipresistance and pile tip displacement shown in Figure 15 it

can be seen that the pile tip resistance and pile tip dis-placement present a double fold line model of work hard-ening +e Nos 2 3 and 4 piles all show a slow deformationfailure and under the action of the first loading step theslope of the curve is larger and the corresponding dis-placement is relatively small in latter steps of loading thecurvature curve becomes smoother the displacement of thepile tips of adjacent loads increases and the soil at the piletips will gradually be compacted

Pile

tip

load

(kN

)

1000 2000 3000 4000 5000 6000 7000 8000 90000Load at pile top (kN)

0250500750

100012501500175020002250250027503000325035003750

No 2No 3No 4

Figure 14 Variation curve of pile tip resistance under differentloading steps (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4pile

Pile

tip

load

(kN

)

5 10 15 20 25 30 35 40 450Pile tip displacement (mm)

0

1000

2000

3000

4000

5000

6000

No 2No 3No 4

Figure 15 Relationship curve between pile tip resistance and piletip displacement (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No4 pile

Advances in Civil Engineering 13

5 Conclusions

Analyses are carried out on data of the in-situ test of unevengravel soil in Baishi Mountain and the following conclu-sions are drawn

(1) Based on the analysis of the results of multichanneltransient surface wave test superheavy (N120) conedynamic sounding test and on-site screening testthe geological environment conditions of the unevengravel soil on the site are further explored +e stratawithin the range from 0 to 15m in Baizhi Mountaincan be divided into four layers ie cultivated soilsandy soil silty clay and gravel soil with depths of1m 2m 3m and 9m respectively

(2) +e axial force and shaft skin resistance of pile bodyin gravel soil are the same as those in other geologicalstructures At the initial loading step the axial forceand shaft skin resistance of pile body play a smallerrole With the increase of loading steps the axialforce and shaft skin resistance of pile body increasecorrespondingly Under the same loading step theaxial force of the pile body and the shaft skin re-sistance do not mobilize in the same time andgradually transfer from the upper part to the lowerpart of the pile body and the upper part of the pilebody reaches the limit value before the middle andlower parts As for the solid-bottom pile the soil atthe pile tip will strengthen the shaft skin resistance ofthe pile

(3) According to this experimental study it is suggestedthat similar projects should adopt concrete manuallybored cast-in-place piles with a diameter of 900mmand a length of 15m the wall-protecting thicknessshall not be less than 175mm and the grade ofconcrete cannot be lower than C35 +e pile end canbe widened or not and the pile bottom shall be freeof sediment

Data Availability

All the data in this paper are obtained from tests in thisstudy and no other data were used to support this study

Conflicts of Interest

+e authors declare that with regard to the publication ofthis paper and the funding for publishing it there are noconflicts of interest

Acknowledgments

+is study was partially supported by the National NaturalScience Foundation of China (Grant no 41302223) ChongqingNo 3 colleges and universities youth backbone teachers fundingplans Chongqing Research Program of Basic Research andFrontier Technology (Grant no cstc2016jcyjA0074cstc2016jcyjA0933 and cstc2015jcyjA90012) Scientific andTechnological Research Program of Chongqing Municipal

Education Commission (Grant no KJ1713327 and KJ1600532)Key Laboratory ofWell Stability and Fluid and RockMechanicsin Oil and Gas Reservoir of Shaanxi Province Xirsquoan ShiyouUniversity (Grant no FRM20190201002) Chongqing Post-graduate Research Innovation Project (Grant no CYS19353)and Chongqing University of Science and Technology GraduateStudentsrsquo Science and Technology Innovation Program (Grantno YKJCX1720601 and YKJCX1920613)

References

[1] F Kirsch and B Plabmann ldquoDynamic methods in pile testingdevelopments in measurement and analysisrdquo in Proceedings ofthe International Deep Foundations Congress 2002 pp 868ndash882 ASCE Orlando FL USA February 2002

[2] S H Ni L Lehmann J J Charng et al ldquoLow-strain integritytesting of drilled piles with high slenderness ratiordquo Computersand Geotechnics vol 33 no 6-7 pp 283ndash293 2006

[3] K Mori A Spagnoli Y Murakami G Kondo and I TorigoeldquoA new non-contacting non-destructive testing method fordefect detection in concreterdquo NDT amp E International vol 35no 6 pp 399ndash406 2002

[4] G Cai S Liu L Tong et al ldquoAssessment of direct CPT andCPTUmethods for predicting the ultimate bearing capacity ofsingle pilesrdquo Engineering Geology vol 104 no 3-4pp 211ndash222 2009

[5] X Y Bai M Y Zhang L Zhu et al ldquoIn-situ test and finiteelement analysis of bearing characteristics of rock-socketedshort piles in strongly weathered graniterdquo Journal of CentralSouth University Natural Science Edition vol 48 no 2pp 512ndash524 2017 in Chinese

[6] E C Leong and M F Randolph ldquoFinite element modelling ofrock-socketed pilesrdquo International Journal for Numerical andAnalytical Methods in Geomechanics vol 18 no 1 pp 25ndash471994

[7] G Gao M Gao Q Chen and J Yang ldquoField load testingstudy of vertical bearing behavior of a large diameter belledcast-in-place pilerdquo KSCE Journal of Civil Engineering vol 23no 5 pp 2009ndash2016 2019

[8] R Berardi and R Bovolenta ldquoPile-settlement evaluation usingfield stiffness non-linearityrdquo Proceedings of the Institution ofCivil Engineers - Geotechnical Engineering vol 158 no 1pp 35ndash44 2005

[9] B H Fellenius Basics of Foundation Design Lulu PressMorrisville NC USA 2016

[10] C Lam and S A Jefferis ldquoCritical assessment of pile modulusdetermination methodsrdquo Canadian Geotechnical Journalvol 48 no 10 pp 1433ndash1448 2011

[11] A Mohammadi T Ebadi and M R Boroomand ldquoPhysicalmodelling of axial compressive bearing capacity of instru-mented piles in oil-contaminated sandy soilrdquo Iranian Journalof Science and Technology Transactions of Civil Engineeringvol 44 no 2 pp 695ndash714 2019

[12] W D Guo and E H Ghee ldquoBehavior of axially loaded pilegroups subjected to lateral soil movementrdquo in Proceedings ofthe GeoShanghai International Conference 2006 pp 174ndash181ASCE Shanghai China June 2006

[13] Z Zhou Y Dong P Jiang D Han and T Liu ldquoCalculation ofpile side friction by multiparameter statistical analysisrdquo Ad-vances in Civil Engineering vol 2019 Article ID 263852012 pages 2019

14 Advances in Civil Engineering

[14] N M Hai and B H Fellenius ldquoO-Cell tests on two 70 m longbored piles in Vietnamrdquo in Proceedings of the Geo-Congress2014 pp 482ndash496 ASCE Atlanta Georgia February 2014

[15] S Gong G Cai S Liu et al ldquoNumerical Simulation of BearingCapacity and Consolidation Characteristics of PHC PileFoundationrdquo GeoShanghai International Conferencepp 178ndash185 Springer Singapore 2018

[16] M A Bashir H Furuuchi T Ueda and M Nauman BashirldquoNumerical simulation of axial anchorage capacity of con-crete-filled steel box footingrdquo Iranian Journal of Science andTechnology Transactions of Civil Engineering vol 40 no 3pp 257ndash262 2016

[17] N A Haskell ldquo+e dispersion of surface waves on multi-layered mediardquo Bulletin of the Seismological Society ofAmerica vol 43 no 1 pp 17ndash34 1953

[18] China Architecture amp Building PressNational Standard of thePeoplersquos Republic of China Code for Investigation of Geo-technical Engineering (GB50021 2016) China Architecture ampBuilding Press Beijing China 2016 in Chinese

[19] China Architecture amp Building Press Industry Standard of thePeoplersquos Republic of China Technical Code for Testing ofBuilding Foundation Piles (JGJ106-2014) China Architectureamp Building Press Beijing China 2014 in Chinese

[20] N Guo Z H Chen X F Huang et al ldquoResearch on the anti-pulling test of large-diameter cloth bag pile in soft rockfoundationrdquo Rock and Soil Mechanics vol 36 no 2pp 603ndash609 2015 in Chinese

[21] Y H Li X Zhu and T H Zhou ldquoField comparative ex-perimental study on the influence of post-tip grouting onlarge-diameter cast-in-situ pilesrdquo Rock and Soil Mechanicsvol 37 no 2 pp 388ndash396 2016 in Chinese

[22] G F Xin Z M Zhang T Xia et al ldquoExperimental study onbearing behavior of ultra-long pile under high loadrdquo ChineseJournal of Rock Mechanics and Engineering vol 13pp 2397ndash2402 2005 in Chinese

[23] Z Ling W Wang J Wu et al ldquoFull-scale loading test on pre-bored precast pile with enlarged base in Shanghairdquo in Pro-ceedings of the GeoShanghai 2018 International ConferenceAdvances in Soil Dynamics and Foundation Engineeringpp 637ndash645 Springer Singapore 2018

[24] J R Dong S T Lin and Y M Dai ldquo+e load tranfer behaviorof large diameter cast-in-situ pile in crushed pebble stratumrdquoChinese Journal of Geotechnical Engineering vol 16 no 6pp 123ndash131 1994

[25] X F Huang J H Zhang L G Ma et al ldquoExperimental studyon reinforcement effect of lateral resistance of collapsible loessrock-socketed perfusion pilerdquo Architectural Structure vol 41no 2 pp 351ndash355 2011 in Chinese

[26] J-J Zhou K-H Wang X-N Gong and R-H ZhangldquoBearing capacity and load transfer mechanism of a static drillrooted nodular pile in soft soil areasrdquo Journal of ZhejiangUniversity Science A vol 14 no 10 pp 705ndash719 2013

Advances in Civil Engineering 15

Table 4 +e characteristic parameters of each tested pile

Pilesrsquo no Pilesrsquo length L (m) Pilesrsquo diameter D (mm) Concrete strength FormNo 1 10 900 C35 Empty-bottomNo 2 10 900 C35 Solid-bottomNo 3 10 900 C35 Solid-bottomNo 4 15 900 C35 Solid-bottom

1700

1700

1700

1700

1700

300

1000

0

Ground

400

300

1000

0

900

Within 3mbelow the pile

top 100

3m belowthe pile top

200

Under thepile top 2m 2000

Strain gauges

Ground

Steel platewith D 850

20 mm thick

Empty-bottom

12 14

(a)

1600

1600

1600

1600

1700

1700

300

1000

0

400

300

1000

0

900

Ground Ground

Under thepile top 2m 2000

Within 3 mbelow the pile

top 100

3m belowthe pile top

200

Strain gauges

12 14

(b)

1600

1600

1600

1600

1800

1800

1600

1600

1600

300

1500

0

400

300

900

1500

0

GroundGround

Under thepile top 2m 2000

Within 3 mbelow the pile

top 100

3m belowthe pile top

200

Strain gauges

12 14

(c)

900

The stirrup of outside

50mm thickconcrete protective layer

Main reinforcement 120iexclatilde

The stirrup of insideStrain gauges

(d)

Figure 5 +e reinforcement layout and strain gauges layout of the test pile (a) Empty-bottom pile No 1 (b) Solid-bottom pile No 2 andNo 3 (c) Solid-bottom pile No 4 (d) Plan of layout of pile

Advances in Civil Engineering 5

resistance +e embedment of strain gauges shall be carriedout strictly in accordance with the operating instructionsand maintenance work shall be done to improve the successrate for the on-site embedding photo of strain gauges seeFigure 6

33 Static LoadMethod of Pile Foundation In the YD10000-200 hydraulic jack applying load the load reaction force isprovided by anchor pile Four FP-50 displacement sensorswere used for settlement which were installed on the ref-erence beam at the top of the test pile +e support of thesensor was fixed on the jack with magnetic bearing and thereference beam was fixed on the steel beam in an inde-pendent position unaffected by deformation calibrating thedisplacement sensor with the displacement sensor calibra-tion table before the test and using a force transducer tocalibrate the accuracy of the load on the jack +e Y-LINKstatic load test system automatically collects and stores testdata+e vertical compressive static load device of single pileis shown in Figure 7(a) and the photo of field static load testis shown in Figure 7(b)

+e test is carried out according to China Technical Codefor Testing of Building Foundation Piles (JGJ106-2014) [19]the specific steps are as follows

(1) In the test the step loading method was adopted (2)measuring the settlement every 5 minutes 10 minutes and15 minutes after each step of loading was applied every 15minutes after that and every 30 minutes after a total of 1hour and then loading stopped (3) +e settlement per hourcould not exceed 01mm When it occurred twice in suc-cession it was considered to be relatively stable and the nextstep of loading could be applied (4) When the settlement ofthe pile was 5 times of the settlement under the previous loadand the total settlement at the top of the pile exceeded40mm or the settlement of the pile was greater than 2 timesof the settlement under the previous load and had notreached a stable state after 24 hours loading stopped

34 Test Results of Pile Foundation +e line type of load-displacement (Q-S) curve in static load test can macro-scopically reflect pile bodyrsquos failure mechanism and failuremode of soil around pile [20] When the load is small thedisplacement curve tends to develop linearly and is in theelastic stage With the increasing load the pile graduallychanges from elastic deformation to elastic-plastic defor-mation As the load continues to increase the displacementincreases sharply and enters the plastic stage and the pilereaches the failure state +e Q-S curves of piles Nos 1 2 3and 4 are shown in Figure 8

It can be seen from Figure 8 that the Q-S curve of No 1pile has obvious inflection point indicating that it is a steepdrop curve +e pile top load corresponding to the inflectionpoint of Q-S curve is taken as the ultimate bearing capacityof the test pile [21] +e Q-S curves of the other three pileshave similar trends and there is no obvious steep dropsection indicating that they are slowly changing curves sothe ultimate bearing capacity is confirmed according to 442in =e Technical Specification of China Building Foundation

Piles +e bearing capacity under ultimate load of No 1 pileis 3600 kN and the corresponding pile top settlement is1289mm the maximum load is 4000 kN and the maximumsettlement is 4068mm similarly the maximum loadingload of Nos 2 3 and 4 piles and their corresponding pile topdisplacement values bearing capacity under ultimate loadand corresponding pile top displacement and settlementvalues are shown in Table 5

Figure 9 shows the S-lgt curve of settlement of each pilewith time which can further explain the value of ultimatebearing capacity At the last step of loading an obviousdownward bending is shown in the displacement of the fourpiles the displacement exceeded 40mm and the pile reachesthe failure state [22]

4 Discussion

41 =e Characteristics of Pile Shaft Force Transfer +e re-inforcement strain εi(microε) at the corresponding depth is di-rectly measured by the strain gauges meter embedded andthen the reinforcement stress σi is calculated according to

σsi K middot ε0 minus εi( 1113857 (1)

where σi is the stress on the reinforcement K is calibrationcoefficient indicated by the manufacturer and its valueranges from 06 to 08 εi is average strain measured by straingauges on the same section i is the serial numbers of theburied sections of the strain gauges i 1 2 L n are arrangedfrom top to bottom

According to Hookersquos law and the stress on the rein-forcement the real strain value of the strain gauges string iscalculated

εsi σsi

Es

(2)

where εsi is the real strain of the strain gauges string Es is theelastic modulus of reinforcement with a value of204 times 104 MPa σi and i are the same as in formula (1)

Assuming that the compression amount of concrete isequal to the real strain value of the strain gauges string andthe cross-sectional area of the pile is unchanged the concretestress can be obtained according to

σci εsi middot Ec (3)

where σci is the concrete stress value εsi is the real strain ofstrain gauges string Ec is the modulus of elasticity ofconcrete with a value of 104MPa i is the same as in formula(1)

Finally the respective internal forces are obtained bymultiplying stress values by the respective cross-sectionalareas and then the pile axial force of this section is obtainedby the stress of reinforcements plus the stress of concreteand the axial force of other cross-sectional pile bodies can beobtained similarly the calculation formula is as follows

Qi σciAci + σsiAsi (4)

where Qi is the axial force of tested section σci and σsi are thesame as in formulas (3) and (1) respectively Aci is the

6 Advances in Civil Engineering

concrete area of pile body 063585m2 Asi is the area ofreinforcement in pile body 184632 times 10minus3 m2

Figures 9(a)ndash9(d) show the axial force distribution of pilesNos 1 2 3 and 4 with depth under different loading steps

respectively With the increase of loading steps axial force isgradually generated at the lower part of each solid-bottompile and pile tip resistance is gradually mobilized [23] Whenreaching the ultimate load pile tip resistance of piles Nos 2 3and 4 is 2100 kN 1418 kN and 3463 kN respectively+e firstpile is an empty-bottom pile with the increase of load axialforce is gradually generated at the lower part of the pile bodyAfter reaching the ultimate bearing capacity the load willcontinue to be applied and the displacement of pile top willrapidly increase from the original 1289mm to 4068mm+is is because the first pile is an empty-bottom pile and thereis no soil at the bottom to generate tip resistance When theshaft skin resistance reaches the maximum value and cannotresist the axial force in the vertical direction if the loadcontinues to be applied after reaching the ultimate bearingcapacity sudden settlement of the pile body occurs and thesettlement exceeds the limit specified in the specificationindicating that pile failure has occurred

It can be seen from the stress conditions of the solid-bottom pile the empty-bottom pile the 10-meter-long pilebody and the 15-meter-long pile body in Figure 10 that theaxial force of the empty-bottom pile and the solid-bottompile is relatively small at the initial loading step With theincrease of loading step the axial force gradually develops

(a) (b)

Figure 6 Site photo of strain gauges embedding (a) Part photo (b) +e overall photo

Displacementmeter

Hydraulicjack

Test beam(s)

Test pile

Steel testplate

Load transfercolumn

Anchor pileMinimum clear

distance is 25m

(a) (b)

Figure 7 Schematic diagram and field photo of static load test (a) Schematic diagram (b) Photograph

0 1200 2400 3600 4800 6000 7200 8400 9600

Disp

lace

men

t (m

m)

Load at pile top (kN)

No 1No 2

No 3No 4

45

40

35

30

25

20

15

10

5

0

Figure 8 Load-displacement curve of static load test

Advances in Civil Engineering 7

Table 5 Results of static load test

Pile number Maximum load (kN) Maximum load settlement (mm) Bearing capacity underultimate load (kN)

Settlement underultimate load (mm) Rebound rate ()

No 1 4000 4068 3600 1289 1006No 2 5400 401 4800 3001 1236No 3 4800 4051 4200 2892 1684No 4 8800 4083 8000 2676 1894

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

lgt (min)

S (m

m)

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

(a)

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

lgt (min)

S (m

m)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

(b)

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

lgt (min)

S (m

m)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

(c)

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

S (m

m)

lgt (min)

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

(d)

Figure 9 S-lgt curve of each pile (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

8 Advances in Civil Engineering

and transmits to the pile bottom and decreases with theincrease of the depth Under different loading steps theattenuation rate of axial force curve is different +e at-tenuation rate of axial force curve reflects the development ofshaft skin resistance In low loading steps the attenuationrate of axial force curve is not fast and relatively uniformWith the increase of loading steps the attenuation rate ofaxial force curve at the upper part of pile body is not fastwhile the attenuation rate of axial force curve at the lowerpart of pile body is obvious which shows that no matter it isan empty-bottom pile or solid-bottom pile the force

transmission characteristics are similar With the increase ofloading steps the axial force gradually transmits to the lowerpart of the pile body If load continues to be applied afterreaching the ultimate load the empty-bottom pile will besuddenly destroyed while the solid-bottom pile will be able tobear due to soil in the lower part and will show a slow change

42 Analysis of Characteristics of Shaft Skin Resistance of PileBody Based on the principle of static balance regardless ofthe influence of pile body weight [24] the pile shaft skin

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

0 400 800 1200 1600 2000 2400 2800 3200 3600 4000 4400Axial force (kN)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

(a)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 600 1200 1800 2400 3000 3600 4200 4800 5400 6000Axial force (kN)

(b)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 600 1200 1800 2400 3000 3600 4200 4800 5400Axial force (kN)

(c)

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

16151413121110

9876543210

Dep

th Z

m

0 800 1600 2400 3200 4000 4800 5600 6400 7200 8000 8800Axial force (kN)

(d)

Figure 10 Distribution curve of axial force of each pile (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Advances in Civil Engineering 9

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

(a)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

(b)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

(c)

16151413121110

9876543210

Dep

th Z

m

0 20 40 60 80 100 120 140 160 180Skin friction of test pile (kPa)

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

(d)

Figure 11 Distribution of shaft skin resistance on each pile (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Table 6 Value of shaft skin resistance of piles at different depths when reaching ultimate bearing capacity

(a) No 1 pileDepth Zm 17 34 51 68 85Skin frictionkPa 60177 112974 133037 139158 135235

(b) No 2 pileDepth Zm 16 32 48 64 81 98Skin frictionkPa 43899 105055 128283 1391 146983 148015

(c) No 3 pileDepth Zm 16 32 48 64 81 98Skin frictionkPa 46948 106907 130723 145894 147938 150828

(d) No 4 pileDepth Zm 16 32 48 64 8 96 112 13 148Skin frictionkPa 63055 93009 117592 129804 141642 144953 145865 151216 154956

10 Advances in Civil Engineering

resistance value can be calculated by the following formulaaccording to the change value of the axial force of the pilebody

qsi Qi minus Qi+1

u middot li (5)

where qsi is the resistance between the i section and the i + 1section i is the same as in formula (1) u is the perimeter ofpile body li is the pile length between the i section and thei + 1 section

Figures 10(a)ndash10(d) show the distribution of shaft skinresistance of piles Nos 1 2 3 and 4 with depth underdifferent steps of loading

As can be seen from Figure 11 in lower loading step theshaft skin resistance of each pile is small with the increase ofloading steps the shaft skin resistance of each pile increasesOn the whole the shaft skin resistance of the pile is alsogradually transmitted from the upper part of the pile body tothe lower part of the pile body but compared with themiddle part of the pile body the shaft skin resistance of thelower part of the first pile body with empty-bottom shows a

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

10

8

6

4

2

0

Dep

th Z

m

(a)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

10

8

6

4

2

0

Dep

th Z

m

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

(b)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

10

8

6

4

2

0

Dep

th Z

m

(c)

Dep

th Z

m

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

151413121110

9876543210

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

(d)

Figure 12 Pile-soil relative displacement curve (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Advances in Civil Engineering 11

decreasing trend because there is no soil bearing the cor-responding load at the pile tip however for the remainingthree solid-bottom piles when the tip resistance is mobilizedto a certain extent the friction resistance in the middle partof the pile body is smaller than that in the lower part whichindicates that the soil at the pile tip can reinforce the shaftskin resistance of the pile [25] When the ultimate bearingcapacity is reached the shaft skin resistance values of thecorresponding piles at various depths are shown in Table 6

43 Relationship between Shaft Skin Resistance and Pile-SoilRelative Displacement +e pile-soil relative displacementobjectively reflects the mobilization of shaft skin resistance

According to the calculation method in [26] the pile-soilrelative displacement curves under different loading stepsare calculated and drawn +e curves of shaft skin resistanceand pile-soil relative displacement are shown inFigures 12(a)ndash12(d) respectively

In the calculation of the pile-soil relative displacement itis assumed that the soil around the pile will be not displacedAs can be seen from Figure 12 the pile-soil relative dis-placement at the same position of the pile body increases withthe increase of loading steps on the pile tipWhen the first twosteps of loading are applied the pile-soil relative displacementof the four piles increases obviously due to the compactioneffect of the first two steps of loading on the gravel soil aroundthe pile As the load level continues to increase the pile-soil

Skin

fric

tion

of te

st pi

le (k

Pa)

5 10 15 20 25 300Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

1-11-21-3

1-41-5

(a)Sk

in fr

ictio

n of

test

pile

(kPa

)

0 10 15 20 25 30 35 405Pile-soil relative displacement (mm)

