behavior of grouted single screw piles under inclined tensile

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Behavior of Grouted Single Screw Piles under Inclined Tensile Loads in Sand

Mohamed A. Sakr Professor of Geotechnical Engineering,

Faculty of Engineering, Tanta University, EGYPT; e-mail: [email protected]

Ashraf K. Nazir

Professor and Director, Geotechnical Engineering Research Laboratory, Faculty of Engineering, Tanta University, EGYPT; e-mail: [email protected]

Waseim R. Azzam Assistant Professor of Geotechnical Engineering, Faculty of Engineering, Tanta

University, EGYPT. e-mail: [email protected]

Ahmed F. Sallam 4Demonstrator of Geotechnical Engineering, Faculty of Engineering, Tanta

University, EGYPT. e-mail: [email protected]

ABSTRACT In this paper, the inclined tensile load capacities of grouted screw piles under constant pressure of

grouting were examined through an experimental investigation that carried on model of screw

piles embedded in dry sand. A total number of sixty models grouted/un grouted screw piles load

tests were carried out. Tests were performed at fixed helix spacing, embedment depth, number of

helices and helix diameter. The inclined load was applied at constant eccentricity above the soil

surface, with different inclination angles varying from 0º to 90º measured from horizontal level for

the point of applied load.The effect of grout bulbs which were carried on screw piles at different

embedment depths below the soil surface was studied. The inclined load capacity of grouted screw

piles were compared with normal screw piles without grouting. The test results showed that the

ultimate capacity for screw pile increases with the increase of sand relative density. While, For the

same grout embedment ratio the efficiency of improvement for ultimate capacity increases as the

sand relative densities increase. At the embedment ratio of (Z/dp = 4) the improvement in the

ultimate load capacity reached to 3, 2.6 and 2 times of normal screw piles without grout at sand

density of (80, 50 and 30%) respectively in case of horizontal loads. It is also found that the soil

between the grouted bulb and the first helix behaves like an embedded shaft that increases the

friction resistance as a result it improves the screw pile capacity.

mailto:[email protected]

Vol. 21 [2016], Bund. 2 572

KEYWORDS: grouted screw pile; inclined load; efficiency of improvement and relative density.

INTRODUCTION Screw piles have been widely used to resist tensile loads in supporting structures such a guyed towers, transmission towers, buried pipelines, retaining wall systems, etc. Screw piles consisting of a circular / square steel tube with one or more helical blades. Small diameter multi-helix screw piles or screw micropiles with one or more helical bearing plates have also many applications, providing the adequate safety against axial compression, uplift and/or lateral loadings. The performance of screw piles has been investigated in several studies including ( Mooney et al. 1985; Mitsch and Clemence 1985 ; Hoyt and Clemence 1989; Ghaly et al. 1991 ; Rao et al. 1991; Rao and Prasad 1993 ; El Naggar and Abdelghany ( 2007a,b ) ; Livneh and El Naggar 2008 ; Sakr 2009 and Abdelghany and El Naggar 2010). An empirical correlation of ultimate axial load with the installation torque of the screw pile is referred by Hoyt and Clemence 1989. Ghaly et al. 1991 have suggested that for a single screw anchors, the failure surface be determined by the ultimate pull-out load, and assumed that the form of a truncated cone with a taper angle (θ = 2/3 ϕ). Sakr 2009 observed that for oil sands, the individual bearing method is more suitable for ultimate capacity calculations. The mechanism of cylindrical shear was suggested by Rao et al. 1991 for clayey soils and screw piles with two or more blades. According to the authors, this mechanism is clearly developed for a ratio (S/dh ≤ 1.5–2.0), where (S) is equal to the vertical distance of the plates and dh is the diameter of the helical plate. El Naggar and Abdelghany (2007a,b) found that for helical piles in clay the load is transferred through a tapered cylindrical shear surface and bearing underneath the lead helix for a spacing ratio (S/dh of 3). Mitsch and Clemence 1985 showed that the coefficient of lateral pressure of the soil is a function of the ratio of the height to the diameter (H/D), and sand relative density. Prasad and Rao 1996 performed laboratory tests on helical piles with 2 and 4 helices and plain shaft, embedded in saturated clay. It was concluded that the lateral capacity of helical piles increases from 1.2 to 1.4 times that of plain shaft pile and it increases with number of helical plates and embedment length. Satyendra. et al., 2010, studied experimentally the behavior of screw piles under lateral loads. It is observed that, the ultimate lateral capacity of piles increases by the increase of the embedment ratio and decreases by the increase of load eccentricity above the surface ground. Gahly and Celemence 1998, studied experimentally and theoretically the pullout performance of inclined screw piles. It is observed that, the pullout capacity of inclined helical screw anchors depends on the installation depth, relative depth ratio, sand characteristics and the inclination angle. On the other hand the grouting technique with helical piles was used to improve its performance in the form of helical pull down micropiles or grouted-screw piles. Screw piles installed with a grout column surrounding the central shaft along the extensions as stated by Vickers and Clemence 2000. This implemented technique was prevented the buckling of long piles and increase both the capacity and resistance to corrosion. In this way Bian et al. 2006 conducted a series of tests on model piers in dry sands using different pier-grout installation schemes to form grout bulbs around the tested pier shafts. It is observed that, additional sets of grout holes aligned on the pier shaft proved unnecessary and the largest grout bulbs were approximately eight times that of the pier shaft diameter. Yasser (2008), Abdelghany and El Naggar (2007a and b), studied several modifications for the grouted helical screw piles (G-HSP) installed in clayey soils. These modifications include, enhanced grout mix, using fiber reinforcement in the grout mix and encasing part of the grout column with relatively rigid fiber reinforced polymer tubes. The results