2-12-22-3

2-42-52-6

0

20

40

60

80

100

120

140

160

(b)

Skin

fric

tion

of te

st pi

le (k

Pa)

5 10 15 20 25 30 35 400Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

3-13-23-3

3-43-53-6

(c)

Skin

fric

tion

of te

st pi

le (k

Pa)

0 10 15 20 25 30 35 405Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

4-14-24-3

4-44-54-6

4-74-84-9

(d)

Figure 13 Distribution curves of shaft skin resistances versus relative displacements (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4pile

12 Advances in Civil Engineering

relative displacement increment is relatively small because theshaft skin resistance gradually mobilizes its function toprevent the pile from moving With the increase of loadingwhen the shaft skin resistance cannot resist the load at theloading sections 4800 kNndash5400 kN 4200 kNndash4800 kN8000 kNndash8800 kN and 3600 kNndash4000 kN the maximumdisplacements of pile tips of Nos 2 3 and 4 solid-bottom pileand the empty-bottom pile increase by 9792mm 7939mm9881mm and 1497mm respectively

+e shaft skin resistance increases with the increase ofpile-soil relative displacement Although continuous loadinghas led to foundation soil destruction it can be concludedfrom Figure 13 that the lateral resistance in the middle andlower parts is not fully mobilized and a certain pile-soilrelative displacement is needed to promote the full mobi-lization of the shaft skin resistance However according tothe development trend that the slope of the shaft skin re-sistance gradually decreases it can be inferred that the shaftskin resistance will reach the maximum value At the firstsection position the ultimate shaft skin resistance of pilesNos 1 2 3 and 4 are 4695 kPa 4390 kPa 6018 kPa and6305 kPa respectively and the corresponding pile-soil rel-ative displacements are 3419mm 3442mm 2633mm and3279mm In the second section the shaft skin resistance ofthe four piles increases greatly so does the pile-soil relativedisplacement Since the soil above the first section position isbasically cultivated soil and it can resist small shaft skinresistance while the content of gravel soil of the secondsection is large and with the increase of depth the sur-rounding pressure of soil around the pile increases thereforethe ability of resisting pile foundation sliding and dis-placement increases+erefore between the first section andthe second section the shaft skin resistance increases greatlywhile the pile-soil relative displacement increases a little Inthe second section and beyond the soil is gravel soil and theshaft skin resistance increases slightly with the increase ofpile depth and the relative displacement decreases gradually

44 Pile Tip Resistance Varies with Load and DisplacementDuring the test the section where the largest embeddeddepth of strain gauges is very close to the bottom of the pilewas shown so the resistance at the pile tip is approximatelyequal to the axial force measured at this section namelyQb Q0 Under the action of all loading steps the resistancevariation curves at the lower tips of the Nos 2 3 and 4 pilesare shown in Figure 14

+e No 2 and No 3 piles in the first two loading stepsand No 4 pile in the first loading step have smaller resistanceat the pile tip +e resistance at the pile tip gradually in-creases until the third and fourth loading steps Under theaction of ultimate load the ratio of pile tip resistance to piletop load of Nos 2 3 and 4 is 377 3008 and 3992respectively all higher 15 which shows that pile tip re-sistance has been brought into play to a certain extent +epile tip resistance is shared by shaft skin resistance and piletip resistance and the pile is of tip bearing friction pile

According to the relationship curve between pile tipresistance and pile tip displacement shown in Figure 15 it

can be seen that the pile tip resistance and pile tip dis-placement present a double fold line model of work hard-ening +e Nos 2 3 and 4 piles all show a slow deformationfailure and under the action of the first loading step theslope of the curve is larger and the corresponding dis-placement is relatively small in latter steps of loading thecurvature curve becomes smoother the displacement of thepile tips of adjacent loads increases and the soil at the piletips will gradually be compacted

Pile

tip

load

(kN

)

1000 2000 3000 4000 5000 6000 7000 8000 90000Load at pile top (kN)

0250500750

100012501500175020002250250027503000325035003750

No 2No 3No 4

Figure 14 Variation curve of pile tip resistance under differentloading steps (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4pile

Pile

tip

load

(kN

)

5 10 15 20 25 30 35 40 450Pile tip displacement (mm)

0

1000

2000

3000

4000

5000

6000

No 2No 3No 4

Figure 15 Relationship curve between pile tip resistance and piletip displacement (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No4 pile

Advances in Civil Engineering 13

5 Conclusions

Analyses are carried out on data of the in-situ test of unevengravel soil in Baishi Mountain and the following conclu-sions are drawn

(1) Based on the analysis of the results of multichanneltransient surface wave test superheavy (N120) conedynamic sounding test and on-site screening testthe geological environment conditions of the unevengravel soil on the site are further explored +e stratawithin the range from 0 to 15m in Baizhi Mountaincan be divided into four layers ie cultivated soilsandy soil silty clay and gravel soil with depths of1m 2m 3m and 9m respectively

(2) +e axial force and shaft skin resistance of pile bodyin gravel soil are the same as those in other geologicalstructures At the initial loading step the axial forceand shaft skin resistance of pile body play a smallerrole With the increase of loading steps the axialforce and shaft skin resistance of pile body increasecorrespondingly Under the same loading step theaxial force of the pile body and the shaft skin re-sistance do not mobilize in the same time andgradually transfer from the upper part to the lowerpart of the pile body and the upper part of the pilebody reaches the limit value before the middle andlower parts As for the solid-bottom pile the soil atthe pile tip will strengthen the shaft skin resistance ofthe pile

(3) According to this experimental study it is suggestedthat similar projects should adopt concrete manuallybored cast-in-place piles with a diameter of 900mmand a length of 15m the wall-protecting thicknessshall not be less than 175mm and the grade ofconcrete cannot be lower than C35 +e pile end canbe widened or not and the pile bottom shall be freeof sediment

Data Availability

All the data in this paper are obtained from tests in thisstudy and no other data were used to support this study

Conflicts of Interest

+e authors declare that with regard to the publication ofthis paper and the funding for publishing it there are noconflicts of interest

Acknowledgments

+is study was partially supported by the National NaturalScience Foundation of China (Grant no 41302223) ChongqingNo 3 colleges and universities youth backbone teachers fundingplans Chongqing Research Program of Basic Research andFrontier Technology (Grant no cstc2016jcyjA0074cstc2016jcyjA0933 and cstc2015jcyjA90012) Scientific andTechnological Research Program of Chongqing Municipal

Education Commission (Grant no KJ1713327 and KJ1600532)Key Laboratory ofWell Stability and Fluid and RockMechanicsin Oil and Gas Reservoir of Shaanxi Province Xirsquoan ShiyouUniversity (Grant no FRM20190201002) Chongqing Post-graduate Research Innovation Project (Grant no CYS19353)and Chongqing University of Science and Technology GraduateStudentsrsquo Science and Technology Innovation Program (Grantno YKJCX1720601 and YKJCX1920613)

References

[1] F Kirsch and B Plabmann ldquoDynamic methods in pile testingdevelopments in measurement and analysisrdquo in Proceedings ofthe International Deep Foundations Congress 2002 pp 868ndash882 ASCE Orlando FL USA February 2002

[2] S H Ni L Lehmann J J Charng et al ldquoLow-strain integritytesting of drilled piles with high slenderness ratiordquo Computersand Geotechnics vol 33 no 6-7 pp 283ndash293 2006

[3] K Mori A Spagnoli Y Murakami G Kondo and I TorigoeldquoA new non-contacting non-destructive testing method fordefect detection in concreterdquo NDT amp E International vol 35no 6 pp 399ndash406 2002

[4] G Cai S Liu L Tong et al ldquoAssessment of direct CPT andCPTUmethods for predicting the ultimate bearing capacity ofsingle pilesrdquo Engineering Geology vol 104 no 3-4pp 211ndash222 2009

[5] X Y Bai M Y Zhang L Zhu et al ldquoIn-situ test and finiteelement analysis of bearing characteristics of rock-socketedshort piles in strongly weathered graniterdquo Journal of CentralSouth University Natural Science Edition vol 48 no 2pp 512ndash524 2017 in Chinese

[6] E C Leong and M F Randolph ldquoFinite element modelling ofrock-socketed pilesrdquo International Journal for Numerical andAnalytical Methods in Geomechanics vol 18 no 1 pp 25ndash471994

[7] G Gao M Gao Q Chen and J Yang ldquoField load testingstudy of vertical bearing behavior of a large diameter belledcast-in-place pilerdquo KSCE Journal of Civil Engineering vol 23no 5 pp 2009ndash2016 2019

[8] R Berardi and R Bovolenta ldquoPile-settlement evaluation usingfield stiffness non-linearityrdquo Proceedings of the Institution ofCivil Engineers - Geotechnical Engineering vol 158 no 1pp 35ndash44 2005

[9] B H Fellenius Basics of Foundation Design Lulu PressMorrisville NC USA 2016

[10] C Lam and S A Jefferis ldquoCritical assessment of pile modulusdetermination methodsrdquo Canadian Geotechnical Journalvol 48 no 10 pp 1433ndash1448 2011

[11] A Mohammadi T Ebadi and M R Boroomand ldquoPhysicalmodelling of axial compressive bearing capacity of instru-mented piles in oil-contaminated sandy soilrdquo Iranian Journalof Science and Technology Transactions of Civil Engineeringvol 44 no 2 pp 695ndash714 2019

[12] W D Guo and E H Ghee ldquoBehavior of axially loaded pilegroups subjected to lateral soil movementrdquo in Proceedings ofthe GeoShanghai International Conference 2006 pp 174ndash181ASCE Shanghai China June 2006

[13] Z Zhou Y Dong P Jiang D Han and T Liu ldquoCalculation ofpile side friction by multiparameter statistical analysisrdquo Ad-vances in Civil Engineering vol 2019 Article ID 263852012 pages 2019

14 Advances in Civil Engineering

[14] N M Hai and B H Fellenius ldquoO-Cell tests on two 70 m longbored piles in Vietnamrdquo in Proceedings of the Geo-Congress2014 pp 482ndash496 ASCE Atlanta Georgia February 2014

[15] S Gong G Cai S Liu et al ldquoNumerical Simulation of BearingCapacity and Consolidation Characteristics of PHC PileFoundationrdquo GeoShanghai International Conferencepp 178ndash185 Springer Singapore 2018

[16] M A Bashir H Furuuchi T Ueda and M Nauman BashirldquoNumerical simulation of axial anchorage capacity of con-crete-filled steel box footingrdquo Iranian Journal of Science andTechnology Transactions of Civil Engineering vol 40 no 3pp 257ndash262 2016

[17] N A Haskell ldquo+e dispersion of surface waves on multi-layered mediardquo Bulletin of the Seismological Society ofAmerica vol 43 no 1 pp 17ndash34 1953

[18] China Architecture amp Building PressNational Standard of thePeoplersquos Republic of China Code for Investigation of Geo-technical Engineering (GB50021 2016) China Architecture ampBuilding Press Beijing China 2016 in Chinese

[19] China Architecture amp Building Press Industry Standard of thePeoplersquos Republic of China Technical Code for Testing ofBuilding Foundation Piles (JGJ106-2014) China Architectureamp Building Press Beijing China 2014 in Chinese

[20] N Guo Z H Chen X F Huang et al ldquoResearch on the anti-pulling test of large-diameter cloth bag pile in soft rockfoundationrdquo Rock and Soil Mechanics vol 36 no 2pp 603ndash609 2015 in Chinese

[21] Y H Li X Zhu and T H Zhou ldquoField comparative ex-perimental study on the influence of post-tip grouting onlarge-diameter cast-in-situ pilesrdquo Rock and Soil Mechanicsvol 37 no 2 pp 388ndash396 2016 in Chinese

[22] G F Xin Z M Zhang T Xia et al ldquoExperimental study onbearing behavior of ultra-long pile under high loadrdquo ChineseJournal of Rock Mechanics and Engineering vol 13pp 2397ndash2402 2005 in Chinese

[23] Z Ling W Wang J Wu et al ldquoFull-scale loading test on pre-bored precast pile with enlarged base in Shanghairdquo in Pro-ceedings of the GeoShanghai 2018 International ConferenceAdvances in Soil Dynamics and Foundation Engineeringpp 637ndash645 Springer Singapore 2018

[24] J R Dong S T Lin and Y M Dai ldquo+e load tranfer behaviorof large diameter cast-in-situ pile in crushed pebble stratumrdquoChinese Journal of Geotechnical Engineering vol 16 no 6pp 123ndash131 1994

[25] X F Huang J H Zhang L G Ma et al ldquoExperimental studyon reinforcement effect of lateral resistance of collapsible loessrock-socketed perfusion pilerdquo Architectural Structure vol 41no 2 pp 351ndash355 2011 in Chinese

[26] J-J Zhou K-H Wang X-N Gong and R-H ZhangldquoBearing capacity and load transfer mechanism of a static drillrooted nodular pile in soft soil areasrdquo Journal of ZhejiangUniversity Science A vol 14 no 10 pp 705ndash719 2013

Advances in Civil Engineering 15

resistance +e embedment of strain gauges shall be carriedout strictly in accordance with the operating instructionsand maintenance work shall be done to improve the successrate for the on-site embedding photo of strain gauges seeFigure 6

33 Static LoadMethod of Pile Foundation In the YD10000-200 hydraulic jack applying load the load reaction force isprovided by anchor pile Four FP-50 displacement sensorswere used for settlement which were installed on the ref-erence beam at the top of the test pile +e support of thesensor was fixed on the jack with magnetic bearing and thereference beam was fixed on the steel beam in an inde-pendent position unaffected by deformation calibrating thedisplacement sensor with the displacement sensor calibra-tion table before the test and using a force transducer tocalibrate the accuracy of the load on the jack +e Y-LINKstatic load test system automatically collects and stores testdata+e vertical compressive static load device of single pileis shown in Figure 7(a) and the photo of field static load testis shown in Figure 7(b)

+e test is carried out according to China Technical Codefor Testing of Building Foundation Piles (JGJ106-2014) [19]the specific steps are as follows

(1) In the test the step loading method was adopted (2)measuring the settlement every 5 minutes 10 minutes and15 minutes after each step of loading was applied every 15minutes after that and every 30 minutes after a total of 1hour and then loading stopped (3) +e settlement per hourcould not exceed 01mm When it occurred twice in suc-cession it was considered to be relatively stable and the nextstep of loading could be applied (4) When the settlement ofthe pile was 5 times of the settlement under the previous loadand the total settlement at the top of the pile exceeded40mm or the settlement of the pile was greater than 2 timesof the settlement under the previous load and had notreached a stable state after 24 hours loading stopped

34 Test Results of Pile Foundation +e line type of load-displacement (Q-S) curve in static load test can macro-scopically reflect pile bodyrsquos failure mechanism and failuremode of soil around pile [20] When the load is small thedisplacement curve tends to develop linearly and is in theelastic stage With the increasing load the pile graduallychanges from elastic deformation to elastic-plastic defor-mation As the load continues to increase the displacementincreases sharply and enters the plastic stage and the pilereaches the failure state +e Q-S curves of piles Nos 1 2 3and 4 are shown in Figure 8

It can be seen from Figure 8 that the Q-S curve of No 1pile has obvious inflection point indicating that it is a steepdrop curve +e pile top load corresponding to the inflectionpoint of Q-S curve is taken as the ultimate bearing capacityof the test pile [21] +e Q-S curves of the other three pileshave similar trends and there is no obvious steep dropsection indicating that they are slowly changing curves sothe ultimate bearing capacity is confirmed according to 442in =e Technical Specification of China Building Foundation

Piles +e bearing capacity under ultimate load of No 1 pileis 3600 kN and the corresponding pile top settlement is1289mm the maximum load is 4000 kN and the maximumsettlement is 4068mm similarly the maximum loadingload of Nos 2 3 and 4 piles and their corresponding pile topdisplacement values bearing capacity under ultimate loadand corresponding pile top displacement and settlementvalues are shown in Table 5

Figure 9 shows the S-lgt curve of settlement of each pilewith time which can further explain the value of ultimatebearing capacity At the last step of loading an obviousdownward bending is shown in the displacement of the fourpiles the displacement exceeded 40mm and the pile reachesthe failure state [22]

4 Discussion

41 =e Characteristics of Pile Shaft Force Transfer +e re-inforcement strain εi(microε) at the corresponding depth is di-rectly measured by the strain gauges meter embedded andthen the reinforcement stress σi is calculated according to

σsi K middot ε0 minus εi( 1113857 (1)

where σi is the stress on the reinforcement K is calibrationcoefficient indicated by the manufacturer and its valueranges from 06 to 08 εi is average strain measured by straingauges on the same section i is the serial numbers of theburied sections of the strain gauges i 1 2 L n are arrangedfrom top to bottom

According to Hookersquos law and the stress on the rein-forcement the real strain value of the strain gauges string iscalculated

εsi σsi

Es

(2)

where εsi is the real strain of the strain gauges string Es is theelastic modulus of reinforcement with a value of204 times 104 MPa σi and i are the same as in formula (1)

Assuming that the compression amount of concrete isequal to the real strain value of the strain gauges string andthe cross-sectional area of the pile is unchanged the concretestress can be obtained according to

σci εsi middot Ec (3)

where σci is the concrete stress value εsi is the real strain ofstrain gauges string Ec is the modulus of elasticity ofconcrete with a value of 104MPa i is the same as in formula(1)

Finally the respective internal forces are obtained bymultiplying stress values by the respective cross-sectionalareas and then the pile axial force of this section is obtainedby the stress of reinforcements plus the stress of concreteand the axial force of other cross-sectional pile bodies can beobtained similarly the calculation formula is as follows

Qi σciAci + σsiAsi (4)

where Qi is the axial force of tested section σci and σsi are thesame as in formulas (3) and (1) respectively Aci is the

6 Advances in Civil Engineering

concrete area of pile body 063585m2 Asi is the area ofreinforcement in pile body 184632 times 10minus3 m2

Figures 9(a)ndash9(d) show the axial force distribution of pilesNos 1 2 3 and 4 with depth under different loading steps

respectively With the increase of loading steps axial force isgradually generated at the lower part of each solid-bottompile and pile tip resistance is gradually mobilized [23] Whenreaching the ultimate load pile tip resistance of piles Nos 2 3and 4 is 2100 kN 1418 kN and 3463 kN respectively+e firstpile is an empty-bottom pile with the increase of load axialforce is gradually generated at the lower part of the pile bodyAfter reaching the ultimate bearing capacity the load willcontinue to be applied and the displacement of pile top willrapidly increase from the original 1289mm to 4068mm+is is because the first pile is an empty-bottom pile and thereis no soil at the bottom to generate tip resistance When theshaft skin resistance reaches the maximum value and cannotresist the axial force in the vertical direction if the loadcontinues to be applied after reaching the ultimate bearingcapacity sudden settlement of the pile body occurs and thesettlement exceeds the limit specified in the specificationindicating that pile failure has occurred

It can be seen from the stress conditions of the solid-bottom pile the empty-bottom pile the 10-meter-long pilebody and the 15-meter-long pile body in Figure 10 that theaxial force of the empty-bottom pile and the solid-bottompile is relatively small at the initial loading step With theincrease of loading step the axial force gradually develops

(a) (b)

Figure 6 Site photo of strain gauges embedding (a) Part photo (b) +e overall photo

Displacementmeter

Hydraulicjack

Test beam(s)

Test pile

Steel testplate

Load transfercolumn

Anchor pileMinimum clear

distance is 25m

(a) (b)

Figure 7 Schematic diagram and field photo of static load test (a) Schematic diagram (b) Photograph

0 1200 2400 3600 4800 6000 7200 8400 9600

Disp

lace

men

t (m

m)

Load at pile top (kN)

No 1No 2

No 3No 4

45

40

35

30

25

20

15

10

5

0

Figure 8 Load-displacement curve of static load test

Advances in Civil Engineering 7

Table 5 Results of static load test

Pile number Maximum load (kN) Maximum load settlement (mm) Bearing capacity underultimate load (kN)

Settlement underultimate load (mm) Rebound rate ()

No 1 4000 4068 3600 1289 1006No 2 5400 401 4800 3001 1236No 3 4800 4051 4200 2892 1684No 4 8800 4083 8000 2676 1894

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

lgt (min)

S (m

m)

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

(a)

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

lgt (min)

S (m

m)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

(b)

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

lgt (min)

S (m

m)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

(c)

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

S (m

m)

lgt (min)

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

(d)

Figure 9 S-lgt curve of each pile (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

8 Advances in Civil Engineering

and transmits to the pile bottom and decreases with theincrease of the depth Under different loading steps theattenuation rate of axial force curve is different +e at-tenuation rate of axial force curve reflects the development ofshaft skin resistance In low loading steps the attenuationrate of axial force curve is not fast and relatively uniformWith the increase of loading steps the attenuation rate ofaxial force curve at the upper part of pile body is not fastwhile the attenuation rate of axial force curve at the lowerpart of pile body is obvious which shows that no matter it isan empty-bottom pile or solid-bottom pile the force

transmission characteristics are similar With the increase ofloading steps the axial force gradually transmits to the lowerpart of the pile body If load continues to be applied afterreaching the ultimate load the empty-bottom pile will besuddenly destroyed while the solid-bottom pile will be able tobear due to soil in the lower part and will show a slow change

42 Analysis of Characteristics of Shaft Skin Resistance of PileBody Based on the principle of static balance regardless ofthe influence of pile body weight [24] the pile shaft skin

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

0 400 800 1200 1600 2000 2400 2800 3200 3600 4000 4400Axial force (kN)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

(a)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 600 1200 1800 2400 3000 3600 4200 4800 5400 6000Axial force (kN)

(b)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 600 1200 1800 2400 3000 3600 4200 4800 5400Axial force (kN)

(c)

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

16151413121110

9876543210

Dep

th Z

m

0 800 1600 2400 3200 4000 4800 5600 6400 7200 8000 8800Axial force (kN)

(d)

Figure 10 Distribution curve of axial force of each pile (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Advances in Civil Engineering 9

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

(a)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

(b)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

(c)

16151413121110

9876543210

Dep

th Z

m

0 20 40 60 80 100 120 140 160 180Skin friction of test pile (kPa)

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

(d)

Figure 11 Distribution of shaft skin resistance on each pile (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Table 6 Value of shaft skin resistance of piles at different depths when reaching ultimate bearing capacity

(a) No 1 pileDepth Zm 17 34 51 68 85Skin frictionkPa 60177 112974 133037 139158 135235

(b) No 2 pileDepth Zm 16 32 48 64 81 98Skin frictionkPa 43899 105055 128283 1391 146983 148015

(c) No 3 pileDepth Zm 16 32 48 64 81 98Skin frictionkPa 46948 106907 130723 145894 147938 150828

(d) No 4 pileDepth Zm 16 32 48 64 8 96 112 13 148Skin frictionkPa 63055 93009 117592 129804 141642 144953 145865 151216 154956

10 Advances in Civil Engineering

resistance value can be calculated by the following formulaaccording to the change value of the axial force of the pilebody

qsi Qi minus Qi+1

u middot li (5)

where qsi is the resistance between the i section and the i + 1section i is the same as in formula (1) u is the perimeter ofpile body li is the pile length between the i section and thei + 1 section

Figures 10(a)ndash10(d) show the distribution of shaft skinresistance of piles Nos 1 2 3 and 4 with depth underdifferent steps of loading

As can be seen from Figure 11 in lower loading step theshaft skin resistance of each pile is small with the increase ofloading steps the shaft skin resistance of each pile increasesOn the whole the shaft skin resistance of the pile is alsogradually transmitted from the upper part of the pile body tothe lower part of the pile body but compared with themiddle part of the pile body the shaft skin resistance of thelower part of the first pile body with empty-bottom shows a

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

10

8

6

4

2

0

Dep

th Z

m

(a)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

10

8

6

4

2

0

Dep

th Z

m

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

(b)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

10

8

6

4

2

0

Dep

th Z

m

(c)

Dep

th Z

m

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

151413121110

9876543210

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

(d)

Figure 12 Pile-soil relative displacement curve (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Advances in Civil Engineering 11

decreasing trend because there is no soil bearing the cor-responding load at the pile tip however for the remainingthree solid-bottom piles when the tip resistance is mobilizedto a certain extent the friction resistance in the middle partof the pile body is smaller than that in the lower part whichindicates that the soil at the pile tip can reinforce the shaftskin resistance of the pile [25] When the ultimate bearingcapacity is reached the shaft skin resistance values of thecorresponding piles at various depths are shown in Table 6

43 Relationship between Shaft Skin Resistance and Pile-SoilRelative Displacement +e pile-soil relative displacementobjectively reflects the mobilization of shaft skin resistance

According to the calculation method in [26] the pile-soilrelative displacement curves under different loading stepsare calculated and drawn +e curves of shaft skin resistanceand pile-soil relative displacement are shown inFigures 12(a)ndash12(d) respectively

In the calculation of the pile-soil relative displacement itis assumed that the soil around the pile will be not displacedAs can be seen from Figure 12 the pile-soil relative dis-placement at the same position of the pile body increases withthe increase of loading steps on the pile tipWhen the first twosteps of loading are applied the pile-soil relative displacementof the four piles increases obviously due to the compactioneffect of the first two steps of loading on the gravel soil aroundthe pile As the load level continues to increase the pile-soil

Skin

fric

tion

of te

st pi

le (k

Pa)

5 10 15 20 25 300Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

1-11-21-3

1-41-5

(a)Sk

in fr

ictio

n of

test

pile

(kPa

)

0 10 15 20 25 30 35 405Pile-soil relative displacement (mm)

2-12-22-3

2-42-52-6

0

20

40

60

80

100

120

140

160

(b)

Skin

fric

tion

of te

st pi

le (k

Pa)

5 10 15 20 25 30 35 400Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

3-13-23-3

3-43-53-6

(c)

Skin

fric

tion

of te

st pi

le (k

Pa)

0 10 15 20 25 30 35 405Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

4-14-24-3

4-44-54-6

4-74-84-9

(d)

Figure 13 Distribution curves of shaft skin resistances versus relative displacements (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4pile

12 Advances in Civil Engineering

relative displacement increment is relatively small because theshaft skin resistance gradually mobilizes its function toprevent the pile from moving With the increase of loadingwhen the shaft skin resistance cannot resist the load at theloading sections 4800 kNndash5400 kN 4200 kNndash4800 kN8000 kNndash8800 kN and 3600 kNndash4000 kN the maximumdisplacements of pile tips of Nos 2 3 and 4 solid-bottom pileand the empty-bottom pile increase by 9792mm 7939mm9881mm and 1497mm respectively

+e shaft skin resistance increases with the increase ofpile-soil relative displacement Although continuous loadinghas led to foundation soil destruction it can be concludedfrom Figure 13 that the lateral resistance in the middle andlower parts is not fully mobilized and a certain pile-soilrelative displacement is needed to promote the full mobi-lization of the shaft skin resistance However according tothe development trend that the slope of the shaft skin re-sistance gradually decreases it can be inferred that the shaftskin resistance will reach the maximum value At the firstsection position the ultimate shaft skin resistance of pilesNos 1 2 3 and 4 are 4695 kPa 4390 kPa 6018 kPa and6305 kPa respectively and the corresponding pile-soil rel-ative displacements are 3419mm 3442mm 2633mm and3279mm In the second section the shaft skin resistance ofthe four piles increases greatly so does the pile-soil relativedisplacement Since the soil above the first section position isbasically cultivated soil and it can resist small shaft skinresistance while the content of gravel soil of the secondsection is large and with the increase of depth the sur-rounding pressure of soil around the pile increases thereforethe ability of resisting pile foundation sliding and dis-placement increases+erefore between the first section andthe second section the shaft skin resistance increases greatlywhile the pile-soil relative displacement increases a little Inthe second section and beyond the soil is gravel soil and theshaft skin resistance increases slightly with the increase ofpile depth and the relative displacement decreases gradually

44 Pile Tip Resistance Varies with Load and DisplacementDuring the test the section where the largest embeddeddepth of strain gauges is very close to the bottom of the pilewas shown so the resistance at the pile tip is approximatelyequal to the axial force measured at this section namelyQb Q0 Under the action of all loading steps the resistancevariation curves at the lower tips of the Nos 2 3 and 4 pilesare shown in Figure 14

+e No 2 and No 3 piles in the first two loading stepsand No 4 pile in the first loading step have smaller resistanceat the pile tip +e resistance at the pile tip gradually in-creases until the third and fourth loading steps Under theaction of ultimate load the ratio of pile tip resistance to piletop load of Nos 2 3 and 4 is 377 3008 and 3992respectively all higher 15 which shows that pile tip re-sistance has been brought into play to a certain extent +epile tip resistance is shared by shaft skin resistance and piletip resistance and the pile is of tip bearing friction pile

According to the relationship curve between pile tipresistance and pile tip displacement shown in Figure 15 it

can be seen that the pile tip resistance and pile tip dis-placement present a double fold line model of work hard-ening +e Nos 2 3 and 4 piles all show a slow deformationfailure and under the action of the first loading step theslope of the curve is larger and the corresponding dis-placement is relatively small in latter steps of loading thecurvature curve becomes smoother the displacement of thepile tips of adjacent loads increases and the soil at the piletips will gradually be compacted

Pile

tip

load

(kN

)

1000 2000 3000 4000 5000 6000 7000 8000 90000Load at pile top (kN)

0250500750

100012501500175020002250250027503000325035003750

No 2No 3No 4

Figure 14 Variation curve of pile tip resistance under differentloading steps (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4pile

Pile

tip

load

(kN

)

5 10 15 20 25 30 35 40 450Pile tip displacement (mm)

0

1000

2000

3000

4000

5000

6000

No 2No 3No 4

Figure 15 Relationship curve between pile tip resistance and piletip displacement (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No4 pile

Advances in Civil Engineering 13

5 Conclusions

Analyses are carried out on data of the in-situ test of unevengravel soil in Baishi Mountain and the following conclu-sions are drawn

(1) Based on the analysis of the results of multichanneltransient surface wave test superheavy (N120) conedynamic sounding test and on-site screening testthe geological environment conditions of the unevengravel soil on the site are further explored +e stratawithin the range from 0 to 15m in Baizhi Mountaincan be divided into four layers ie cultivated soilsandy soil silty clay and gravel soil with depths of1m 2m 3m and 9m respectively

(2) +e axial force and shaft skin resistance of pile bodyin gravel soil are the same as those in other geologicalstructures At the initial loading step the axial forceand shaft skin resistance of pile body play a smallerrole With the increase of loading steps the axialforce and shaft skin resistance of pile body increasecorrespondingly Under the same loading step theaxial force of the pile body and the shaft skin re-sistance do not mobilize in the same time andgradually transfer from the upper part to the lowerpart of the pile body and the upper part of the pilebody reaches the limit value before the middle andlower parts As for the solid-bottom pile the soil atthe pile tip will strengthen the shaft skin resistance ofthe pile

(3) According to this experimental study it is suggestedthat similar projects should adopt concrete manuallybored cast-in-place piles with a diameter of 900mmand a length of 15m the wall-protecting thicknessshall not be less than 175mm and the grade ofconcrete cannot be lower than C35 +e pile end canbe widened or not and the pile bottom shall be freeof sediment

Data Availability

All the data in this paper are obtained from tests in thisstudy and no other data were used to support this study

Conflicts of Interest

+e authors declare that with regard to the publication ofthis paper and the funding for publishing it there are noconflicts of interest

Acknowledgments

+is study was partially supported by the National NaturalScience Foundation of China (Grant no 41302223) ChongqingNo 3 colleges and universities youth backbone teachers fundingplans Chongqing Research Program of Basic Research andFrontier Technology (Grant no cstc2016jcyjA0074cstc2016jcyjA0933 and cstc2015jcyjA90012) Scientific andTechnological Research Program of Chongqing Municipal

Education Commission (Grant no KJ1713327 and KJ1600532)Key Laboratory ofWell Stability and Fluid and RockMechanicsin Oil and Gas Reservoir of Shaanxi Province Xirsquoan ShiyouUniversity (Grant no FRM20190201002) Chongqing Post-graduate Research Innovation Project (Grant no CYS19353)and Chongqing University of Science and Technology GraduateStudentsrsquo Science and Technology Innovation Program (Grantno YKJCX1720601 and YKJCX1920613)

References

[1] F Kirsch and B Plabmann ldquoDynamic methods in pile testingdevelopments in measurement and analysisrdquo in Proceedings ofthe International Deep Foundations Congress 2002 pp 868ndash882 ASCE Orlando FL USA February 2002

[2] S H Ni L Lehmann J J Charng et al ldquoLow-strain integritytesting of drilled piles with high slenderness ratiordquo Computersand Geotechnics vol 33 no 6-7 pp 283ndash293 2006

[3] K Mori A Spagnoli Y Murakami G Kondo and I TorigoeldquoA new non-contacting non-destructive testing method fordefect detection in concreterdquo NDT amp E International vol 35no 6 pp 399ndash406 2002

[4] G Cai S Liu L Tong et al ldquoAssessment of direct CPT andCPTUmethods for predicting the ultimate bearing capacity ofsingle pilesrdquo Engineering Geology vol 104 no 3-4pp 211ndash222 2009

[5] X Y Bai M Y Zhang L Zhu et al ldquoIn-situ test and finiteelement analysis of bearing characteristics of rock-socketedshort piles in strongly weathered graniterdquo Journal of CentralSouth University Natural Science Edition vol 48 no 2pp 512ndash524 2017 in Chinese

[6] E C Leong and M F Randolph ldquoFinite element modelling ofrock-socketed pilesrdquo International Journal for Numerical andAnalytical Methods in Geomechanics vol 18 no 1 pp 25ndash471994

[7] G Gao M Gao Q Chen and J Yang ldquoField load testingstudy of vertical bearing behavior of a large diameter belledcast-in-place pilerdquo KSCE Journal of Civil Engineering vol 23no 5 pp 2009ndash2016 2019

[8] R Berardi and R Bovolenta ldquoPile-settlement evaluation usingfield stiffness non-linearityrdquo Proceedings of the Institution ofCivil Engineers - Geotechnical Engineering vol 158 no 1pp 35ndash44 2005

[9] B H Fellenius Basics of Foundation Design Lulu PressMorrisville NC USA 2016

[10] C Lam and S A Jefferis ldquoCritical assessment of pile modulusdetermination methodsrdquo Canadian Geotechnical Journalvol 48 no 10 pp 1433ndash1448 2011

[11] A Mohammadi T Ebadi and M R Boroomand ldquoPhysicalmodelling of axial compressive bearing capacity of instru-mented piles in oil-contaminated sandy soilrdquo Iranian Journalof Science and Technology Transactions of Civil Engineeringvol 44 no 2 pp 695ndash714 2019

[12] W D Guo and E H Ghee ldquoBehavior of axially loaded pilegroups subjected to lateral soil movementrdquo in Proceedings ofthe GeoShanghai International Conference 2006 pp 174ndash181ASCE Shanghai China June 2006

[13] Z Zhou Y Dong P Jiang D Han and T Liu ldquoCalculation ofpile side friction by multiparameter statistical analysisrdquo Ad-vances in Civil Engineering vol 2019 Article ID 263852012 pages 2019

14 Advances in Civil Engineering

[14] N M Hai and B H Fellenius ldquoO-Cell tests on two 70 m longbored piles in Vietnamrdquo in Proceedings of the Geo-Congress2014 pp 482ndash496 ASCE Atlanta Georgia February 2014

[15] S Gong G Cai S Liu et al ldquoNumerical Simulation of BearingCapacity and Consolidation Characteristics of PHC PileFoundationrdquo GeoShanghai International Conferencepp 178ndash185 Springer Singapore 2018

[16] M A Bashir H Furuuchi T Ueda and M Nauman BashirldquoNumerical simulation of axial anchorage capacity of con-crete-filled steel box footingrdquo Iranian Journal of Science andTechnology Transactions of Civil Engineering vol 40 no 3pp 257ndash262 2016

[17] N A Haskell ldquo+e dispersion of surface waves on multi-layered mediardquo Bulletin of the Seismological Society ofAmerica vol 43 no 1 pp 17ndash34 1953

[18] China Architecture amp Building PressNational Standard of thePeoplersquos Republic of China Code for Investigation of Geo-technical Engineering (GB50021 2016) China Architecture ampBuilding Press Beijing China 2016 in Chinese

[19] China Architecture amp Building Press Industry Standard of thePeoplersquos Republic of China Technical Code for Testing ofBuilding Foundation Piles (JGJ106-2014) China Architectureamp Building Press Beijing China 2014 in Chinese

[20] N Guo Z H Chen X F Huang et al ldquoResearch on the anti-pulling test of large-diameter cloth bag pile in soft rockfoundationrdquo Rock and Soil Mechanics vol 36 no 2pp 603ndash609 2015 in Chinese

[21] Y H Li X Zhu and T H Zhou ldquoField comparative ex-perimental study on the influence of post-tip grouting onlarge-diameter cast-in-situ pilesrdquo Rock and Soil Mechanicsvol 37 no 2 pp 388ndash396 2016 in Chinese

[22] G F Xin Z M Zhang T Xia et al ldquoExperimental study onbearing behavior of ultra-long pile under high loadrdquo ChineseJournal of Rock Mechanics and Engineering vol 13pp 2397ndash2402 2005 in Chinese

[23] Z Ling W Wang J Wu et al ldquoFull-scale loading test on pre-bored precast pile with enlarged base in Shanghairdquo in Pro-ceedings of the GeoShanghai 2018 International ConferenceAdvances in Soil Dynamics and Foundation Engineeringpp 637ndash645 Springer Singapore 2018

[24] J R Dong S T Lin and Y M Dai ldquo+e load tranfer behaviorof large diameter cast-in-situ pile in crushed pebble stratumrdquoChinese Journal of Geotechnical Engineering vol 16 no 6pp 123ndash131 1994

[25] X F Huang J H Zhang L G Ma et al ldquoExperimental studyon reinforcement effect of lateral resistance of collapsible loessrock-socketed perfusion pilerdquo Architectural Structure vol 41no 2 pp 351ndash355 2011 in Chinese

[26] J-J Zhou K-H Wang X-N Gong and R-H ZhangldquoBearing capacity and load transfer mechanism of a static drillrooted nodular pile in soft soil areasrdquo Journal of ZhejiangUniversity Science A vol 14 no 10 pp 705ndash719 2013

Advances in Civil Engineering 15

concrete area of pile body 063585m2 Asi is the area ofreinforcement in pile body 184632 times 10minus3 m2

Figures 9(a)ndash9(d) show the axial force distribution of pilesNos 1 2 3 and 4 with depth under different loading steps

respectively With the increase of loading steps axial force isgradually generated at the lower part of each solid-bottompile and pile tip resistance is gradually mobilized [23] Whenreaching the ultimate load pile tip resistance of piles Nos 2 3and 4 is 2100 kN 1418 kN and 3463 kN respectively+e firstpile is an empty-bottom pile with the increase of load axialforce is gradually generated at the lower part of the pile bodyAfter reaching the ultimate bearing capacity the load willcontinue to be applied and the displacement of pile top willrapidly increase from the original 1289mm to 4068mm+is is because the first pile is an empty-bottom pile and thereis no soil at the bottom to generate tip resistance When theshaft skin resistance reaches the maximum value and cannotresist the axial force in the vertical direction if the loadcontinues to be applied after reaching the ultimate bearingcapacity sudden settlement of the pile body occurs and thesettlement exceeds the limit specified in the specificationindicating that pile failure has occurred

It can be seen from the stress conditions of the solid-bottom pile the empty-bottom pile the 10-meter-long pilebody and the 15-meter-long pile body in Figure 10 that theaxial force of the empty-bottom pile and the solid-bottompile is relatively small at the initial loading step With theincrease of loading step the axial force gradually develops

(a) (b)

Figure 6 Site photo of strain gauges embedding (a) Part photo (b) +e overall photo

Displacementmeter

Hydraulicjack

Test beam(s)

Test pile

Steel testplate

Load transfercolumn

Anchor pileMinimum clear

distance is 25m

(a) (b)

Figure 7 Schematic diagram and field photo of static load test (a) Schematic diagram (b) Photograph

0 1200 2400 3600 4800 6000 7200 8400 9600

Disp

lace

men

t (m

m)

Load at pile top (kN)

No 1No 2

No 3No 4

45

40

35

30

25

20

15

10

5

0

Figure 8 Load-displacement curve of static load test

Advances in Civil Engineering 7

Table 5 Results of static load test

Pile number Maximum load (kN) Maximum load settlement (mm) Bearing capacity underultimate load (kN)

Settlement underultimate load (mm) Rebound rate ()

No 1 4000 4068 3600 1289 1006No 2 5400 401 4800 3001 1236No 3 4800 4051 4200 2892 1684No 4 8800 4083 8000 2676 1894

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

lgt (min)

S (m

m)

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

(a)

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

lgt (min)

S (m

m)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

(b)

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

lgt (min)

S (m

m)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

(c)

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

S (m

m)

lgt (min)

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

(d)

Figure 9 S-lgt curve of each pile (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

8 Advances in Civil Engineering

and transmits to the pile bottom and decreases with theincrease of the depth Under different loading steps theattenuation rate of axial force curve is different +e at-tenuation rate of axial force curve reflects the development ofshaft skin resistance In low loading steps the attenuationrate of axial force curve is not fast and relatively uniformWith the increase of loading steps the attenuation rate ofaxial force curve at the upper part of pile body is not fastwhile the attenuation rate of axial force curve at the lowerpart of pile body is obvious which shows that no matter it isan empty-bottom pile or solid-bottom pile the force

transmission characteristics are similar With the increase ofloading steps the axial force gradually transmits to the lowerpart of the pile body If load continues to be applied afterreaching the ultimate load the empty-bottom pile will besuddenly destroyed while the solid-bottom pile will be able tobear due to soil in the lower part and will show a slow change

42 Analysis of Characteristics of Shaft Skin Resistance of PileBody Based on the principle of static balance regardless ofthe influence of pile body weight [24] the pile shaft skin

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

0 400 800 1200 1600 2000 2400 2800 3200 3600 4000 4400Axial force (kN)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

(a)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 600 1200 1800 2400 3000 3600 4200 4800 5400 6000Axial force (kN)

(b)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 600 1200 1800 2400 3000 3600 4200 4800 5400Axial force (kN)

(c)

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

16151413121110

9876543210

Dep

th Z

m

0 800 1600 2400 3200 4000 4800 5600 6400 7200 8000 8800Axial force (kN)

(d)

Figure 10 Distribution curve of axial force of each pile (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Advances in Civil Engineering 9

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

(a)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

(b)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

(c)

16151413121110

9876543210

Dep

th Z

m

0 20 40 60 80 100 120 140 160 180Skin friction of test pile (kPa)

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

(d)

Figure 11 Distribution of shaft skin resistance on each pile (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Table 6 Value of shaft skin resistance of piles at different depths when reaching ultimate bearing capacity

(a) No 1 pileDepth Zm 17 34 51 68 85Skin frictionkPa 60177 112974 133037 139158 135235

(b) No 2 pileDepth Zm 16 32 48 64 81 98Skin frictionkPa 43899 105055 128283 1391 146983 148015

(c) No 3 pileDepth Zm 16 32 48 64 81 98Skin frictionkPa 46948 106907 130723 145894 147938 150828

(d) No 4 pileDepth Zm 16 32 48 64 8 96 112 13 148Skin frictionkPa 63055 93009 117592 129804 141642 144953 145865 151216 154956

10 Advances in Civil Engineering

resistance value can be calculated by the following formulaaccording to the change value of the axial force of the pilebody

qsi Qi minus Qi+1

u middot li (5)

where qsi is the resistance between the i section and the i + 1section i is the same as in formula (1) u is the perimeter ofpile body li is the pile length between the i section and thei + 1 section

Figures 10(a)ndash10(d) show the distribution of shaft skinresistance of piles Nos 1 2 3 and 4 with depth underdifferent steps of loading

As can be seen from Figure 11 in lower loading step theshaft skin resistance of each pile is small with the increase ofloading steps the shaft skin resistance of each pile increasesOn the whole the shaft skin resistance of the pile is alsogradually transmitted from the upper part of the pile body tothe lower part of the pile body but compared with themiddle part of the pile body the shaft skin resistance of thelower part of the first pile body with empty-bottom shows a

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

10

8

6

4

2

0

Dep

th Z

m

(a)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

10

8

6

4

2

0

Dep

th Z

m

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

(b)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

10

8

6

4

2

0

Dep

th Z

m

(c)

Dep

th Z

m

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

151413121110

9876543210

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

(d)

Figure 12 Pile-soil relative displacement curve (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Advances in Civil Engineering 11

decreasing trend because there is no soil bearing the cor-responding load at the pile tip however for the remainingthree solid-bottom piles when the tip resistance is mobilizedto a certain extent the friction resistance in the middle partof the pile body is smaller than that in the lower part whichindicates that the soil at the pile tip can reinforce the shaftskin resistance of the pile [25] When the ultimate bearingcapacity is reached the shaft skin resistance values of thecorresponding piles at various depths are shown in Table 6

43 Relationship between Shaft Skin Resistance and Pile-SoilRelative Displacement +e pile-soil relative displacementobjectively reflects the mobilization of shaft skin resistance

According to the calculation method in [26] the pile-soilrelative displacement curves under different loading stepsare calculated and drawn +e curves of shaft skin resistanceand pile-soil relative displacement are shown inFigures 12(a)ndash12(d) respectively

In the calculation of the pile-soil relative displacement itis assumed that the soil around the pile will be not displacedAs can be seen from Figure 12 the pile-soil relative dis-placement at the same position of the pile body increases withthe increase of loading steps on the pile tipWhen the first twosteps of loading are applied the pile-soil relative displacementof the four piles increases obviously due to the compactioneffect of the first two steps of loading on the gravel soil aroundthe pile As the load level continues to increase the pile-soil

Skin

fric

tion

of te

st pi

le (k

Pa)

5 10 15 20 25 300Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

1-11-21-3

1-41-5

(a)Sk

in fr

ictio

n of

test

pile

(kPa

)

0 10 15 20 25 30 35 405Pile-soil relative displacement (mm)

2-12-22-3

2-42-52-6

0

20

40

60

80

100

120

140

160

(b)

Skin

fric

tion

of te

st pi

le (k

Pa)

5 10 15 20 25 30 35 400Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

3-13-23-3

3-43-53-6

(c)

Skin

fric

tion

of te

st pi

le (k

Pa)

0 10 15 20 25 30 35 405Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

4-14-24-3

4-44-54-6

4-74-84-9

(d)

Figure 13 Distribution curves of shaft skin resistances versus relative displacements (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4pile

12 Advances in Civil Engineering

relative displacement increment is relatively small because theshaft skin resistance gradually mobilizes its function toprevent the pile from moving With the increase of loadingwhen the shaft skin resistance cannot resist the load at theloading sections 4800 kNndash5400 kN 4200 kNndash4800 kN8000 kNndash8800 kN and 3600 kNndash4000 kN the maximumdisplacements of pile tips of Nos 2 3 and 4 solid-bottom pileand the empty-bottom pile increase by 9792mm 7939mm9881mm and 1497mm respectively

+e shaft skin resistance increases with the increase ofpile-soil relative displacement Although continuous loadinghas led to foundation soil destruction it can be concludedfrom Figure 13 that the lateral resistance in the middle andlower parts is not fully mobilized and a certain pile-soilrelative displacement is needed to promote the full mobi-lization of the shaft skin resistance However according tothe development trend that the slope of the shaft skin re-sistance gradually decreases it can be inferred that the shaftskin resistance will reach the maximum value At the firstsection position the ultimate shaft skin resistance of pilesNos 1 2 3 and 4 are 4695 kPa 4390 kPa 6018 kPa and6305 kPa respectively and the corresponding pile-soil rel-ative displacements are 3419mm 3442mm 2633mm and3279mm In the second section the shaft skin resistance ofthe four piles increases greatly so does the pile-soil relativedisplacement Since the soil above the first section position isbasically cultivated soil and it can resist small shaft skinresistance while the content of gravel soil of the secondsection is large and with the increase of depth the sur-rounding pressure of soil around the pile increases thereforethe ability of resisting pile foundation sliding and dis-placement increases+erefore between the first section andthe second section the shaft skin resistance increases greatlywhile the pile-soil relative displacement increases a little Inthe second section and beyond the soil is gravel soil and theshaft skin resistance increases slightly with the increase ofpile depth and the relative displacement decreases gradually

44 Pile Tip Resistance Varies with Load and DisplacementDuring the test the section where the largest embeddeddepth of strain gauges is very close to the bottom of the pilewas shown so the resistance at the pile tip is approximatelyequal to the axial force measured at this section namelyQb Q0 Under the action of all loading steps the resistancevariation curves at the lower tips of the Nos 2 3 and 4 pilesare shown in Figure 14

+e No 2 and No 3 piles in the first two loading stepsand No 4 pile in the first loading step have smaller resistanceat the pile tip +e resistance at the pile tip gradually in-creases until the third and fourth loading steps Under theaction of ultimate load the ratio of pile tip resistance to piletop load of Nos 2 3 and 4 is 377 3008 and 3992respectively all higher 15 which shows that pile tip re-sistance has been brought into play to a certain extent +epile tip resistance is shared by shaft skin resistance and piletip resistance and the pile is of tip bearing friction pile

According to the relationship curve between pile tipresistance and pile tip displacement shown in Figure 15 it

can be seen that the pile tip resistance and pile tip dis-placement present a double fold line model of work hard-ening +e Nos 2 3 and 4 piles all show a slow deformationfailure and under the action of the first loading step theslope of the curve is larger and the corresponding dis-placement is relatively small in latter steps of loading thecurvature curve becomes smoother the displacement of thepile tips of adjacent loads increases and the soil at the piletips will gradually be compacted

Pile

tip

load

(kN

)

1000 2000 3000 4000 5000 6000 7000 8000 90000Load at pile top (kN)

0250500750

100012501500175020002250250027503000325035003750

No 2No 3No 4

Figure 14 Variation curve of pile tip resistance under differentloading steps (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4pile

Pile

tip

load

(kN

)

5 10 15 20 25 30 35 40 450Pile tip displacement (mm)

0

1000

2000

3000

4000

5000

6000

No 2No 3No 4

Figure 15 Relationship curve between pile tip resistance and piletip displacement (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No4 pile

Advances in Civil Engineering 13

5 Conclusions

Analyses are carried out on data of the in-situ test of unevengravel soil in Baishi Mountain and the following conclu-sions are drawn

(1) Based on the analysis of the results of multichanneltransient surface wave test superheavy (N120) conedynamic sounding test and on-site screening testthe geological environment conditions of the unevengravel soil on the site are further explored +e stratawithin the range from 0 to 15m in Baizhi Mountaincan be divided into four layers ie cultivated soilsandy soil silty clay and gravel soil with depths of1m 2m 3m and 9m respectively

(2) +e axial force and shaft skin resistance of pile bodyin gravel soil are the same as those in other geologicalstructures At the initial loading step the axial forceand shaft skin resistance of pile body play a smallerrole With the increase of loading steps the axialforce and shaft skin resistance of pile body increasecorrespondingly Under the same loading step theaxial force of the pile body and the shaft skin re-sistance do not mobilize in the same time andgradually transfer from the upper part to the lowerpart of the pile body and the upper part of the pilebody reaches the limit value before the middle andlower parts As for the solid-bottom pile the soil atthe pile tip will strengthen the shaft skin resistance ofthe pile

(3) According to this experimental study it is suggestedthat similar projects should adopt concrete manuallybored cast-in-place piles with a diameter of 900mmand a length of 15m the wall-protecting thicknessshall not be less than 175mm and the grade ofconcrete cannot be lower than C35 +e pile end canbe widened or not and the pile bottom shall be freeof sediment

Data Availability

All the data in this paper are obtained from tests in thisstudy and no other data were used to support this study

Conflicts of Interest

+e authors declare that with regard to the publication ofthis paper and the funding for publishing it there are noconflicts of interest

Acknowledgments

+is study was partially supported by the National NaturalScience Foundation of China (Grant no 41302223) ChongqingNo 3 colleges and universities youth backbone teachers fundingplans Chongqing Research Program of Basic Research andFrontier Technology (Grant no cstc2016jcyjA0074cstc2016jcyjA0933 and cstc2015jcyjA90012) Scientific andTechnological Research Program of Chongqing Municipal

Education Commission (Grant no KJ1713327 and KJ1600532)Key Laboratory ofWell Stability and Fluid and RockMechanicsin Oil and Gas Reservoir of Shaanxi Province Xirsquoan ShiyouUniversity (Grant no FRM20190201002) Chongqing Post-graduate Research Innovation Project (Grant no CYS19353)and Chongqing University of Science and Technology GraduateStudentsrsquo Science and Technology Innovation Program (Grantno YKJCX1720601 and YKJCX1920613)

References

[1] F Kirsch and B Plabmann ldquoDynamic methods in pile testingdevelopments in measurement and analysisrdquo in Proceedings ofthe International Deep Foundations Congress 2002 pp 868ndash882 ASCE Orlando FL USA February 2002

[2] S H Ni L Lehmann J J Charng et al ldquoLow-strain integritytesting of drilled piles with high slenderness ratiordquo Computersand Geotechnics vol 33 no 6-7 pp 283ndash293 2006

[3] K Mori A Spagnoli Y Murakami G Kondo and I TorigoeldquoA new non-contacting non-destructive testing method fordefect detection in concreterdquo NDT amp E International vol 35no 6 pp 399ndash406 2002

[4] G Cai S Liu L Tong et al ldquoAssessment of direct CPT andCPTUmethods for predicting the ultimate bearing capacity ofsingle pilesrdquo Engineering Geology vol 104 no 3-4pp 211ndash222 2009

[5] X Y Bai M Y Zhang L Zhu et al ldquoIn-situ test and finiteelement analysis of bearing characteristics of rock-socketedshort piles in strongly weathered graniterdquo Journal of CentralSouth University Natural Science Edition vol 48 no 2pp 512ndash524 2017 in Chinese

[6] E C Leong and M F Randolph ldquoFinite element modelling ofrock-socketed pilesrdquo International Journal for Numerical andAnalytical Methods in Geomechanics vol 18 no 1 pp 25ndash471994

[7] G Gao M Gao Q Chen and J Yang ldquoField load testingstudy of vertical bearing behavior of a large diameter belledcast-in-place pilerdquo KSCE Journal of Civil Engineering vol 23no 5 pp 2009ndash2016 2019

[8] R Berardi and R Bovolenta ldquoPile-settlement evaluation usingfield stiffness non-linearityrdquo Proceedings of the Institution ofCivil Engineers - Geotechnical Engineering vol 158 no 1pp 35ndash44 2005

[9] B H Fellenius Basics of Foundation Design Lulu PressMorrisville NC USA 2016

[10] C Lam and S A Jefferis ldquoCritical assessment of pile modulusdetermination methodsrdquo Canadian Geotechnical Journalvol 48 no 10 pp 1433ndash1448 2011

[11] A Mohammadi T Ebadi and M R Boroomand ldquoPhysicalmodelling of axial compressive bearing capacity of instru-mented piles in oil-contaminated sandy soilrdquo Iranian Journalof Science and Technology Transactions of Civil Engineeringvol 44 no 2 pp 695ndash714 2019

[12] W D Guo and E H Ghee ldquoBehavior of axially loaded pilegroups subjected to lateral soil movementrdquo in Proceedings ofthe GeoShanghai International Conference 2006 pp 174ndash181ASCE Shanghai China June 2006

[13] Z Zhou Y Dong P Jiang D Han and T Liu ldquoCalculation ofpile side friction by multiparameter statistical analysisrdquo Ad-vances in Civil Engineering vol 2019 Article ID 263852012 pages 2019

14 Advances in Civil Engineering

[14] N M Hai and B H Fellenius ldquoO-Cell tests on two 70 m longbored piles in Vietnamrdquo in Proceedings of the Geo-Congress2014 pp 482ndash496 ASCE Atlanta Georgia February 2014

[15] S Gong G Cai S Liu et al ldquoNumerical Simulation of BearingCapacity and Consolidation Characteristics of PHC PileFoundationrdquo GeoShanghai International Conferencepp 178ndash185 Springer Singapore 2018

[16] M A Bashir H Furuuchi T Ueda and M Nauman BashirldquoNumerical simulation of axial anchorage capacity of con-crete-filled steel box footingrdquo Iranian Journal of Science andTechnology Transactions of Civil Engineering vol 40 no 3pp 257ndash262 2016

[17] N A Haskell ldquo+e dispersion of surface waves on multi-layered mediardquo Bulletin of the Seismological Society ofAmerica vol 43 no 1 pp 17ndash34 1953

[18] China Architecture amp Building PressNational Standard of thePeoplersquos Republic of China Code for Investigation of Geo-technical Engineering (GB50021 2016) China Architecture ampBuilding Press Beijing China 2016 in Chinese

[19] China Architecture amp Building Press Industry Standard of thePeoplersquos Republic of China Technical Code for Testing ofBuilding Foundation Piles (JGJ106-2014) China Architectureamp Building Press Beijing China 2014 in Chinese

[20] N Guo Z H Chen X F Huang et al ldquoResearch on the anti-pulling test of large-diameter cloth bag pile in soft rockfoundationrdquo Rock and Soil Mechanics vol 36 no 2pp 603ndash609 2015 in Chinese

[21] Y H Li X Zhu and T H Zhou ldquoField comparative ex-perimental study on the influence of post-tip grouting onlarge-diameter cast-in-situ pilesrdquo Rock and Soil Mechanicsvol 37 no 2 pp 388ndash396 2016 in Chinese

[22] G F Xin Z M Zhang T Xia et al ldquoExperimental study onbearing behavior of ultra-long pile under high loadrdquo ChineseJournal of Rock Mechanics and Engineering vol 13pp 2397ndash2402 2005 in Chinese

[23] Z Ling W Wang J Wu et al ldquoFull-scale loading test on pre-bored precast pile with enlarged base in Shanghairdquo in Pro-ceedings of the GeoShanghai 2018 International ConferenceAdvances in Soil Dynamics and Foundation Engineeringpp 637ndash645 Springer Singapore 2018

[24] J R Dong S T Lin and Y M Dai ldquo+e load tranfer behaviorof large diameter cast-in-situ pile in crushed pebble stratumrdquoChinese Journal of Geotechnical Engineering vol 16 no 6pp 123ndash131 1994

[25] X F Huang J H Zhang L G Ma et al ldquoExperimental studyon reinforcement effect of lateral resistance of collapsible loessrock-socketed perfusion pilerdquo Architectural Structure vol 41no 2 pp 351ndash355 2011 in Chinese

[26] J-J Zhou K-H Wang X-N Gong and R-H ZhangldquoBearing capacity and load transfer mechanism of a static drillrooted nodular pile in soft soil areasrdquo Journal of ZhejiangUniversity Science A vol 14 no 10 pp 705ndash719 2013

Advances in Civil Engineering 15

Table 5 Results of static load test

Pile number Maximum load (kN) Maximum load settlement (mm) Bearing capacity underultimate load (kN)

Settlement underultimate load (mm) Rebound rate ()

No 1 4000 4068 3600 1289 1006No 2 5400 401 4800 3001 1236No 3 4800 4051 4200 2892 1684No 4 8800 4083 8000 2676 1894

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

lgt (min)

S (m

m)

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

(a)

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

lgt (min)

S (m

m)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

(b)

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

lgt (min)

S (m

m)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

(c)

1 10 100 1000

45

40

35

30

25

20

15

10

5

0

S (m

m)

lgt (min)

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

(d)

Figure 9 S-lgt curve of each pile (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

8 Advances in Civil Engineering

and transmits to the pile bottom and decreases with theincrease of the depth Under different loading steps theattenuation rate of axial force curve is different +e at-tenuation rate of axial force curve reflects the development ofshaft skin resistance In low loading steps the attenuationrate of axial force curve is not fast and relatively uniformWith the increase of loading steps the attenuation rate ofaxial force curve at the upper part of pile body is not fastwhile the attenuation rate of axial force curve at the lowerpart of pile body is obvious which shows that no matter it isan empty-bottom pile or solid-bottom pile the force

transmission characteristics are similar With the increase ofloading steps the axial force gradually transmits to the lowerpart of the pile body If load continues to be applied afterreaching the ultimate load the empty-bottom pile will besuddenly destroyed while the solid-bottom pile will be able tobear due to soil in the lower part and will show a slow change

42 Analysis of Characteristics of Shaft Skin Resistance of PileBody Based on the principle of static balance regardless ofthe influence of pile body weight [24] the pile shaft skin

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

0 400 800 1200 1600 2000 2400 2800 3200 3600 4000 4400Axial force (kN)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

(a)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 600 1200 1800 2400 3000 3600 4200 4800 5400 6000Axial force (kN)

(b)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 600 1200 1800 2400 3000 3600 4200 4800 5400Axial force (kN)

(c)

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

16151413121110

9876543210

Dep

th Z

m

0 800 1600 2400 3200 4000 4800 5600 6400 7200 8000 8800Axial force (kN)

(d)

Figure 10 Distribution curve of axial force of each pile (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Advances in Civil Engineering 9

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

(a)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

(b)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

(c)

16151413121110

9876543210

Dep

th Z

m

0 20 40 60 80 100 120 140 160 180Skin friction of test pile (kPa)

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

(d)

Figure 11 Distribution of shaft skin resistance on each pile (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Table 6 Value of shaft skin resistance of piles at different depths when reaching ultimate bearing capacity

(a) No 1 pileDepth Zm 17 34 51 68 85Skin frictionkPa 60177 112974 133037 139158 135235

(b) No 2 pileDepth Zm 16 32 48 64 81 98Skin frictionkPa 43899 105055 128283 1391 146983 148015

(c) No 3 pileDepth Zm 16 32 48 64 81 98Skin frictionkPa 46948 106907 130723 145894 147938 150828

(d) No 4 pileDepth Zm 16 32 48 64 8 96 112 13 148Skin frictionkPa 63055 93009 117592 129804 141642 144953 145865 151216 154956

10 Advances in Civil Engineering

resistance value can be calculated by the following formulaaccording to the change value of the axial force of the pilebody

qsi Qi minus Qi+1

u middot li (5)

where qsi is the resistance between the i section and the i + 1section i is the same as in formula (1) u is the perimeter ofpile body li is the pile length between the i section and thei + 1 section

Figures 10(a)ndash10(d) show the distribution of shaft skinresistance of piles Nos 1 2 3 and 4 with depth underdifferent steps of loading

As can be seen from Figure 11 in lower loading step theshaft skin resistance of each pile is small with the increase ofloading steps the shaft skin resistance of each pile increasesOn the whole the shaft skin resistance of the pile is alsogradually transmitted from the upper part of the pile body tothe lower part of the pile body but compared with themiddle part of the pile body the shaft skin resistance of thelower part of the first pile body with empty-bottom shows a

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

10

8

6

4

2

0

Dep

th Z

m

(a)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

10

8

6

4

2

0

Dep

th Z

m

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

(b)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

10

8

6

4

2

0

Dep

th Z

m

(c)

Dep

th Z

m

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

151413121110

9876543210

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

(d)

Figure 12 Pile-soil relative displacement curve (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Advances in Civil Engineering 11

decreasing trend because there is no soil bearing the cor-responding load at the pile tip however for the remainingthree solid-bottom piles when the tip resistance is mobilizedto a certain extent the friction resistance in the middle partof the pile body is smaller than that in the lower part whichindicates that the soil at the pile tip can reinforce the shaftskin resistance of the pile [25] When the ultimate bearingcapacity is reached the shaft skin resistance values of thecorresponding piles at various depths are shown in Table 6

43 Relationship between Shaft Skin Resistance and Pile-SoilRelative Displacement +e pile-soil relative displacementobjectively reflects the mobilization of shaft skin resistance

According to the calculation method in [26] the pile-soilrelative displacement curves under different loading stepsare calculated and drawn +e curves of shaft skin resistanceand pile-soil relative displacement are shown inFigures 12(a)ndash12(d) respectively

In the calculation of the pile-soil relative displacement itis assumed that the soil around the pile will be not displacedAs can be seen from Figure 12 the pile-soil relative dis-placement at the same position of the pile body increases withthe increase of loading steps on the pile tipWhen the first twosteps of loading are applied the pile-soil relative displacementof the four piles increases obviously due to the compactioneffect of the first two steps of loading on the gravel soil aroundthe pile As the load level continues to increase the pile-soil

Skin

fric

tion

of te

st pi

le (k

Pa)

5 10 15 20 25 300Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

1-11-21-3

1-41-5

(a)Sk

in fr

ictio

n of

test

pile

(kPa

)

0 10 15 20 25 30 35 405Pile-soil relative displacement (mm)

2-12-22-3

2-42-52-6

0

20

40

60

80

100

120

140

160

(b)

Skin

fric

tion

of te

st pi

le (k

Pa)

5 10 15 20 25 30 35 400Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

3-13-23-3

3-43-53-6

(c)

Skin

fric

tion

of te

st pi

le (k

Pa)

0 10 15 20 25 30 35 405Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

4-14-24-3

4-44-54-6

4-74-84-9

(d)

Figure 13 Distribution curves of shaft skin resistances versus relative displacements (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4pile

12 Advances in Civil Engineering

relative displacement increment is relatively small because theshaft skin resistance gradually mobilizes its function toprevent the pile from moving With the increase of loadingwhen the shaft skin resistance cannot resist the load at theloading sections 4800 kNndash5400 kN 4200 kNndash4800 kN8000 kNndash8800 kN and 3600 kNndash4000 kN the maximumdisplacements of pile tips of Nos 2 3 and 4 solid-bottom pileand the empty-bottom pile increase by 9792mm 7939mm9881mm and 1497mm respectively

+e shaft skin resistance increases with the increase ofpile-soil relative displacement Although continuous loadinghas led to foundation soil destruction it can be concludedfrom Figure 13 that the lateral resistance in the middle andlower parts is not fully mobilized and a certain pile-soilrelative displacement is needed to promote the full mobi-lization of the shaft skin resistance However according tothe development trend that the slope of the shaft skin re-sistance gradually decreases it can be inferred that the shaftskin resistance will reach the maximum value At the firstsection position the ultimate shaft skin resistance of pilesNos 1 2 3 and 4 are 4695 kPa 4390 kPa 6018 kPa and6305 kPa respectively and the corresponding pile-soil rel-ative displacements are 3419mm 3442mm 2633mm and3279mm In the second section the shaft skin resistance ofthe four piles increases greatly so does the pile-soil relativedisplacement Since the soil above the first section position isbasically cultivated soil and it can resist small shaft skinresistance while the content of gravel soil of the secondsection is large and with the increase of depth the sur-rounding pressure of soil around the pile increases thereforethe ability of resisting pile foundation sliding and dis-placement increases+erefore between the first section andthe second section the shaft skin resistance increases greatlywhile the pile-soil relative displacement increases a little Inthe second section and beyond the soil is gravel soil and theshaft skin resistance increases slightly with the increase ofpile depth and the relative displacement decreases gradually

44 Pile Tip Resistance Varies with Load and DisplacementDuring the test the section where the largest embeddeddepth of strain gauges is very close to the bottom of the pilewas shown so the resistance at the pile tip is approximatelyequal to the axial force measured at this section namelyQb Q0 Under the action of all loading steps the resistancevariation curves at the lower tips of the Nos 2 3 and 4 pilesare shown in Figure 14

+e No 2 and No 3 piles in the first two loading stepsand No 4 pile in the first loading step have smaller resistanceat the pile tip +e resistance at the pile tip gradually in-creases until the third and fourth loading steps Under theaction of ultimate load the ratio of pile tip resistance to piletop load of Nos 2 3 and 4 is 377 3008 and 3992respectively all higher 15 which shows that pile tip re-sistance has been brought into play to a certain extent +epile tip resistance is shared by shaft skin resistance and piletip resistance and the pile is of tip bearing friction pile

According to the relationship curve between pile tipresistance and pile tip displacement shown in Figure 15 it

can be seen that the pile tip resistance and pile tip dis-placement present a double fold line model of work hard-ening +e Nos 2 3 and 4 piles all show a slow deformationfailure and under the action of the first loading step theslope of the curve is larger and the corresponding dis-placement is relatively small in latter steps of loading thecurvature curve becomes smoother the displacement of thepile tips of adjacent loads increases and the soil at the piletips will gradually be compacted

Pile

tip

load

(kN

)

1000 2000 3000 4000 5000 6000 7000 8000 90000Load at pile top (kN)

0250500750

100012501500175020002250250027503000325035003750

No 2No 3No 4

Figure 14 Variation curve of pile tip resistance under differentloading steps (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4pile

Pile

tip

load

(kN

)

5 10 15 20 25 30 35 40 450Pile tip displacement (mm)

0

1000

2000

3000

4000

5000

6000

No 2No 3No 4

Figure 15 Relationship curve between pile tip resistance and piletip displacement (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No4 pile

Advances in Civil Engineering 13

5 Conclusions

Analyses are carried out on data of the in-situ test of unevengravel soil in Baishi Mountain and the following conclu-sions are drawn

(1) Based on the analysis of the results of multichanneltransient surface wave test superheavy (N120) conedynamic sounding test and on-site screening testthe geological environment conditions of the unevengravel soil on the site are further explored +e stratawithin the range from 0 to 15m in Baizhi Mountaincan be divided into four layers ie cultivated soilsandy soil silty clay and gravel soil with depths of1m 2m 3m and 9m respectively

(2) +e axial force and shaft skin resistance of pile bodyin gravel soil are the same as those in other geologicalstructures At the initial loading step the axial forceand shaft skin resistance of pile body play a smallerrole With the increase of loading steps the axialforce and shaft skin resistance of pile body increasecorrespondingly Under the same loading step theaxial force of the pile body and the shaft skin re-sistance do not mobilize in the same time andgradually transfer from the upper part to the lowerpart of the pile body and the upper part of the pilebody reaches the limit value before the middle andlower parts As for the solid-bottom pile the soil atthe pile tip will strengthen the shaft skin resistance ofthe pile

(3) According to this experimental study it is suggestedthat similar projects should adopt concrete manuallybored cast-in-place piles with a diameter of 900mmand a length of 15m the wall-protecting thicknessshall not be less than 175mm and the grade ofconcrete cannot be lower than C35 +e pile end canbe widened or not and the pile bottom shall be freeof sediment

Data Availability

All the data in this paper are obtained from tests in thisstudy and no other data were used to support this study

Conflicts of Interest

+e authors declare that with regard to the publication ofthis paper and the funding for publishing it there are noconflicts of interest

Acknowledgments

+is study was partially supported by the National NaturalScience Foundation of China (Grant no 41302223) ChongqingNo 3 colleges and universities youth backbone teachers fundingplans Chongqing Research Program of Basic Research andFrontier Technology (Grant no cstc2016jcyjA0074cstc2016jcyjA0933 and cstc2015jcyjA90012) Scientific andTechnological Research Program of Chongqing Municipal

Education Commission (Grant no KJ1713327 and KJ1600532)Key Laboratory ofWell Stability and Fluid and RockMechanicsin Oil and Gas Reservoir of Shaanxi Province Xirsquoan ShiyouUniversity (Grant no FRM20190201002) Chongqing Post-graduate Research Innovation Project (Grant no CYS19353)and Chongqing University of Science and Technology GraduateStudentsrsquo Science and Technology Innovation Program (Grantno YKJCX1720601 and YKJCX1920613)

References

[1] F Kirsch and B Plabmann ldquoDynamic methods in pile testingdevelopments in measurement and analysisrdquo in Proceedings ofthe International Deep Foundations Congress 2002 pp 868ndash882 ASCE Orlando FL USA February 2002

[2] S H Ni L Lehmann J J Charng et al ldquoLow-strain integritytesting of drilled piles with high slenderness ratiordquo Computersand Geotechnics vol 33 no 6-7 pp 283ndash293 2006

[3] K Mori A Spagnoli Y Murakami G Kondo and I TorigoeldquoA new non-contacting non-destructive testing method fordefect detection in concreterdquo NDT amp E International vol 35no 6 pp 399ndash406 2002

[4] G Cai S Liu L Tong et al ldquoAssessment of direct CPT andCPTUmethods for predicting the ultimate bearing capacity ofsingle pilesrdquo Engineering Geology vol 104 no 3-4pp 211ndash222 2009

[5] X Y Bai M Y Zhang L Zhu et al ldquoIn-situ test and finiteelement analysis of bearing characteristics of rock-socketedshort piles in strongly weathered graniterdquo Journal of CentralSouth University Natural Science Edition vol 48 no 2pp 512ndash524 2017 in Chinese

[6] E C Leong and M F Randolph ldquoFinite element modelling ofrock-socketed pilesrdquo International Journal for Numerical andAnalytical Methods in Geomechanics vol 18 no 1 pp 25ndash471994

[7] G Gao M Gao Q Chen and J Yang ldquoField load testingstudy of vertical bearing behavior of a large diameter belledcast-in-place pilerdquo KSCE Journal of Civil Engineering vol 23no 5 pp 2009ndash2016 2019

[8] R Berardi and R Bovolenta ldquoPile-settlement evaluation usingfield stiffness non-linearityrdquo Proceedings of the Institution ofCivil Engineers - Geotechnical Engineering vol 158 no 1pp 35ndash44 2005

[9] B H Fellenius Basics of Foundation Design Lulu PressMorrisville NC USA 2016

[10] C Lam and S A Jefferis ldquoCritical assessment of pile modulusdetermination methodsrdquo Canadian Geotechnical Journalvol 48 no 10 pp 1433ndash1448 2011

[11] A Mohammadi T Ebadi and M R Boroomand ldquoPhysicalmodelling of axial compressive bearing capacity of instru-mented piles in oil-contaminated sandy soilrdquo Iranian Journalof Science and Technology Transactions of Civil Engineeringvol 44 no 2 pp 695ndash714 2019

[12] W D Guo and E H Ghee ldquoBehavior of axially loaded pilegroups subjected to lateral soil movementrdquo in Proceedings ofthe GeoShanghai International Conference 2006 pp 174ndash181ASCE Shanghai China June 2006

[13] Z Zhou Y Dong P Jiang D Han and T Liu ldquoCalculation ofpile side friction by multiparameter statistical analysisrdquo Ad-vances in Civil Engineering vol 2019 Article ID 263852012 pages 2019

14 Advances in Civil Engineering

[14] N M Hai and B H Fellenius ldquoO-Cell tests on two 70 m longbored piles in Vietnamrdquo in Proceedings of the Geo-Congress2014 pp 482ndash496 ASCE Atlanta Georgia February 2014

[15] S Gong G Cai S Liu et al ldquoNumerical Simulation of BearingCapacity and Consolidation Characteristics of PHC PileFoundationrdquo GeoShanghai International Conferencepp 178ndash185 Springer Singapore 2018

[16] M A Bashir H Furuuchi T Ueda and M Nauman BashirldquoNumerical simulation of axial anchorage capacity of con-crete-filled steel box footingrdquo Iranian Journal of Science andTechnology Transactions of Civil Engineering vol 40 no 3pp 257ndash262 2016

[17] N A Haskell ldquo+e dispersion of surface waves on multi-layered mediardquo Bulletin of the Seismological Society ofAmerica vol 43 no 1 pp 17ndash34 1953

[18] China Architecture amp Building PressNational Standard of thePeoplersquos Republic of China Code for Investigation of Geo-technical Engineering (GB50021 2016) China Architecture ampBuilding Press Beijing China 2016 in Chinese

[19] China Architecture amp Building Press Industry Standard of thePeoplersquos Republic of China Technical Code for Testing ofBuilding Foundation Piles (JGJ106-2014) China Architectureamp Building Press Beijing China 2014 in Chinese

[20] N Guo Z H Chen X F Huang et al ldquoResearch on the anti-pulling test of large-diameter cloth bag pile in soft rockfoundationrdquo Rock and Soil Mechanics vol 36 no 2pp 603ndash609 2015 in Chinese

[21] Y H Li X Zhu and T H Zhou ldquoField comparative ex-perimental study on the influence of post-tip grouting onlarge-diameter cast-in-situ pilesrdquo Rock and Soil Mechanicsvol 37 no 2 pp 388ndash396 2016 in Chinese

[22] G F Xin Z M Zhang T Xia et al ldquoExperimental study onbearing behavior of ultra-long pile under high loadrdquo ChineseJournal of Rock Mechanics and Engineering vol 13pp 2397ndash2402 2005 in Chinese

[23] Z Ling W Wang J Wu et al ldquoFull-scale loading test on pre-bored precast pile with enlarged base in Shanghairdquo in Pro-ceedings of the GeoShanghai 2018 International ConferenceAdvances in Soil Dynamics and Foundation Engineeringpp 637ndash645 Springer Singapore 2018

[24] J R Dong S T Lin and Y M Dai ldquo+e load tranfer behaviorof large diameter cast-in-situ pile in crushed pebble stratumrdquoChinese Journal of Geotechnical Engineering vol 16 no 6pp 123ndash131 1994

[25] X F Huang J H Zhang L G Ma et al ldquoExperimental studyon reinforcement effect of lateral resistance of collapsible loessrock-socketed perfusion pilerdquo Architectural Structure vol 41no 2 pp 351ndash355 2011 in Chinese

[26] J-J Zhou K-H Wang X-N Gong and R-H ZhangldquoBearing capacity and load transfer mechanism of a static drillrooted nodular pile in soft soil areasrdquo Journal of ZhejiangUniversity Science A vol 14 no 10 pp 705ndash719 2013

Advances in Civil Engineering 15

and transmits to the pile bottom and decreases with theincrease of the depth Under different loading steps theattenuation rate of axial force curve is different +e at-tenuation rate of axial force curve reflects the development ofshaft skin resistance In low loading steps the attenuationrate of axial force curve is not fast and relatively uniformWith the increase of loading steps the attenuation rate ofaxial force curve at the upper part of pile body is not fastwhile the attenuation rate of axial force curve at the lowerpart of pile body is obvious which shows that no matter it isan empty-bottom pile or solid-bottom pile the force

transmission characteristics are similar With the increase ofloading steps the axial force gradually transmits to the lowerpart of the pile body If load continues to be applied afterreaching the ultimate load the empty-bottom pile will besuddenly destroyed while the solid-bottom pile will be able tobear due to soil in the lower part and will show a slow change

42 Analysis of Characteristics of Shaft Skin Resistance of PileBody Based on the principle of static balance regardless ofthe influence of pile body weight [24] the pile shaft skin

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

0 400 800 1200 1600 2000 2400 2800 3200 3600 4000 4400Axial force (kN)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

(a)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 600 1200 1800 2400 3000 3600 4200 4800 5400 6000Axial force (kN)

(b)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 600 1200 1800 2400 3000 3600 4200 4800 5400Axial force (kN)

(c)

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

16151413121110

9876543210

Dep

th Z

m

0 800 1600 2400 3200 4000 4800 5600 6400 7200 8000 8800Axial force (kN)

(d)

Figure 10 Distribution curve of axial force of each pile (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Advances in Civil Engineering 9

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

(a)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

(b)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

(c)

16151413121110

9876543210

Dep

th Z

m

0 20 40 60 80 100 120 140 160 180Skin friction of test pile (kPa)

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

(d)

Figure 11 Distribution of shaft skin resistance on each pile (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Table 6 Value of shaft skin resistance of piles at different depths when reaching ultimate bearing capacity

(a) No 1 pileDepth Zm 17 34 51 68 85Skin frictionkPa 60177 112974 133037 139158 135235

(b) No 2 pileDepth Zm 16 32 48 64 81 98Skin frictionkPa 43899 105055 128283 1391 146983 148015

(c) No 3 pileDepth Zm 16 32 48 64 81 98Skin frictionkPa 46948 106907 130723 145894 147938 150828

(d) No 4 pileDepth Zm 16 32 48 64 8 96 112 13 148Skin frictionkPa 63055 93009 117592 129804 141642 144953 145865 151216 154956

10 Advances in Civil Engineering

resistance value can be calculated by the following formulaaccording to the change value of the axial force of the pilebody

qsi Qi minus Qi+1

u middot li (5)

where qsi is the resistance between the i section and the i + 1section i is the same as in formula (1) u is the perimeter ofpile body li is the pile length between the i section and thei + 1 section

Figures 10(a)ndash10(d) show the distribution of shaft skinresistance of piles Nos 1 2 3 and 4 with depth underdifferent steps of loading

As can be seen from Figure 11 in lower loading step theshaft skin resistance of each pile is small with the increase ofloading steps the shaft skin resistance of each pile increasesOn the whole the shaft skin resistance of the pile is alsogradually transmitted from the upper part of the pile body tothe lower part of the pile body but compared with themiddle part of the pile body the shaft skin resistance of thelower part of the first pile body with empty-bottom shows a

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

10

8

6

4

2

0

Dep

th Z

m

(a)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

10

8

6

4

2

0

Dep

th Z

m

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

(b)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

10

8

6

4

2

0

Dep

th Z

m

(c)

Dep

th Z

m

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

151413121110

9876543210

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

(d)

Figure 12 Pile-soil relative displacement curve (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Advances in Civil Engineering 11

decreasing trend because there is no soil bearing the cor-responding load at the pile tip however for the remainingthree solid-bottom piles when the tip resistance is mobilizedto a certain extent the friction resistance in the middle partof the pile body is smaller than that in the lower part whichindicates that the soil at the pile tip can reinforce the shaftskin resistance of the pile [25] When the ultimate bearingcapacity is reached the shaft skin resistance values of thecorresponding piles at various depths are shown in Table 6

43 Relationship between Shaft Skin Resistance and Pile-SoilRelative Displacement +e pile-soil relative displacementobjectively reflects the mobilization of shaft skin resistance

According to the calculation method in [26] the pile-soilrelative displacement curves under different loading stepsare calculated and drawn +e curves of shaft skin resistanceand pile-soil relative displacement are shown inFigures 12(a)ndash12(d) respectively

In the calculation of the pile-soil relative displacement itis assumed that the soil around the pile will be not displacedAs can be seen from Figure 12 the pile-soil relative dis-placement at the same position of the pile body increases withthe increase of loading steps on the pile tipWhen the first twosteps of loading are applied the pile-soil relative displacementof the four piles increases obviously due to the compactioneffect of the first two steps of loading on the gravel soil aroundthe pile As the load level continues to increase the pile-soil

Skin

fric

tion

of te

st pi

le (k

Pa)

5 10 15 20 25 300Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

1-11-21-3

1-41-5

(a)Sk

in fr

ictio

n of

test

pile

(kPa

)

0 10 15 20 25 30 35 405Pile-soil relative displacement (mm)

2-12-22-3

2-42-52-6

0

20

40

60

80

100

120

140

160

(b)

Skin

fric

tion

of te

st pi

le (k

Pa)

5 10 15 20 25 30 35 400Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

3-13-23-3

3-43-53-6

(c)

Skin

fric

tion

of te

st pi

le (k

Pa)

0 10 15 20 25 30 35 405Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

4-14-24-3

4-44-54-6

4-74-84-9

(d)

Figure 13 Distribution curves of shaft skin resistances versus relative displacements (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4pile

12 Advances in Civil Engineering

relative displacement increment is relatively small because theshaft skin resistance gradually mobilizes its function toprevent the pile from moving With the increase of loadingwhen the shaft skin resistance cannot resist the load at theloading sections 4800 kNndash5400 kN 4200 kNndash4800 kN8000 kNndash8800 kN and 3600 kNndash4000 kN the maximumdisplacements of pile tips of Nos 2 3 and 4 solid-bottom pileand the empty-bottom pile increase by 9792mm 7939mm9881mm and 1497mm respectively

+e shaft skin resistance increases with the increase ofpile-soil relative displacement Although continuous loadinghas led to foundation soil destruction it can be concludedfrom Figure 13 that the lateral resistance in the middle andlower parts is not fully mobilized and a certain pile-soilrelative displacement is needed to promote the full mobi-lization of the shaft skin resistance However according tothe development trend that the slope of the shaft skin re-sistance gradually decreases it can be inferred that the shaftskin resistance will reach the maximum value At the firstsection position the ultimate shaft skin resistance of pilesNos 1 2 3 and 4 are 4695 kPa 4390 kPa 6018 kPa and6305 kPa respectively and the corresponding pile-soil rel-ative displacements are 3419mm 3442mm 2633mm and3279mm In the second section the shaft skin resistance ofthe four piles increases greatly so does the pile-soil relativedisplacement Since the soil above the first section position isbasically cultivated soil and it can resist small shaft skinresistance while the content of gravel soil of the secondsection is large and with the increase of depth the sur-rounding pressure of soil around the pile increases thereforethe ability of resisting pile foundation sliding and dis-placement increases+erefore between the first section andthe second section the shaft skin resistance increases greatlywhile the pile-soil relative displacement increases a little Inthe second section and beyond the soil is gravel soil and theshaft skin resistance increases slightly with the increase ofpile depth and the relative displacement decreases gradually

44 Pile Tip Resistance Varies with Load and DisplacementDuring the test the section where the largest embeddeddepth of strain gauges is very close to the bottom of the pilewas shown so the resistance at the pile tip is approximatelyequal to the axial force measured at this section namelyQb Q0 Under the action of all loading steps the resistancevariation curves at the lower tips of the Nos 2 3 and 4 pilesare shown in Figure 14

+e No 2 and No 3 piles in the first two loading stepsand No 4 pile in the first loading step have smaller resistanceat the pile tip +e resistance at the pile tip gradually in-creases until the third and fourth loading steps Under theaction of ultimate load the ratio of pile tip resistance to piletop load of Nos 2 3 and 4 is 377 3008 and 3992respectively all higher 15 which shows that pile tip re-sistance has been brought into play to a certain extent +epile tip resistance is shared by shaft skin resistance and piletip resistance and the pile is of tip bearing friction pile

According to the relationship curve between pile tipresistance and pile tip displacement shown in Figure 15 it

can be seen that the pile tip resistance and pile tip dis-placement present a double fold line model of work hard-ening +e Nos 2 3 and 4 piles all show a slow deformationfailure and under the action of the first loading step theslope of the curve is larger and the corresponding dis-placement is relatively small in latter steps of loading thecurvature curve becomes smoother the displacement of thepile tips of adjacent loads increases and the soil at the piletips will gradually be compacted

Pile

tip

load

(kN

)

1000 2000 3000 4000 5000 6000 7000 8000 90000Load at pile top (kN)

0250500750

100012501500175020002250250027503000325035003750

No 2No 3No 4

Figure 14 Variation curve of pile tip resistance under differentloading steps (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4pile

Pile

tip

load

(kN

)

5 10 15 20 25 30 35 40 450Pile tip displacement (mm)

0

1000

2000

3000

4000

5000

6000

No 2No 3No 4

Figure 15 Relationship curve between pile tip resistance and piletip displacement (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No4 pile

Advances in Civil Engineering 13

5 Conclusions

Analyses are carried out on data of the in-situ test of unevengravel soil in Baishi Mountain and the following conclu-sions are drawn

(1) Based on the analysis of the results of multichanneltransient surface wave test superheavy (N120) conedynamic sounding test and on-site screening testthe geological environment conditions of the unevengravel soil on the site are further explored +e stratawithin the range from 0 to 15m in Baizhi Mountaincan be divided into four layers ie cultivated soilsandy soil silty clay and gravel soil with depths of1m 2m 3m and 9m respectively

(2) +e axial force and shaft skin resistance of pile bodyin gravel soil are the same as those in other geologicalstructures At the initial loading step the axial forceand shaft skin resistance of pile body play a smallerrole With the increase of loading steps the axialforce and shaft skin resistance of pile body increasecorrespondingly Under the same loading step theaxial force of the pile body and the shaft skin re-sistance do not mobilize in the same time andgradually transfer from the upper part to the lowerpart of the pile body and the upper part of the pilebody reaches the limit value before the middle andlower parts As for the solid-bottom pile the soil atthe pile tip will strengthen the shaft skin resistance ofthe pile

(3) According to this experimental study it is suggestedthat similar projects should adopt concrete manuallybored cast-in-place piles with a diameter of 900mmand a length of 15m the wall-protecting thicknessshall not be less than 175mm and the grade ofconcrete cannot be lower than C35 +e pile end canbe widened or not and the pile bottom shall be freeof sediment

Data Availability

All the data in this paper are obtained from tests in thisstudy and no other data were used to support this study

Conflicts of Interest

+e authors declare that with regard to the publication ofthis paper and the funding for publishing it there are noconflicts of interest

Acknowledgments

+is study was partially supported by the National NaturalScience Foundation of China (Grant no 41302223) ChongqingNo 3 colleges and universities youth backbone teachers fundingplans Chongqing Research Program of Basic Research andFrontier Technology (Grant no cstc2016jcyjA0074cstc2016jcyjA0933 and cstc2015jcyjA90012) Scientific andTechnological Research Program of Chongqing Municipal

Education Commission (Grant no KJ1713327 and KJ1600532)Key Laboratory ofWell Stability and Fluid and RockMechanicsin Oil and Gas Reservoir of Shaanxi Province Xirsquoan ShiyouUniversity (Grant no FRM20190201002) Chongqing Post-graduate Research Innovation Project (Grant no CYS19353)and Chongqing University of Science and Technology GraduateStudentsrsquo Science and Technology Innovation Program (Grantno YKJCX1720601 and YKJCX1920613)

References

[1] F Kirsch and B Plabmann ldquoDynamic methods in pile testingdevelopments in measurement and analysisrdquo in Proceedings ofthe International Deep Foundations Congress 2002 pp 868ndash882 ASCE Orlando FL USA February 2002

[2] S H Ni L Lehmann J J Charng et al ldquoLow-strain integritytesting of drilled piles with high slenderness ratiordquo Computersand Geotechnics vol 33 no 6-7 pp 283ndash293 2006

[3] K Mori A Spagnoli Y Murakami G Kondo and I TorigoeldquoA new non-contacting non-destructive testing method fordefect detection in concreterdquo NDT amp E International vol 35no 6 pp 399ndash406 2002

[4] G Cai S Liu L Tong et al ldquoAssessment of direct CPT andCPTUmethods for predicting the ultimate bearing capacity ofsingle pilesrdquo Engineering Geology vol 104 no 3-4pp 211ndash222 2009

[5] X Y Bai M Y Zhang L Zhu et al ldquoIn-situ test and finiteelement analysis of bearing characteristics of rock-socketedshort piles in strongly weathered graniterdquo Journal of CentralSouth University Natural Science Edition vol 48 no 2pp 512ndash524 2017 in Chinese

[6] E C Leong and M F Randolph ldquoFinite element modelling ofrock-socketed pilesrdquo International Journal for Numerical andAnalytical Methods in Geomechanics vol 18 no 1 pp 25ndash471994

[7] G Gao M Gao Q Chen and J Yang ldquoField load testingstudy of vertical bearing behavior of a large diameter belledcast-in-place pilerdquo KSCE Journal of Civil Engineering vol 23no 5 pp 2009ndash2016 2019

[8] R Berardi and R Bovolenta ldquoPile-settlement evaluation usingfield stiffness non-linearityrdquo Proceedings of the Institution ofCivil Engineers - Geotechnical Engineering vol 158 no 1pp 35ndash44 2005

[9] B H Fellenius Basics of Foundation Design Lulu PressMorrisville NC USA 2016

[10] C Lam and S A Jefferis ldquoCritical assessment of pile modulusdetermination methodsrdquo Canadian Geotechnical Journalvol 48 no 10 pp 1433ndash1448 2011

[11] A Mohammadi T Ebadi and M R Boroomand ldquoPhysicalmodelling of axial compressive bearing capacity of instru-mented piles in oil-contaminated sandy soilrdquo Iranian Journalof Science and Technology Transactions of Civil Engineeringvol 44 no 2 pp 695ndash714 2019

[12] W D Guo and E H Ghee ldquoBehavior of axially loaded pilegroups subjected to lateral soil movementrdquo in Proceedings ofthe GeoShanghai International Conference 2006 pp 174ndash181ASCE Shanghai China June 2006

[13] Z Zhou Y Dong P Jiang D Han and T Liu ldquoCalculation ofpile side friction by multiparameter statistical analysisrdquo Ad-vances in Civil Engineering vol 2019 Article ID 263852012 pages 2019

14 Advances in Civil Engineering

[14] N M Hai and B H Fellenius ldquoO-Cell tests on two 70 m longbored piles in Vietnamrdquo in Proceedings of the Geo-Congress2014 pp 482ndash496 ASCE Atlanta Georgia February 2014

[15] S Gong G Cai S Liu et al ldquoNumerical Simulation of BearingCapacity and Consolidation Characteristics of PHC PileFoundationrdquo GeoShanghai International Conferencepp 178ndash185 Springer Singapore 2018

[16] M A Bashir H Furuuchi T Ueda and M Nauman BashirldquoNumerical simulation of axial anchorage capacity of con-crete-filled steel box footingrdquo Iranian Journal of Science andTechnology Transactions of Civil Engineering vol 40 no 3pp 257ndash262 2016

[17] N A Haskell ldquo+e dispersion of surface waves on multi-layered mediardquo Bulletin of the Seismological Society ofAmerica vol 43 no 1 pp 17ndash34 1953

[18] China Architecture amp Building PressNational Standard of thePeoplersquos Republic of China Code for Investigation of Geo-technical Engineering (GB50021 2016) China Architecture ampBuilding Press Beijing China 2016 in Chinese

[19] China Architecture amp Building Press Industry Standard of thePeoplersquos Republic of China Technical Code for Testing ofBuilding Foundation Piles (JGJ106-2014) China Architectureamp Building Press Beijing China 2014 in Chinese

[20] N Guo Z H Chen X F Huang et al ldquoResearch on the anti-pulling test of large-diameter cloth bag pile in soft rockfoundationrdquo Rock and Soil Mechanics vol 36 no 2pp 603ndash609 2015 in Chinese

[21] Y H Li X Zhu and T H Zhou ldquoField comparative ex-perimental study on the influence of post-tip grouting onlarge-diameter cast-in-situ pilesrdquo Rock and Soil Mechanicsvol 37 no 2 pp 388ndash396 2016 in Chinese

[22] G F Xin Z M Zhang T Xia et al ldquoExperimental study onbearing behavior of ultra-long pile under high loadrdquo ChineseJournal of Rock Mechanics and Engineering vol 13pp 2397ndash2402 2005 in Chinese

[23] Z Ling W Wang J Wu et al ldquoFull-scale loading test on pre-bored precast pile with enlarged base in Shanghairdquo in Pro-ceedings of the GeoShanghai 2018 International ConferenceAdvances in Soil Dynamics and Foundation Engineeringpp 637ndash645 Springer Singapore 2018

[24] J R Dong S T Lin and Y M Dai ldquo+e load tranfer behaviorof large diameter cast-in-situ pile in crushed pebble stratumrdquoChinese Journal of Geotechnical Engineering vol 16 no 6pp 123ndash131 1994

[25] X F Huang J H Zhang L G Ma et al ldquoExperimental studyon reinforcement effect of lateral resistance of collapsible loessrock-socketed perfusion pilerdquo Architectural Structure vol 41no 2 pp 351ndash355 2011 in Chinese

[26] J-J Zhou K-H Wang X-N Gong and R-H ZhangldquoBearing capacity and load transfer mechanism of a static drillrooted nodular pile in soft soil areasrdquo Journal of ZhejiangUniversity Science A vol 14 no 10 pp 705ndash719 2013

Advances in Civil Engineering 15

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

(a)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

(b)

10

9

8

7

6

5

4

3

2

1

0

Dep

th Z

m

0 20 40 60 80 100 120 140 160Skin friction of test pile (kPa)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

(c)

16151413121110

9876543210

Dep

th Z

m

0 20 40 60 80 100 120 140 160 180Skin friction of test pile (kPa)

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

(d)

Figure 11 Distribution of shaft skin resistance on each pile (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Table 6 Value of shaft skin resistance of piles at different depths when reaching ultimate bearing capacity

(a) No 1 pileDepth Zm 17 34 51 68 85Skin frictionkPa 60177 112974 133037 139158 135235

(b) No 2 pileDepth Zm 16 32 48 64 81 98Skin frictionkPa 43899 105055 128283 1391 146983 148015

(c) No 3 pileDepth Zm 16 32 48 64 81 98Skin frictionkPa 46948 106907 130723 145894 147938 150828

(d) No 4 pileDepth Zm 16 32 48 64 8 96 112 13 148Skin frictionkPa 63055 93009 117592 129804 141642 144953 145865 151216 154956

10 Advances in Civil Engineering

resistance value can be calculated by the following formulaaccording to the change value of the axial force of the pilebody

qsi Qi minus Qi+1

u middot li (5)

where qsi is the resistance between the i section and the i + 1section i is the same as in formula (1) u is the perimeter ofpile body li is the pile length between the i section and thei + 1 section

Figures 10(a)ndash10(d) show the distribution of shaft skinresistance of piles Nos 1 2 3 and 4 with depth underdifferent steps of loading

As can be seen from Figure 11 in lower loading step theshaft skin resistance of each pile is small with the increase ofloading steps the shaft skin resistance of each pile increasesOn the whole the shaft skin resistance of the pile is alsogradually transmitted from the upper part of the pile body tothe lower part of the pile body but compared with themiddle part of the pile body the shaft skin resistance of thelower part of the first pile body with empty-bottom shows a

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

10

8

6

4

2

0

Dep

th Z

m

(a)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

10

8

6

4

2

0

Dep

th Z

m

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

(b)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

10

8

6

4

2

0

Dep

th Z

m

(c)

Dep

th Z

m

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

151413121110

9876543210

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

(d)

Figure 12 Pile-soil relative displacement curve (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Advances in Civil Engineering 11

decreasing trend because there is no soil bearing the cor-responding load at the pile tip however for the remainingthree solid-bottom piles when the tip resistance is mobilizedto a certain extent the friction resistance in the middle partof the pile body is smaller than that in the lower part whichindicates that the soil at the pile tip can reinforce the shaftskin resistance of the pile [25] When the ultimate bearingcapacity is reached the shaft skin resistance values of thecorresponding piles at various depths are shown in Table 6

43 Relationship between Shaft Skin Resistance and Pile-SoilRelative Displacement +e pile-soil relative displacementobjectively reflects the mobilization of shaft skin resistance

According to the calculation method in [26] the pile-soilrelative displacement curves under different loading stepsare calculated and drawn +e curves of shaft skin resistanceand pile-soil relative displacement are shown inFigures 12(a)ndash12(d) respectively

In the calculation of the pile-soil relative displacement itis assumed that the soil around the pile will be not displacedAs can be seen from Figure 12 the pile-soil relative dis-placement at the same position of the pile body increases withthe increase of loading steps on the pile tipWhen the first twosteps of loading are applied the pile-soil relative displacementof the four piles increases obviously due to the compactioneffect of the first two steps of loading on the gravel soil aroundthe pile As the load level continues to increase the pile-soil

Skin

fric

tion

of te

st pi

le (k

Pa)

5 10 15 20 25 300Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

1-11-21-3

1-41-5

(a)Sk

in fr

ictio

n of

test

pile

(kPa

)

0 10 15 20 25 30 35 405Pile-soil relative displacement (mm)

2-12-22-3

2-42-52-6

0

20

40

60

80

100

120

140

160

(b)

Skin

fric

tion

of te

st pi

le (k

Pa)

5 10 15 20 25 30 35 400Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

3-13-23-3

3-43-53-6

(c)

Skin

fric

tion

of te

st pi

le (k

Pa)

0 10 15 20 25 30 35 405Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

4-14-24-3

4-44-54-6

4-74-84-9

(d)

Figure 13 Distribution curves of shaft skin resistances versus relative displacements (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4pile

12 Advances in Civil Engineering

relative displacement increment is relatively small because theshaft skin resistance gradually mobilizes its function toprevent the pile from moving With the increase of loadingwhen the shaft skin resistance cannot resist the load at theloading sections 4800 kNndash5400 kN 4200 kNndash4800 kN8000 kNndash8800 kN and 3600 kNndash4000 kN the maximumdisplacements of pile tips of Nos 2 3 and 4 solid-bottom pileand the empty-bottom pile increase by 9792mm 7939mm9881mm and 1497mm respectively

+e shaft skin resistance increases with the increase ofpile-soil relative displacement Although continuous loadinghas led to foundation soil destruction it can be concludedfrom Figure 13 that the lateral resistance in the middle andlower parts is not fully mobilized and a certain pile-soilrelative displacement is needed to promote the full mobi-lization of the shaft skin resistance However according tothe development trend that the slope of the shaft skin re-sistance gradually decreases it can be inferred that the shaftskin resistance will reach the maximum value At the firstsection position the ultimate shaft skin resistance of pilesNos 1 2 3 and 4 are 4695 kPa 4390 kPa 6018 kPa and6305 kPa respectively and the corresponding pile-soil rel-ative displacements are 3419mm 3442mm 2633mm and3279mm In the second section the shaft skin resistance ofthe four piles increases greatly so does the pile-soil relativedisplacement Since the soil above the first section position isbasically cultivated soil and it can resist small shaft skinresistance while the content of gravel soil of the secondsection is large and with the increase of depth the sur-rounding pressure of soil around the pile increases thereforethe ability of resisting pile foundation sliding and dis-placement increases+erefore between the first section andthe second section the shaft skin resistance increases greatlywhile the pile-soil relative displacement increases a little Inthe second section and beyond the soil is gravel soil and theshaft skin resistance increases slightly with the increase ofpile depth and the relative displacement decreases gradually

44 Pile Tip Resistance Varies with Load and DisplacementDuring the test the section where the largest embeddeddepth of strain gauges is very close to the bottom of the pilewas shown so the resistance at the pile tip is approximatelyequal to the axial force measured at this section namelyQb Q0 Under the action of all loading steps the resistancevariation curves at the lower tips of the Nos 2 3 and 4 pilesare shown in Figure 14

+e No 2 and No 3 piles in the first two loading stepsand No 4 pile in the first loading step have smaller resistanceat the pile tip +e resistance at the pile tip gradually in-creases until the third and fourth loading steps Under theaction of ultimate load the ratio of pile tip resistance to piletop load of Nos 2 3 and 4 is 377 3008 and 3992respectively all higher 15 which shows that pile tip re-sistance has been brought into play to a certain extent +epile tip resistance is shared by shaft skin resistance and piletip resistance and the pile is of tip bearing friction pile

According to the relationship curve between pile tipresistance and pile tip displacement shown in Figure 15 it

can be seen that the pile tip resistance and pile tip dis-placement present a double fold line model of work hard-ening +e Nos 2 3 and 4 piles all show a slow deformationfailure and under the action of the first loading step theslope of the curve is larger and the corresponding dis-placement is relatively small in latter steps of loading thecurvature curve becomes smoother the displacement of thepile tips of adjacent loads increases and the soil at the piletips will gradually be compacted

Pile

tip

load

(kN

)

1000 2000 3000 4000 5000 6000 7000 8000 90000Load at pile top (kN)

0250500750

100012501500175020002250250027503000325035003750

No 2No 3No 4

Figure 14 Variation curve of pile tip resistance under differentloading steps (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4pile

Pile

tip

load

(kN

)

5 10 15 20 25 30 35 40 450Pile tip displacement (mm)

0

1000

2000

3000

4000

5000

6000

No 2No 3No 4

Figure 15 Relationship curve between pile tip resistance and piletip displacement (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No4 pile

Advances in Civil Engineering 13

5 Conclusions

Analyses are carried out on data of the in-situ test of unevengravel soil in Baishi Mountain and the following conclu-sions are drawn

(1) Based on the analysis of the results of multichanneltransient surface wave test superheavy (N120) conedynamic sounding test and on-site screening testthe geological environment conditions of the unevengravel soil on the site are further explored +e stratawithin the range from 0 to 15m in Baizhi Mountaincan be divided into four layers ie cultivated soilsandy soil silty clay and gravel soil with depths of1m 2m 3m and 9m respectively

(2) +e axial force and shaft skin resistance of pile bodyin gravel soil are the same as those in other geologicalstructures At the initial loading step the axial forceand shaft skin resistance of pile body play a smallerrole With the increase of loading steps the axialforce and shaft skin resistance of pile body increasecorrespondingly Under the same loading step theaxial force of the pile body and the shaft skin re-sistance do not mobilize in the same time andgradually transfer from the upper part to the lowerpart of the pile body and the upper part of the pilebody reaches the limit value before the middle andlower parts As for the solid-bottom pile the soil atthe pile tip will strengthen the shaft skin resistance ofthe pile

(3) According to this experimental study it is suggestedthat similar projects should adopt concrete manuallybored cast-in-place piles with a diameter of 900mmand a length of 15m the wall-protecting thicknessshall not be less than 175mm and the grade ofconcrete cannot be lower than C35 +e pile end canbe widened or not and the pile bottom shall be freeof sediment

Data Availability

All the data in this paper are obtained from tests in thisstudy and no other data were used to support this study

Conflicts of Interest

+e authors declare that with regard to the publication ofthis paper and the funding for publishing it there are noconflicts of interest

Acknowledgments

+is study was partially supported by the National NaturalScience Foundation of China (Grant no 41302223) ChongqingNo 3 colleges and universities youth backbone teachers fundingplans Chongqing Research Program of Basic Research andFrontier Technology (Grant no cstc2016jcyjA0074cstc2016jcyjA0933 and cstc2015jcyjA90012) Scientific andTechnological Research Program of Chongqing Municipal

Education Commission (Grant no KJ1713327 and KJ1600532)Key Laboratory ofWell Stability and Fluid and RockMechanicsin Oil and Gas Reservoir of Shaanxi Province Xirsquoan ShiyouUniversity (Grant no FRM20190201002) Chongqing Post-graduate Research Innovation Project (Grant no CYS19353)and Chongqing University of Science and Technology GraduateStudentsrsquo Science and Technology Innovation Program (Grantno YKJCX1720601 and YKJCX1920613)

References

[1] F Kirsch and B Plabmann ldquoDynamic methods in pile testingdevelopments in measurement and analysisrdquo in Proceedings ofthe International Deep Foundations Congress 2002 pp 868ndash882 ASCE Orlando FL USA February 2002

[2] S H Ni L Lehmann J J Charng et al ldquoLow-strain integritytesting of drilled piles with high slenderness ratiordquo Computersand Geotechnics vol 33 no 6-7 pp 283ndash293 2006

[3] K Mori A Spagnoli Y Murakami G Kondo and I TorigoeldquoA new non-contacting non-destructive testing method fordefect detection in concreterdquo NDT amp E International vol 35no 6 pp 399ndash406 2002

[4] G Cai S Liu L Tong et al ldquoAssessment of direct CPT andCPTUmethods for predicting the ultimate bearing capacity ofsingle pilesrdquo Engineering Geology vol 104 no 3-4pp 211ndash222 2009

[5] X Y Bai M Y Zhang L Zhu et al ldquoIn-situ test and finiteelement analysis of bearing characteristics of rock-socketedshort piles in strongly weathered graniterdquo Journal of CentralSouth University Natural Science Edition vol 48 no 2pp 512ndash524 2017 in Chinese

[6] E C Leong and M F Randolph ldquoFinite element modelling ofrock-socketed pilesrdquo International Journal for Numerical andAnalytical Methods in Geomechanics vol 18 no 1 pp 25ndash471994

[7] G Gao M Gao Q Chen and J Yang ldquoField load testingstudy of vertical bearing behavior of a large diameter belledcast-in-place pilerdquo KSCE Journal of Civil Engineering vol 23no 5 pp 2009ndash2016 2019

[8] R Berardi and R Bovolenta ldquoPile-settlement evaluation usingfield stiffness non-linearityrdquo Proceedings of the Institution ofCivil Engineers - Geotechnical Engineering vol 158 no 1pp 35ndash44 2005

[9] B H Fellenius Basics of Foundation Design Lulu PressMorrisville NC USA 2016

[10] C Lam and S A Jefferis ldquoCritical assessment of pile modulusdetermination methodsrdquo Canadian Geotechnical Journalvol 48 no 10 pp 1433ndash1448 2011

[11] A Mohammadi T Ebadi and M R Boroomand ldquoPhysicalmodelling of axial compressive bearing capacity of instru-mented piles in oil-contaminated sandy soilrdquo Iranian Journalof Science and Technology Transactions of Civil Engineeringvol 44 no 2 pp 695ndash714 2019

[12] W D Guo and E H Ghee ldquoBehavior of axially loaded pilegroups subjected to lateral soil movementrdquo in Proceedings ofthe GeoShanghai International Conference 2006 pp 174ndash181ASCE Shanghai China June 2006

[13] Z Zhou Y Dong P Jiang D Han and T Liu ldquoCalculation ofpile side friction by multiparameter statistical analysisrdquo Ad-vances in Civil Engineering vol 2019 Article ID 263852012 pages 2019

14 Advances in Civil Engineering

[14] N M Hai and B H Fellenius ldquoO-Cell tests on two 70 m longbored piles in Vietnamrdquo in Proceedings of the Geo-Congress2014 pp 482ndash496 ASCE Atlanta Georgia February 2014

[15] S Gong G Cai S Liu et al ldquoNumerical Simulation of BearingCapacity and Consolidation Characteristics of PHC PileFoundationrdquo GeoShanghai International Conferencepp 178ndash185 Springer Singapore 2018

[16] M A Bashir H Furuuchi T Ueda and M Nauman BashirldquoNumerical simulation of axial anchorage capacity of con-crete-filled steel box footingrdquo Iranian Journal of Science andTechnology Transactions of Civil Engineering vol 40 no 3pp 257ndash262 2016

[17] N A Haskell ldquo+e dispersion of surface waves on multi-layered mediardquo Bulletin of the Seismological Society ofAmerica vol 43 no 1 pp 17ndash34 1953

[18] China Architecture amp Building PressNational Standard of thePeoplersquos Republic of China Code for Investigation of Geo-technical Engineering (GB50021 2016) China Architecture ampBuilding Press Beijing China 2016 in Chinese

[19] China Architecture amp Building Press Industry Standard of thePeoplersquos Republic of China Technical Code for Testing ofBuilding Foundation Piles (JGJ106-2014) China Architectureamp Building Press Beijing China 2014 in Chinese

[20] N Guo Z H Chen X F Huang et al ldquoResearch on the anti-pulling test of large-diameter cloth bag pile in soft rockfoundationrdquo Rock and Soil Mechanics vol 36 no 2pp 603ndash609 2015 in Chinese

[21] Y H Li X Zhu and T H Zhou ldquoField comparative ex-perimental study on the influence of post-tip grouting onlarge-diameter cast-in-situ pilesrdquo Rock and Soil Mechanicsvol 37 no 2 pp 388ndash396 2016 in Chinese

[22] G F Xin Z M Zhang T Xia et al ldquoExperimental study onbearing behavior of ultra-long pile under high loadrdquo ChineseJournal of Rock Mechanics and Engineering vol 13pp 2397ndash2402 2005 in Chinese

[23] Z Ling W Wang J Wu et al ldquoFull-scale loading test on pre-bored precast pile with enlarged base in Shanghairdquo in Pro-ceedings of the GeoShanghai 2018 International ConferenceAdvances in Soil Dynamics and Foundation Engineeringpp 637ndash645 Springer Singapore 2018

[24] J R Dong S T Lin and Y M Dai ldquo+e load tranfer behaviorof large diameter cast-in-situ pile in crushed pebble stratumrdquoChinese Journal of Geotechnical Engineering vol 16 no 6pp 123ndash131 1994

[25] X F Huang J H Zhang L G Ma et al ldquoExperimental studyon reinforcement effect of lateral resistance of collapsible loessrock-socketed perfusion pilerdquo Architectural Structure vol 41no 2 pp 351ndash355 2011 in Chinese

[26] J-J Zhou K-H Wang X-N Gong and R-H ZhangldquoBearing capacity and load transfer mechanism of a static drillrooted nodular pile in soft soil areasrdquo Journal of ZhejiangUniversity Science A vol 14 no 10 pp 705ndash719 2013

Advances in Civil Engineering 15

resistance value can be calculated by the following formulaaccording to the change value of the axial force of the pilebody

qsi Qi minus Qi+1

u middot li (5)

where qsi is the resistance between the i section and the i + 1section i is the same as in formula (1) u is the perimeter ofpile body li is the pile length between the i section and thei + 1 section

Figures 10(a)ndash10(d) show the distribution of shaft skinresistance of piles Nos 1 2 3 and 4 with depth underdifferent steps of loading

As can be seen from Figure 11 in lower loading step theshaft skin resistance of each pile is small with the increase ofloading steps the shaft skin resistance of each pile increasesOn the whole the shaft skin resistance of the pile is alsogradually transmitted from the upper part of the pile body tothe lower part of the pile body but compared with themiddle part of the pile body the shaft skin resistance of thelower part of the first pile body with empty-bottom shows a

800kN1200kN1600kN2000kN2400kN

2800kN3200kN3600kN4000kN

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

10

8

6

4

2

0

Dep

th Z

m

(a)

600kN1200kN1800kN2400kN3000kN

3600kN4200kN4800kN5400kN

10

8

6

4

2

0

Dep

th Z

m

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

(b)

600kN1200kN1800kN2400kN

3000kN3600kN4200kN4800kN

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

10

8

6

4

2

0

Dep

th Z

m

(c)

Dep

th Z

m

1600kN2400kN3200kN4000kN4800kN

5600kN6400kN7200kN8000kN8800kN

151413121110

9876543210

0 5 10 15 20 25 30 35 40 45Pile-soil relative displacement (mm)

(d)

Figure 12 Pile-soil relative displacement curve (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4 pile

Advances in Civil Engineering 11

decreasing trend because there is no soil bearing the cor-responding load at the pile tip however for the remainingthree solid-bottom piles when the tip resistance is mobilizedto a certain extent the friction resistance in the middle partof the pile body is smaller than that in the lower part whichindicates that the soil at the pile tip can reinforce the shaftskin resistance of the pile [25] When the ultimate bearingcapacity is reached the shaft skin resistance values of thecorresponding piles at various depths are shown in Table 6

43 Relationship between Shaft Skin Resistance and Pile-SoilRelative Displacement +e pile-soil relative displacementobjectively reflects the mobilization of shaft skin resistance

According to the calculation method in [26] the pile-soilrelative displacement curves under different loading stepsare calculated and drawn +e curves of shaft skin resistanceand pile-soil relative displacement are shown inFigures 12(a)ndash12(d) respectively

In the calculation of the pile-soil relative displacement itis assumed that the soil around the pile will be not displacedAs can be seen from Figure 12 the pile-soil relative dis-placement at the same position of the pile body increases withthe increase of loading steps on the pile tipWhen the first twosteps of loading are applied the pile-soil relative displacementof the four piles increases obviously due to the compactioneffect of the first two steps of loading on the gravel soil aroundthe pile As the load level continues to increase the pile-soil

Skin

fric

tion

of te

st pi

le (k

Pa)

5 10 15 20 25 300Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

1-11-21-3

1-41-5

(a)Sk

in fr

ictio

n of

test

pile

(kPa

)

0 10 15 20 25 30 35 405Pile-soil relative displacement (mm)

2-12-22-3

2-42-52-6

0

20

40

60

80

100

120

140

160

(b)

Skin

fric

tion

of te

st pi

le (k

Pa)

5 10 15 20 25 30 35 400Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

3-13-23-3

3-43-53-6

(c)

Skin

fric

tion

of te

st pi

le (k

Pa)

0 10 15 20 25 30 35 405Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

4-14-24-3

4-44-54-6

4-74-84-9

(d)

Figure 13 Distribution curves of shaft skin resistances versus relative displacements (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4pile

12 Advances in Civil Engineering

relative displacement increment is relatively small because theshaft skin resistance gradually mobilizes its function toprevent the pile from moving With the increase of loadingwhen the shaft skin resistance cannot resist the load at theloading sections 4800 kNndash5400 kN 4200 kNndash4800 kN8000 kNndash8800 kN and 3600 kNndash4000 kN the maximumdisplacements of pile tips of Nos 2 3 and 4 solid-bottom pileand the empty-bottom pile increase by 9792mm 7939mm9881mm and 1497mm respectively

+e shaft skin resistance increases with the increase ofpile-soil relative displacement Although continuous loadinghas led to foundation soil destruction it can be concludedfrom Figure 13 that the lateral resistance in the middle andlower parts is not fully mobilized and a certain pile-soilrelative displacement is needed to promote the full mobi-lization of the shaft skin resistance However according tothe development trend that the slope of the shaft skin re-sistance gradually decreases it can be inferred that the shaftskin resistance will reach the maximum value At the firstsection position the ultimate shaft skin resistance of pilesNos 1 2 3 and 4 are 4695 kPa 4390 kPa 6018 kPa and6305 kPa respectively and the corresponding pile-soil rel-ative displacements are 3419mm 3442mm 2633mm and3279mm In the second section the shaft skin resistance ofthe four piles increases greatly so does the pile-soil relativedisplacement Since the soil above the first section position isbasically cultivated soil and it can resist small shaft skinresistance while the content of gravel soil of the secondsection is large and with the increase of depth the sur-rounding pressure of soil around the pile increases thereforethe ability of resisting pile foundation sliding and dis-placement increases+erefore between the first section andthe second section the shaft skin resistance increases greatlywhile the pile-soil relative displacement increases a little Inthe second section and beyond the soil is gravel soil and theshaft skin resistance increases slightly with the increase ofpile depth and the relative displacement decreases gradually

44 Pile Tip Resistance Varies with Load and DisplacementDuring the test the section where the largest embeddeddepth of strain gauges is very close to the bottom of the pilewas shown so the resistance at the pile tip is approximatelyequal to the axial force measured at this section namelyQb Q0 Under the action of all loading steps the resistancevariation curves at the lower tips of the Nos 2 3 and 4 pilesare shown in Figure 14

+e No 2 and No 3 piles in the first two loading stepsand No 4 pile in the first loading step have smaller resistanceat the pile tip +e resistance at the pile tip gradually in-creases until the third and fourth loading steps Under theaction of ultimate load the ratio of pile tip resistance to piletop load of Nos 2 3 and 4 is 377 3008 and 3992respectively all higher 15 which shows that pile tip re-sistance has been brought into play to a certain extent +epile tip resistance is shared by shaft skin resistance and piletip resistance and the pile is of tip bearing friction pile

According to the relationship curve between pile tipresistance and pile tip displacement shown in Figure 15 it

can be seen that the pile tip resistance and pile tip dis-placement present a double fold line model of work hard-ening +e Nos 2 3 and 4 piles all show a slow deformationfailure and under the action of the first loading step theslope of the curve is larger and the corresponding dis-placement is relatively small in latter steps of loading thecurvature curve becomes smoother the displacement of thepile tips of adjacent loads increases and the soil at the piletips will gradually be compacted

Pile

tip

load

(kN

)

1000 2000 3000 4000 5000 6000 7000 8000 90000Load at pile top (kN)

0250500750

100012501500175020002250250027503000325035003750

No 2No 3No 4

Figure 14 Variation curve of pile tip resistance under differentloading steps (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4pile

Pile

tip

load

(kN

)

5 10 15 20 25 30 35 40 450Pile tip displacement (mm)

0

1000

2000

3000

4000

5000

6000

No 2No 3No 4

Figure 15 Relationship curve between pile tip resistance and piletip displacement (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No4 pile

Advances in Civil Engineering 13

5 Conclusions

Analyses are carried out on data of the in-situ test of unevengravel soil in Baishi Mountain and the following conclu-sions are drawn

(1) Based on the analysis of the results of multichanneltransient surface wave test superheavy (N120) conedynamic sounding test and on-site screening testthe geological environment conditions of the unevengravel soil on the site are further explored +e stratawithin the range from 0 to 15m in Baizhi Mountaincan be divided into four layers ie cultivated soilsandy soil silty clay and gravel soil with depths of1m 2m 3m and 9m respectively

(2) +e axial force and shaft skin resistance of pile bodyin gravel soil are the same as those in other geologicalstructures At the initial loading step the axial forceand shaft skin resistance of pile body play a smallerrole With the increase of loading steps the axialforce and shaft skin resistance of pile body increasecorrespondingly Under the same loading step theaxial force of the pile body and the shaft skin re-sistance do not mobilize in the same time andgradually transfer from the upper part to the lowerpart of the pile body and the upper part of the pilebody reaches the limit value before the middle andlower parts As for the solid-bottom pile the soil atthe pile tip will strengthen the shaft skin resistance ofthe pile

(3) According to this experimental study it is suggestedthat similar projects should adopt concrete manuallybored cast-in-place piles with a diameter of 900mmand a length of 15m the wall-protecting thicknessshall not be less than 175mm and the grade ofconcrete cannot be lower than C35 +e pile end canbe widened or not and the pile bottom shall be freeof sediment

Data Availability

All the data in this paper are obtained from tests in thisstudy and no other data were used to support this study

Conflicts of Interest

+e authors declare that with regard to the publication ofthis paper and the funding for publishing it there are noconflicts of interest

Acknowledgments

+is study was partially supported by the National NaturalScience Foundation of China (Grant no 41302223) ChongqingNo 3 colleges and universities youth backbone teachers fundingplans Chongqing Research Program of Basic Research andFrontier Technology (Grant no cstc2016jcyjA0074cstc2016jcyjA0933 and cstc2015jcyjA90012) Scientific andTechnological Research Program of Chongqing Municipal

Education Commission (Grant no KJ1713327 and KJ1600532)Key Laboratory ofWell Stability and Fluid and RockMechanicsin Oil and Gas Reservoir of Shaanxi Province Xirsquoan ShiyouUniversity (Grant no FRM20190201002) Chongqing Post-graduate Research Innovation Project (Grant no CYS19353)and Chongqing University of Science and Technology GraduateStudentsrsquo Science and Technology Innovation Program (Grantno YKJCX1720601 and YKJCX1920613)

References

[1] F Kirsch and B Plabmann ldquoDynamic methods in pile testingdevelopments in measurement and analysisrdquo in Proceedings ofthe International Deep Foundations Congress 2002 pp 868ndash882 ASCE Orlando FL USA February 2002

[2] S H Ni L Lehmann J J Charng et al ldquoLow-strain integritytesting of drilled piles with high slenderness ratiordquo Computersand Geotechnics vol 33 no 6-7 pp 283ndash293 2006

[3] K Mori A Spagnoli Y Murakami G Kondo and I TorigoeldquoA new non-contacting non-destructive testing method fordefect detection in concreterdquo NDT amp E International vol 35no 6 pp 399ndash406 2002

[4] G Cai S Liu L Tong et al ldquoAssessment of direct CPT andCPTUmethods for predicting the ultimate bearing capacity ofsingle pilesrdquo Engineering Geology vol 104 no 3-4pp 211ndash222 2009

[5] X Y Bai M Y Zhang L Zhu et al ldquoIn-situ test and finiteelement analysis of bearing characteristics of rock-socketedshort piles in strongly weathered graniterdquo Journal of CentralSouth University Natural Science Edition vol 48 no 2pp 512ndash524 2017 in Chinese

[6] E C Leong and M F Randolph ldquoFinite element modelling ofrock-socketed pilesrdquo International Journal for Numerical andAnalytical Methods in Geomechanics vol 18 no 1 pp 25ndash471994

[7] G Gao M Gao Q Chen and J Yang ldquoField load testingstudy of vertical bearing behavior of a large diameter belledcast-in-place pilerdquo KSCE Journal of Civil Engineering vol 23no 5 pp 2009ndash2016 2019

[8] R Berardi and R Bovolenta ldquoPile-settlement evaluation usingfield stiffness non-linearityrdquo Proceedings of the Institution ofCivil Engineers - Geotechnical Engineering vol 158 no 1pp 35ndash44 2005

[9] B H Fellenius Basics of Foundation Design Lulu PressMorrisville NC USA 2016

[10] C Lam and S A Jefferis ldquoCritical assessment of pile modulusdetermination methodsrdquo Canadian Geotechnical Journalvol 48 no 10 pp 1433ndash1448 2011

[11] A Mohammadi T Ebadi and M R Boroomand ldquoPhysicalmodelling of axial compressive bearing capacity of instru-mented piles in oil-contaminated sandy soilrdquo Iranian Journalof Science and Technology Transactions of Civil Engineeringvol 44 no 2 pp 695ndash714 2019

[12] W D Guo and E H Ghee ldquoBehavior of axially loaded pilegroups subjected to lateral soil movementrdquo in Proceedings ofthe GeoShanghai International Conference 2006 pp 174ndash181ASCE Shanghai China June 2006

[13] Z Zhou Y Dong P Jiang D Han and T Liu ldquoCalculation ofpile side friction by multiparameter statistical analysisrdquo Ad-vances in Civil Engineering vol 2019 Article ID 263852012 pages 2019

14 Advances in Civil Engineering

[14] N M Hai and B H Fellenius ldquoO-Cell tests on two 70 m longbored piles in Vietnamrdquo in Proceedings of the Geo-Congress2014 pp 482ndash496 ASCE Atlanta Georgia February 2014

[15] S Gong G Cai S Liu et al ldquoNumerical Simulation of BearingCapacity and Consolidation Characteristics of PHC PileFoundationrdquo GeoShanghai International Conferencepp 178ndash185 Springer Singapore 2018

[16] M A Bashir H Furuuchi T Ueda and M Nauman BashirldquoNumerical simulation of axial anchorage capacity of con-crete-filled steel box footingrdquo Iranian Journal of Science andTechnology Transactions of Civil Engineering vol 40 no 3pp 257ndash262 2016

[17] N A Haskell ldquo+e dispersion of surface waves on multi-layered mediardquo Bulletin of the Seismological Society ofAmerica vol 43 no 1 pp 17ndash34 1953

[18] China Architecture amp Building PressNational Standard of thePeoplersquos Republic of China Code for Investigation of Geo-technical Engineering (GB50021 2016) China Architecture ampBuilding Press Beijing China 2016 in Chinese

[19] China Architecture amp Building Press Industry Standard of thePeoplersquos Republic of China Technical Code for Testing ofBuilding Foundation Piles (JGJ106-2014) China Architectureamp Building Press Beijing China 2014 in Chinese

[20] N Guo Z H Chen X F Huang et al ldquoResearch on the anti-pulling test of large-diameter cloth bag pile in soft rockfoundationrdquo Rock and Soil Mechanics vol 36 no 2pp 603ndash609 2015 in Chinese

[21] Y H Li X Zhu and T H Zhou ldquoField comparative ex-perimental study on the influence of post-tip grouting onlarge-diameter cast-in-situ pilesrdquo Rock and Soil Mechanicsvol 37 no 2 pp 388ndash396 2016 in Chinese

[22] G F Xin Z M Zhang T Xia et al ldquoExperimental study onbearing behavior of ultra-long pile under high loadrdquo ChineseJournal of Rock Mechanics and Engineering vol 13pp 2397ndash2402 2005 in Chinese

[23] Z Ling W Wang J Wu et al ldquoFull-scale loading test on pre-bored precast pile with enlarged base in Shanghairdquo in Pro-ceedings of the GeoShanghai 2018 International ConferenceAdvances in Soil Dynamics and Foundation Engineeringpp 637ndash645 Springer Singapore 2018

[24] J R Dong S T Lin and Y M Dai ldquo+e load tranfer behaviorof large diameter cast-in-situ pile in crushed pebble stratumrdquoChinese Journal of Geotechnical Engineering vol 16 no 6pp 123ndash131 1994

[25] X F Huang J H Zhang L G Ma et al ldquoExperimental studyon reinforcement effect of lateral resistance of collapsible loessrock-socketed perfusion pilerdquo Architectural Structure vol 41no 2 pp 351ndash355 2011 in Chinese

[26] J-J Zhou K-H Wang X-N Gong and R-H ZhangldquoBearing capacity and load transfer mechanism of a static drillrooted nodular pile in soft soil areasrdquo Journal of ZhejiangUniversity Science A vol 14 no 10 pp 705ndash719 2013

Advances in Civil Engineering 15

decreasing trend because there is no soil bearing the cor-responding load at the pile tip however for the remainingthree solid-bottom piles when the tip resistance is mobilizedto a certain extent the friction resistance in the middle partof the pile body is smaller than that in the lower part whichindicates that the soil at the pile tip can reinforce the shaftskin resistance of the pile [25] When the ultimate bearingcapacity is reached the shaft skin resistance values of thecorresponding piles at various depths are shown in Table 6

43 Relationship between Shaft Skin Resistance and Pile-SoilRelative Displacement +e pile-soil relative displacementobjectively reflects the mobilization of shaft skin resistance

According to the calculation method in [26] the pile-soilrelative displacement curves under different loading stepsare calculated and drawn +e curves of shaft skin resistanceand pile-soil relative displacement are shown inFigures 12(a)ndash12(d) respectively

In the calculation of the pile-soil relative displacement itis assumed that the soil around the pile will be not displacedAs can be seen from Figure 12 the pile-soil relative dis-placement at the same position of the pile body increases withthe increase of loading steps on the pile tipWhen the first twosteps of loading are applied the pile-soil relative displacementof the four piles increases obviously due to the compactioneffect of the first two steps of loading on the gravel soil aroundthe pile As the load level continues to increase the pile-soil

Skin

fric

tion

of te

st pi

le (k

Pa)

5 10 15 20 25 300Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

1-11-21-3

1-41-5

(a)Sk

in fr

ictio

n of

test

pile

(kPa

)

0 10 15 20 25 30 35 405Pile-soil relative displacement (mm)

2-12-22-3

2-42-52-6

0

20

40

60

80

100

120

140

160

(b)

Skin

fric

tion

of te

st pi

le (k

Pa)

5 10 15 20 25 30 35 400Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

3-13-23-3

3-43-53-6

(c)

Skin

fric

tion

of te

st pi

le (k

Pa)

0 10 15 20 25 30 35 405Pile-soil relative displacement (mm)

0

20

40

60

80

100

120

140

160

4-14-24-3

4-44-54-6

4-74-84-9

(d)

Figure 13 Distribution curves of shaft skin resistances versus relative displacements (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4pile

12 Advances in Civil Engineering

relative displacement increment is relatively small because theshaft skin resistance gradually mobilizes its function toprevent the pile from moving With the increase of loadingwhen the shaft skin resistance cannot resist the load at theloading sections 4800 kNndash5400 kN 4200 kNndash4800 kN8000 kNndash8800 kN and 3600 kNndash4000 kN the maximumdisplacements of pile tips of Nos 2 3 and 4 solid-bottom pileand the empty-bottom pile increase by 9792mm 7939mm9881mm and 1497mm respectively

+e shaft skin resistance increases with the increase ofpile-soil relative displacement Although continuous loadinghas led to foundation soil destruction it can be concludedfrom Figure 13 that the lateral resistance in the middle andlower parts is not fully mobilized and a certain pile-soilrelative displacement is needed to promote the full mobi-lization of the shaft skin resistance However according tothe development trend that the slope of the shaft skin re-sistance gradually decreases it can be inferred that the shaftskin resistance will reach the maximum value At the firstsection position the ultimate shaft skin resistance of pilesNos 1 2 3 and 4 are 4695 kPa 4390 kPa 6018 kPa and6305 kPa respectively and the corresponding pile-soil rel-ative displacements are 3419mm 3442mm 2633mm and3279mm In the second section the shaft skin resistance ofthe four piles increases greatly so does the pile-soil relativedisplacement Since the soil above the first section position isbasically cultivated soil and it can resist small shaft skinresistance while the content of gravel soil of the secondsection is large and with the increase of depth the sur-rounding pressure of soil around the pile increases thereforethe ability of resisting pile foundation sliding and dis-placement increases+erefore between the first section andthe second section the shaft skin resistance increases greatlywhile the pile-soil relative displacement increases a little Inthe second section and beyond the soil is gravel soil and theshaft skin resistance increases slightly with the increase ofpile depth and the relative displacement decreases gradually

44 Pile Tip Resistance Varies with Load and DisplacementDuring the test the section where the largest embeddeddepth of strain gauges is very close to the bottom of the pilewas shown so the resistance at the pile tip is approximatelyequal to the axial force measured at this section namelyQb Q0 Under the action of all loading steps the resistancevariation curves at the lower tips of the Nos 2 3 and 4 pilesare shown in Figure 14

+e No 2 and No 3 piles in the first two loading stepsand No 4 pile in the first loading step have smaller resistanceat the pile tip +e resistance at the pile tip gradually in-creases until the third and fourth loading steps Under theaction of ultimate load the ratio of pile tip resistance to piletop load of Nos 2 3 and 4 is 377 3008 and 3992respectively all higher 15 which shows that pile tip re-sistance has been brought into play to a certain extent +epile tip resistance is shared by shaft skin resistance and piletip resistance and the pile is of tip bearing friction pile

According to the relationship curve between pile tipresistance and pile tip displacement shown in Figure 15 it

can be seen that the pile tip resistance and pile tip dis-placement present a double fold line model of work hard-ening +e Nos 2 3 and 4 piles all show a slow deformationfailure and under the action of the first loading step theslope of the curve is larger and the corresponding dis-placement is relatively small in latter steps of loading thecurvature curve becomes smoother the displacement of thepile tips of adjacent loads increases and the soil at the piletips will gradually be compacted

Pile

tip

load

(kN

)

1000 2000 3000 4000 5000 6000 7000 8000 90000Load at pile top (kN)

0250500750

100012501500175020002250250027503000325035003750

No 2No 3No 4

Figure 14 Variation curve of pile tip resistance under differentloading steps (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4pile

Pile

tip

load

(kN

)

5 10 15 20 25 30 35 40 450Pile tip displacement (mm)

0

1000

2000

3000

4000

5000

6000

No 2No 3No 4

Figure 15 Relationship curve between pile tip resistance and piletip displacement (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No4 pile

Advances in Civil Engineering 13

5 Conclusions

Analyses are carried out on data of the in-situ test of unevengravel soil in Baishi Mountain and the following conclu-sions are drawn

(1) Based on the analysis of the results of multichanneltransient surface wave test superheavy (N120) conedynamic sounding test and on-site screening testthe geological environment conditions of the unevengravel soil on the site are further explored +e stratawithin the range from 0 to 15m in Baizhi Mountaincan be divided into four layers ie cultivated soilsandy soil silty clay and gravel soil with depths of1m 2m 3m and 9m respectively

(2) +e axial force and shaft skin resistance of pile bodyin gravel soil are the same as those in other geologicalstructures At the initial loading step the axial forceand shaft skin resistance of pile body play a smallerrole With the increase of loading steps the axialforce and shaft skin resistance of pile body increasecorrespondingly Under the same loading step theaxial force of the pile body and the shaft skin re-sistance do not mobilize in the same time andgradually transfer from the upper part to the lowerpart of the pile body and the upper part of the pilebody reaches the limit value before the middle andlower parts As for the solid-bottom pile the soil atthe pile tip will strengthen the shaft skin resistance ofthe pile

(3) According to this experimental study it is suggestedthat similar projects should adopt concrete manuallybored cast-in-place piles with a diameter of 900mmand a length of 15m the wall-protecting thicknessshall not be less than 175mm and the grade ofconcrete cannot be lower than C35 +e pile end canbe widened or not and the pile bottom shall be freeof sediment

Data Availability

All the data in this paper are obtained from tests in thisstudy and no other data were used to support this study

Conflicts of Interest

+e authors declare that with regard to the publication ofthis paper and the funding for publishing it there are noconflicts of interest

Acknowledgments

+is study was partially supported by the National NaturalScience Foundation of China (Grant no 41302223) ChongqingNo 3 colleges and universities youth backbone teachers fundingplans Chongqing Research Program of Basic Research andFrontier Technology (Grant no cstc2016jcyjA0074cstc2016jcyjA0933 and cstc2015jcyjA90012) Scientific andTechnological Research Program of Chongqing Municipal

Education Commission (Grant no KJ1713327 and KJ1600532)Key Laboratory ofWell Stability and Fluid and RockMechanicsin Oil and Gas Reservoir of Shaanxi Province Xirsquoan ShiyouUniversity (Grant no FRM20190201002) Chongqing Post-graduate Research Innovation Project (Grant no CYS19353)and Chongqing University of Science and Technology GraduateStudentsrsquo Science and Technology Innovation Program (Grantno YKJCX1720601 and YKJCX1920613)

References

[1] F Kirsch and B Plabmann ldquoDynamic methods in pile testingdevelopments in measurement and analysisrdquo in Proceedings ofthe International Deep Foundations Congress 2002 pp 868ndash882 ASCE Orlando FL USA February 2002

[2] S H Ni L Lehmann J J Charng et al ldquoLow-strain integritytesting of drilled piles with high slenderness ratiordquo Computersand Geotechnics vol 33 no 6-7 pp 283ndash293 2006

[3] K Mori A Spagnoli Y Murakami G Kondo and I TorigoeldquoA new non-contacting non-destructive testing method fordefect detection in concreterdquo NDT amp E International vol 35no 6 pp 399ndash406 2002

[4] G Cai S Liu L Tong et al ldquoAssessment of direct CPT andCPTUmethods for predicting the ultimate bearing capacity ofsingle pilesrdquo Engineering Geology vol 104 no 3-4pp 211ndash222 2009

[5] X Y Bai M Y Zhang L Zhu et al ldquoIn-situ test and finiteelement analysis of bearing characteristics of rock-socketedshort piles in strongly weathered graniterdquo Journal of CentralSouth University Natural Science Edition vol 48 no 2pp 512ndash524 2017 in Chinese

[6] E C Leong and M F Randolph ldquoFinite element modelling ofrock-socketed pilesrdquo International Journal for Numerical andAnalytical Methods in Geomechanics vol 18 no 1 pp 25ndash471994

[7] G Gao M Gao Q Chen and J Yang ldquoField load testingstudy of vertical bearing behavior of a large diameter belledcast-in-place pilerdquo KSCE Journal of Civil Engineering vol 23no 5 pp 2009ndash2016 2019

[8] R Berardi and R Bovolenta ldquoPile-settlement evaluation usingfield stiffness non-linearityrdquo Proceedings of the Institution ofCivil Engineers - Geotechnical Engineering vol 158 no 1pp 35ndash44 2005

[9] B H Fellenius Basics of Foundation Design Lulu PressMorrisville NC USA 2016

[10] C Lam and S A Jefferis ldquoCritical assessment of pile modulusdetermination methodsrdquo Canadian Geotechnical Journalvol 48 no 10 pp 1433ndash1448 2011

[11] A Mohammadi T Ebadi and M R Boroomand ldquoPhysicalmodelling of axial compressive bearing capacity of instru-mented piles in oil-contaminated sandy soilrdquo Iranian Journalof Science and Technology Transactions of Civil Engineeringvol 44 no 2 pp 695ndash714 2019

[12] W D Guo and E H Ghee ldquoBehavior of axially loaded pilegroups subjected to lateral soil movementrdquo in Proceedings ofthe GeoShanghai International Conference 2006 pp 174ndash181ASCE Shanghai China June 2006

[13] Z Zhou Y Dong P Jiang D Han and T Liu ldquoCalculation ofpile side friction by multiparameter statistical analysisrdquo Ad-vances in Civil Engineering vol 2019 Article ID 263852012 pages 2019

14 Advances in Civil Engineering

[14] N M Hai and B H Fellenius ldquoO-Cell tests on two 70 m longbored piles in Vietnamrdquo in Proceedings of the Geo-Congress2014 pp 482ndash496 ASCE Atlanta Georgia February 2014

[15] S Gong G Cai S Liu et al ldquoNumerical Simulation of BearingCapacity and Consolidation Characteristics of PHC PileFoundationrdquo GeoShanghai International Conferencepp 178ndash185 Springer Singapore 2018

[16] M A Bashir H Furuuchi T Ueda and M Nauman BashirldquoNumerical simulation of axial anchorage capacity of con-crete-filled steel box footingrdquo Iranian Journal of Science andTechnology Transactions of Civil Engineering vol 40 no 3pp 257ndash262 2016

[17] N A Haskell ldquo+e dispersion of surface waves on multi-layered mediardquo Bulletin of the Seismological Society ofAmerica vol 43 no 1 pp 17ndash34 1953

[18] China Architecture amp Building PressNational Standard of thePeoplersquos Republic of China Code for Investigation of Geo-technical Engineering (GB50021 2016) China Architecture ampBuilding Press Beijing China 2016 in Chinese

[19] China Architecture amp Building Press Industry Standard of thePeoplersquos Republic of China Technical Code for Testing ofBuilding Foundation Piles (JGJ106-2014) China Architectureamp Building Press Beijing China 2014 in Chinese

[20] N Guo Z H Chen X F Huang et al ldquoResearch on the anti-pulling test of large-diameter cloth bag pile in soft rockfoundationrdquo Rock and Soil Mechanics vol 36 no 2pp 603ndash609 2015 in Chinese

[21] Y H Li X Zhu and T H Zhou ldquoField comparative ex-perimental study on the influence of post-tip grouting onlarge-diameter cast-in-situ pilesrdquo Rock and Soil Mechanicsvol 37 no 2 pp 388ndash396 2016 in Chinese

[22] G F Xin Z M Zhang T Xia et al ldquoExperimental study onbearing behavior of ultra-long pile under high loadrdquo ChineseJournal of Rock Mechanics and Engineering vol 13pp 2397ndash2402 2005 in Chinese

[23] Z Ling W Wang J Wu et al ldquoFull-scale loading test on pre-bored precast pile with enlarged base in Shanghairdquo in Pro-ceedings of the GeoShanghai 2018 International ConferenceAdvances in Soil Dynamics and Foundation Engineeringpp 637ndash645 Springer Singapore 2018

[24] J R Dong S T Lin and Y M Dai ldquo+e load tranfer behaviorof large diameter cast-in-situ pile in crushed pebble stratumrdquoChinese Journal of Geotechnical Engineering vol 16 no 6pp 123ndash131 1994

[25] X F Huang J H Zhang L G Ma et al ldquoExperimental studyon reinforcement effect of lateral resistance of collapsible loessrock-socketed perfusion pilerdquo Architectural Structure vol 41no 2 pp 351ndash355 2011 in Chinese

[26] J-J Zhou K-H Wang X-N Gong and R-H ZhangldquoBearing capacity and load transfer mechanism of a static drillrooted nodular pile in soft soil areasrdquo Journal of ZhejiangUniversity Science A vol 14 no 10 pp 705ndash719 2013

Advances in Civil Engineering 15

relative displacement increment is relatively small because theshaft skin resistance gradually mobilizes its function toprevent the pile from moving With the increase of loadingwhen the shaft skin resistance cannot resist the load at theloading sections 4800 kNndash5400 kN 4200 kNndash4800 kN8000 kNndash8800 kN and 3600 kNndash4000 kN the maximumdisplacements of pile tips of Nos 2 3 and 4 solid-bottom pileand the empty-bottom pile increase by 9792mm 7939mm9881mm and 1497mm respectively

+e shaft skin resistance increases with the increase ofpile-soil relative displacement Although continuous loadinghas led to foundation soil destruction it can be concludedfrom Figure 13 that the lateral resistance in the middle andlower parts is not fully mobilized and a certain pile-soilrelative displacement is needed to promote the full mobi-lization of the shaft skin resistance However according tothe development trend that the slope of the shaft skin re-sistance gradually decreases it can be inferred that the shaftskin resistance will reach the maximum value At the firstsection position the ultimate shaft skin resistance of pilesNos 1 2 3 and 4 are 4695 kPa 4390 kPa 6018 kPa and6305 kPa respectively and the corresponding pile-soil rel-ative displacements are 3419mm 3442mm 2633mm and3279mm In the second section the shaft skin resistance ofthe four piles increases greatly so does the pile-soil relativedisplacement Since the soil above the first section position isbasically cultivated soil and it can resist small shaft skinresistance while the content of gravel soil of the secondsection is large and with the increase of depth the sur-rounding pressure of soil around the pile increases thereforethe ability of resisting pile foundation sliding and dis-placement increases+erefore between the first section andthe second section the shaft skin resistance increases greatlywhile the pile-soil relative displacement increases a little Inthe second section and beyond the soil is gravel soil and theshaft skin resistance increases slightly with the increase ofpile depth and the relative displacement decreases gradually

44 Pile Tip Resistance Varies with Load and DisplacementDuring the test the section where the largest embeddeddepth of strain gauges is very close to the bottom of the pilewas shown so the resistance at the pile tip is approximatelyequal to the axial force measured at this section namelyQb Q0 Under the action of all loading steps the resistancevariation curves at the lower tips of the Nos 2 3 and 4 pilesare shown in Figure 14

+e No 2 and No 3 piles in the first two loading stepsand No 4 pile in the first loading step have smaller resistanceat the pile tip +e resistance at the pile tip gradually in-creases until the third and fourth loading steps Under theaction of ultimate load the ratio of pile tip resistance to piletop load of Nos 2 3 and 4 is 377 3008 and 3992respectively all higher 15 which shows that pile tip re-sistance has been brought into play to a certain extent +epile tip resistance is shared by shaft skin resistance and piletip resistance and the pile is of tip bearing friction pile

According to the relationship curve between pile tipresistance and pile tip displacement shown in Figure 15 it

can be seen that the pile tip resistance and pile tip dis-placement present a double fold line model of work hard-ening +e Nos 2 3 and 4 piles all show a slow deformationfailure and under the action of the first loading step theslope of the curve is larger and the corresponding dis-placement is relatively small in latter steps of loading thecurvature curve becomes smoother the displacement of thepile tips of adjacent loads increases and the soil at the piletips will gradually be compacted

Pile

tip

load

(kN

)

1000 2000 3000 4000 5000 6000 7000 8000 90000Load at pile top (kN)

0250500750

100012501500175020002250250027503000325035003750

No 2No 3No 4

Figure 14 Variation curve of pile tip resistance under differentloading steps (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No 4pile

Pile

tip

load

(kN

)

5 10 15 20 25 30 35 40 450Pile tip displacement (mm)

0

1000

2000

3000

4000

5000

6000

No 2No 3No 4

Figure 15 Relationship curve between pile tip resistance and piletip displacement (a) No 1 pile (b) No 2 pile (c) No 3 pile (d) No4 pile

Advances in Civil Engineering 13

5 Conclusions

Analyses are carried out on data of the in-situ test of unevengravel soil in Baishi Mountain and the following conclu-sions are drawn

(1) Based on the analysis of the results of multichanneltransient surface wave test superheavy (N120) conedynamic sounding test and on-site screening testthe geological environment conditions of the unevengravel soil on the site are further explored +e stratawithin the range from 0 to 15m in Baizhi Mountaincan be divided into four layers ie cultivated soilsandy soil silty clay and gravel soil with depths of1m 2m 3m and 9m respectively

(2) +e axial force and shaft skin resistance of pile bodyin gravel soil are the same as those in other geologicalstructures At the initial loading step the axial forceand shaft skin resistance of pile body play a smallerrole With the increase of loading steps the axialforce and shaft skin resistance of pile body increasecorrespondingly Under the same loading step theaxial force of the pile body and the shaft skin re-sistance do not mobilize in the same time andgradually transfer from the upper part to the lowerpart of the pile body and the upper part of the pilebody reaches the limit value before the middle andlower parts As for the solid-bottom pile the soil atthe pile tip will strengthen the shaft skin resistance ofthe pile

(3) According to this experimental study it is suggestedthat similar projects should adopt concrete manuallybored cast-in-place piles with a diameter of 900mmand a length of 15m the wall-protecting thicknessshall not be less than 175mm and the grade ofconcrete cannot be lower than C35 +e pile end canbe widened or not and the pile bottom shall be freeof sediment

Data Availability

All the data in this paper are obtained from tests in thisstudy and no other data were used to support this study

Conflicts of Interest

+e authors declare that with regard to the publication ofthis paper and the funding for publishing it there are noconflicts of interest

Acknowledgments

+is study was partially supported by the National NaturalScience Foundation of China (Grant no 41302223) ChongqingNo 3 colleges and universities youth backbone teachers fundingplans Chongqing Research Program of Basic Research andFrontier Technology (Grant no cstc2016jcyjA0074cstc2016jcyjA0933 and cstc2015jcyjA90012) Scientific andTechnological Research Program of Chongqing Municipal

Education Commission (Grant no KJ1713327 and KJ1600532)Key Laboratory ofWell Stability and Fluid and RockMechanicsin Oil and Gas Reservoir of Shaanxi Province Xirsquoan ShiyouUniversity (Grant no FRM20190201002) Chongqing Post-graduate Research Innovation Project (Grant no CYS19353)and Chongqing University of Science and Technology GraduateStudentsrsquo Science and Technology Innovation Program (Grantno YKJCX1720601 and YKJCX1920613)

References

[1] F Kirsch and B Plabmann ldquoDynamic methods in pile testingdevelopments in measurement and analysisrdquo in Proceedings ofthe International Deep Foundations Congress 2002 pp 868ndash882 ASCE Orlando FL USA February 2002

[2] S H Ni L Lehmann J J Charng et al ldquoLow-strain integritytesting of drilled piles with high slenderness ratiordquo Computersand Geotechnics vol 33 no 6-7 pp 283ndash293 2006

[3] K Mori A Spagnoli Y Murakami G Kondo and I TorigoeldquoA new non-contacting non-destructive testing method fordefect detection in concreterdquo NDT amp E International vol 35no 6 pp 399ndash406 2002

[4] G Cai S Liu L Tong et al ldquoAssessment of direct CPT andCPTUmethods for predicting the ultimate bearing capacity ofsingle pilesrdquo Engineering Geology vol 104 no 3-4pp 211ndash222 2009

[5] X Y Bai M Y Zhang L Zhu et al ldquoIn-situ test and finiteelement analysis of bearing characteristics of rock-socketedshort piles in strongly weathered graniterdquo Journal of CentralSouth University Natural Science Edition vol 48 no 2pp 512ndash524 2017 in Chinese

[6] E C Leong and M F Randolph ldquoFinite element modelling ofrock-socketed pilesrdquo International Journal for Numerical andAnalytical Methods in Geomechanics vol 18 no 1 pp 25ndash471994

[7] G Gao M Gao Q Chen and J Yang ldquoField load testingstudy of vertical bearing behavior of a large diameter belledcast-in-place pilerdquo KSCE Journal of Civil Engineering vol 23no 5 pp 2009ndash2016 2019

[8] R Berardi and R Bovolenta ldquoPile-settlement evaluation usingfield stiffness non-linearityrdquo Proceedings of the Institution ofCivil Engineers - Geotechnical Engineering vol 158 no 1pp 35ndash44 2005

[9] B H Fellenius Basics of Foundation Design Lulu PressMorrisville NC USA 2016

[10] C Lam and S A Jefferis ldquoCritical assessment of pile modulusdetermination methodsrdquo Canadian Geotechnical Journalvol 48 no 10 pp 1433ndash1448 2011

[11] A Mohammadi T Ebadi and M R Boroomand ldquoPhysicalmodelling of axial compressive bearing capacity of instru-mented piles in oil-contaminated sandy soilrdquo Iranian Journalof Science and Technology Transactions of Civil Engineeringvol 44 no 2 pp 695ndash714 2019

[12] W D Guo and E H Ghee ldquoBehavior of axially loaded pilegroups subjected to lateral soil movementrdquo in Proceedings ofthe GeoShanghai International Conference 2006 pp 174ndash181ASCE Shanghai China June 2006

[13] Z Zhou Y Dong P Jiang D Han and T Liu ldquoCalculation ofpile side friction by multiparameter statistical analysisrdquo Ad-vances in Civil Engineering vol 2019 Article ID 263852012 pages 2019

14 Advances in Civil Engineering

[14] N M Hai and B H Fellenius ldquoO-Cell tests on two 70 m longbored piles in Vietnamrdquo in Proceedings of the Geo-Congress2014 pp 482ndash496 ASCE Atlanta Georgia February 2014

[15] S Gong G Cai S Liu et al ldquoNumerical Simulation of BearingCapacity and Consolidation Characteristics of PHC PileFoundationrdquo GeoShanghai International Conferencepp 178ndash185 Springer Singapore 2018

[16] M A Bashir H Furuuchi T Ueda and M Nauman BashirldquoNumerical simulation of axial anchorage capacity of con-crete-filled steel box footingrdquo Iranian Journal of Science andTechnology Transactions of Civil Engineering vol 40 no 3pp 257ndash262 2016

[17] N A Haskell ldquo+e dispersion of surface waves on multi-layered mediardquo Bulletin of the Seismological Society ofAmerica vol 43 no 1 pp 17ndash34 1953

[18] China Architecture amp Building PressNational Standard of thePeoplersquos Republic of China Code for Investigation of Geo-technical Engineering (GB50021 2016) China Architecture ampBuilding Press Beijing China 2016 in Chinese

[19] China Architecture amp Building Press Industry Standard of thePeoplersquos Republic of China Technical Code for Testing ofBuilding Foundation Piles (JGJ106-2014) China Architectureamp Building Press Beijing China 2014 in Chinese

[20] N Guo Z H Chen X F Huang et al ldquoResearch on the anti-pulling test of large-diameter cloth bag pile in soft rockfoundationrdquo Rock and Soil Mechanics vol 36 no 2pp 603ndash609 2015 in Chinese

[21] Y H Li X Zhu and T H Zhou ldquoField comparative ex-perimental study on the influence of post-tip grouting onlarge-diameter cast-in-situ pilesrdquo Rock and Soil Mechanicsvol 37 no 2 pp 388ndash396 2016 in Chinese

[22] G F Xin Z M Zhang T Xia et al ldquoExperimental study onbearing behavior of ultra-long pile under high loadrdquo ChineseJournal of Rock Mechanics and Engineering vol 13pp 2397ndash2402 2005 in Chinese

[23] Z Ling W Wang J Wu et al ldquoFull-scale loading test on pre-bored precast pile with enlarged base in Shanghairdquo in Pro-ceedings of the GeoShanghai 2018 International ConferenceAdvances in Soil Dynamics and Foundation Engineeringpp 637ndash645 Springer Singapore 2018

[24] J R Dong S T Lin and Y M Dai ldquo+e load tranfer behaviorof large diameter cast-in-situ pile in crushed pebble stratumrdquoChinese Journal of Geotechnical Engineering vol 16 no 6pp 123ndash131 1994

[25] X F Huang J H Zhang L G Ma et al ldquoExperimental studyon reinforcement effect of lateral resistance of collapsible loessrock-socketed perfusion pilerdquo Architectural Structure vol 41no 2 pp 351ndash355 2011 in Chinese

[26] J-J Zhou K-H Wang X-N Gong and R-H ZhangldquoBearing capacity and load transfer mechanism of a static drillrooted nodular pile in soft soil areasrdquo Journal of ZhejiangUniversity Science A vol 14 no 10 pp 705ndash719 2013

Advances in Civil Engineering 15

5 Conclusions

Analyses are carried out on data of the in-situ test of unevengravel soil in Baishi Mountain and the following conclu-sions are drawn

(1) Based on the analysis of the results of multichanneltransient surface wave test superheavy (N120) conedynamic sounding test and on-site screening testthe geological environment conditions of the unevengravel soil on the site are further explored +e stratawithin the range from 0 to 15m in Baizhi Mountaincan be divided into four layers ie cultivated soilsandy soil silty clay and gravel soil with depths of1m 2m 3m and 9m respectively

(2) +e axial force and shaft skin resistance of pile bodyin gravel soil are the same as those in other geologicalstructures At the initial loading step the axial forceand shaft skin resistance of pile body play a smallerrole With the increase of loading steps the axialforce and shaft skin resistance of pile body increasecorrespondingly Under the same loading step theaxial force of the pile body and the shaft skin re-sistance do not mobilize in the same time andgradually transfer from the upper part to the lowerpart of the pile body and the upper part of the pilebody reaches the limit value before the middle andlower parts As for the solid-bottom pile the soil atthe pile tip will strengthen the shaft skin resistance ofthe pile

(3) According to this experimental study it is suggestedthat similar projects should adopt concrete manuallybored cast-in-place piles with a diameter of 900mmand a length of 15m the wall-protecting thicknessshall not be less than 175mm and the grade ofconcrete cannot be lower than C35 +e pile end canbe widened or not and the pile bottom shall be freeof sediment

Data Availability

All the data in this paper are obtained from tests in thisstudy and no other data were used to support this study

Conflicts of Interest

+e authors declare that with regard to the publication ofthis paper and the funding for publishing it there are noconflicts of interest

Acknowledgments

+is study was partially supported by the National NaturalScience Foundation of China (Grant no 41302223) ChongqingNo 3 colleges and universities youth backbone teachers fundingplans Chongqing Research Program of Basic Research andFrontier Technology (Grant no cstc2016jcyjA0074cstc2016jcyjA0933 and cstc2015jcyjA90012) Scientific andTechnological Research Program of Chongqing Municipal

Education Commission (Grant no KJ1713327 and KJ1600532)Key Laboratory ofWell Stability and Fluid and RockMechanicsin Oil and Gas Reservoir of Shaanxi Province Xirsquoan ShiyouUniversity (Grant no FRM20190201002) Chongqing Post-graduate Research Innovation Project (Grant no CYS19353)and Chongqing University of Science and Technology GraduateStudentsrsquo Science and Technology Innovation Program (Grantno YKJCX1720601 and YKJCX1920613)

References

[1] F Kirsch and B Plabmann ldquoDynamic methods in pile testingdevelopments in measurement and analysisrdquo in Proceedings ofthe International Deep Foundations Congress 2002 pp 868ndash882 ASCE Orlando FL USA February 2002

[2] S H Ni L Lehmann J J Charng et al ldquoLow-strain integritytesting of drilled piles with high slenderness ratiordquo Computersand Geotechnics vol 33 no 6-7 pp 283ndash293 2006

[3] K Mori A Spagnoli Y Murakami G Kondo and I TorigoeldquoA new non-contacting non-destructive testing method fordefect detection in concreterdquo NDT amp E International vol 35no 6 pp 399ndash406 2002

[4] G Cai S Liu L Tong et al ldquoAssessment of direct CPT andCPTUmethods for predicting the ultimate bearing capacity ofsingle pilesrdquo Engineering Geology vol 104 no 3-4pp 211ndash222 2009

[5] X Y Bai M Y Zhang L Zhu et al ldquoIn-situ test and finiteelement analysis of bearing characteristics of rock-socketedshort piles in strongly weathered graniterdquo Journal of CentralSouth University Natural Science Edition vol 48 no 2pp 512ndash524 2017 in Chinese

[6] E C Leong and M F Randolph ldquoFinite element modelling ofrock-socketed pilesrdquo International Journal for Numerical andAnalytical Methods in Geomechanics vol 18 no 1 pp 25ndash471994

[7] G Gao M Gao Q Chen and J Yang ldquoField load testingstudy of vertical bearing behavior of a large diameter belledcast-in-place pilerdquo KSCE Journal of Civil Engineering vol 23no 5 pp 2009ndash2016 2019

[8] R Berardi and R Bovolenta ldquoPile-settlement evaluation usingfield stiffness non-linearityrdquo Proceedings of the Institution ofCivil Engineers - Geotechnical Engineering vol 158 no 1pp 35ndash44 2005

[9] B H Fellenius Basics of Foundation Design Lulu PressMorrisville NC USA 2016

[10] C Lam and S A Jefferis ldquoCritical assessment of pile modulusdetermination methodsrdquo Canadian Geotechnical Journalvol 48 no 10 pp 1433ndash1448 2011

[11] A Mohammadi T Ebadi and M R Boroomand ldquoPhysicalmodelling of axial compressive bearing capacity of instru-mented piles in oil-contaminated sandy soilrdquo Iranian Journalof Science and Technology Transactions of Civil Engineeringvol 44 no 2 pp 695ndash714 2019

[12] W D Guo and E H Ghee ldquoBehavior of axially loaded pilegroups subjected to lateral soil movementrdquo in Proceedings ofthe GeoShanghai International Conference 2006 pp 174ndash181ASCE Shanghai China June 2006

[13] Z Zhou Y Dong P Jiang D Han and T Liu ldquoCalculation ofpile side friction by multiparameter statistical analysisrdquo Ad-vances in Civil Engineering vol 2019 Article ID 263852012 pages 2019

14 Advances in Civil Engineering

[14] N M Hai and B H Fellenius ldquoO-Cell tests on two 70 m longbored piles in Vietnamrdquo in Proceedings of the Geo-Congress2014 pp 482ndash496 ASCE Atlanta Georgia February 2014

[15] S Gong G Cai S Liu et al ldquoNumerical Simulation of BearingCapacity and Consolidation Characteristics of PHC PileFoundationrdquo GeoShanghai International Conferencepp 178ndash185 Springer Singapore 2018

[16] M A Bashir H Furuuchi T Ueda and M Nauman BashirldquoNumerical simulation of axial anchorage capacity of con-crete-filled steel box footingrdquo Iranian Journal of Science andTechnology Transactions of Civil Engineering vol 40 no 3pp 257ndash262 2016

[17] N A Haskell ldquo+e dispersion of surface waves on multi-layered mediardquo Bulletin of the Seismological Society ofAmerica vol 43 no 1 pp 17ndash34 1953

[18] China Architecture amp Building PressNational Standard of thePeoplersquos Republic of China Code for Investigation of Geo-technical Engineering (GB50021 2016) China Architecture ampBuilding Press Beijing China 2016 in Chinese

[19] China Architecture amp Building Press Industry Standard of thePeoplersquos Republic of China Technical Code for Testing ofBuilding Foundation Piles (JGJ106-2014) China Architectureamp Building Press Beijing China 2014 in Chinese

[20] N Guo Z H Chen X F Huang et al ldquoResearch on the anti-pulling test of large-diameter cloth bag pile in soft rockfoundationrdquo Rock and Soil Mechanics vol 36 no 2pp 603ndash609 2015 in Chinese

[21] Y H Li X Zhu and T H Zhou ldquoField comparative ex-perimental study on the influence of post-tip grouting onlarge-diameter cast-in-situ pilesrdquo Rock and Soil Mechanicsvol 37 no 2 pp 388ndash396 2016 in Chinese

[22] G F Xin Z M Zhang T Xia et al ldquoExperimental study onbearing behavior of ultra-long pile under high loadrdquo ChineseJournal of Rock Mechanics and Engineering vol 13pp 2397ndash2402 2005 in Chinese

[23] Z Ling W Wang J Wu et al ldquoFull-scale loading test on pre-bored precast pile with enlarged base in Shanghairdquo in Pro-ceedings of the GeoShanghai 2018 International ConferenceAdvances in Soil Dynamics and Foundation Engineeringpp 637ndash645 Springer Singapore 2018

[24] J R Dong S T Lin and Y M Dai ldquo+e load tranfer behaviorof large diameter cast-in-situ pile in crushed pebble stratumrdquoChinese Journal of Geotechnical Engineering vol 16 no 6pp 123ndash131 1994

[25] X F Huang J H Zhang L G Ma et al ldquoExperimental studyon reinforcement effect of lateral resistance of collapsible loessrock-socketed perfusion pilerdquo Architectural Structure vol 41no 2 pp 351ndash355 2011 in Chinese

[26] J-J Zhou K-H Wang X-N Gong and R-H ZhangldquoBearing capacity and load transfer mechanism of a static drillrooted nodular pile in soft soil areasrdquo Journal of ZhejiangUniversity Science A vol 14 no 10 pp 705ndash719 2013

Advances in Civil Engineering 15

[14] N M Hai and B H Fellenius ldquoO-Cell tests on two 70 m longbored piles in Vietnamrdquo in Proceedings of the Geo-Congress2014 pp 482ndash496 ASCE Atlanta Georgia February 2014

[15] S Gong G Cai S Liu et al ldquoNumerical Simulation of BearingCapacity and Consolidation Characteristics of PHC PileFoundationrdquo GeoShanghai International Conferencepp 178ndash185 Springer Singapore 2018

[16] M A Bashir H Furuuchi T Ueda and M Nauman BashirldquoNumerical simulation of axial anchorage capacity of con-crete-filled steel box footingrdquo Iranian Journal of Science andTechnology Transactions of Civil Engineering vol 40 no 3pp 257ndash262 2016

[17] N A Haskell ldquo+e dispersion of surface waves on multi-layered mediardquo Bulletin of the Seismological Society ofAmerica vol 43 no 1 pp 17ndash34 1953

[18] China Architecture amp Building PressNational Standard of thePeoplersquos Republic of China Code for Investigation of Geo-technical Engineering (GB50021 2016) China Architecture ampBuilding Press Beijing China 2016 in Chinese

[19] China Architecture amp Building Press Industry Standard of thePeoplersquos Republic of China Technical Code for Testing ofBuilding Foundation Piles (JGJ106-2014) China Architectureamp Building Press Beijing China 2014 in Chinese

[20] N Guo Z H Chen X F Huang et al ldquoResearch on the anti-pulling test of large-diameter cloth bag pile in soft rockfoundationrdquo Rock and Soil Mechanics vol 36 no 2pp 603ndash609 2015 in Chinese

[21] Y H Li X Zhu and T H Zhou ldquoField comparative ex-perimental study on the influence of post-tip grouting onlarge-diameter cast-in-situ pilesrdquo Rock and Soil Mechanicsvol 37 no 2 pp 388ndash396 2016 in Chinese

[22] G F Xin Z M Zhang T Xia et al ldquoExperimental study onbearing behavior of ultra-long pile under high loadrdquo ChineseJournal of Rock Mechanics and Engineering vol 13pp 2397ndash2402 2005 in Chinese

[23] Z Ling W Wang J Wu et al ldquoFull-scale loading test on pre-bored precast pile with enlarged base in Shanghairdquo in Pro-ceedings of the GeoShanghai 2018 International ConferenceAdvances in Soil Dynamics and Foundation Engineeringpp 637ndash645 Springer Singapore 2018

[24] J R Dong S T Lin and Y M Dai ldquo+e load tranfer behaviorof large diameter cast-in-situ pile in crushed pebble stratumrdquoChinese Journal of Geotechnical Engineering vol 16 no 6pp 123ndash131 1994

[25] X F Huang J H Zhang L G Ma et al ldquoExperimental studyon reinforcement effect of lateral resistance of collapsible loessrock-socketed perfusion pilerdquo Architectural Structure vol 41no 2 pp 351ndash355 2011 in Chinese

[26] J-J Zhou K-H Wang X-N Gong and R-H ZhangldquoBearing capacity and load transfer mechanism of a static drillrooted nodular pile in soft soil areasrdquo Journal of ZhejiangUniversity Science A vol 14 no 10 pp 705ndash719 2013

Advances in Civil Engineering 15