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indicated that in all cases, the axial capacity increased compared to the plain helical pile and the cyclic performance is satisfactory. Abdelghany and El Naggar 2014, presented experimental and theoretical studies on Composite Helical Screw Piles under Axial, Lateral Monotonic and Cyclic Loadings. It is observed that The RG-HSP piles axial capacity was more than twice that for P-HSP, with minimal reduction after cyclic loading, and their lateral capacity was more than 3 times the P-HSPs capacity.

Most of paper in literature deals with studying the behavior of single screw piles in sand under different load condition .While, minor of the researchers using injection of grouting beneath of helixes to improve the capacity and performance of screw piles as discussed above. Based on the paper in literature, it has been found that the screw pile has small axial stiffness and low lateral resistance. Therefore, the present research aims to investigate an alternative technique to improve the ultimate capacity of single screw piles under tensile inclined loads in dry sand using injection of grouting at a different zone in the top surface of the screw pile.

EXPERIMENTAL WORK

Experimental set up

A cylindrical test tank with 950.0 mm diameter, 900.0 mm height and is of 3 mm thick which is sufficiently thick and rigid enough to prevent any lateral deformation of the side walls was used to conduct the experimental investigation. The internal dimensions of the tank were taken so as to eliminate the boundary effect. According to Satyendra et al. (2010) the plan dimensions for the tank to avoid boundary effect which was considered to be minimal of three times the diameter of the helix. The diameter of the test pile is 21 mm with helix diameter of 60 mm. An electric motor is fixed on the top of the loading frame to create the torque required to push the piles into the soil. Three smooth pulleys system and wire are used to apply the inclined load on the piles. The axial vertical and horizontal displacement is measured using 0.01 mm accuracy dial gauge. The experimental set up is shown in Fig. 1.

Pressurized grout chamber A grout delivery mechanism was designed to provide variable pressure control and easy

filling of model pile-soil system .The injection technique of grout was done through four nozzles with diameter of 5mm on the tested model piles and located at studied depth from the ground surface. The grout was injected inside the screw pipe piles and dissipated to the surrounding soil through these nozzles which located at the same level. Tests were done at different pressure of injection (2, 4 and 5 bars). It has been found that the used grout pressure of 5 bars made the soil collapsed and raised above. So, A system to fix the soil was used however, big cracks appeared and the diameter of grout bulb was very big compared with the diameter of the central shaft. Finally, at pressurized grout 2 bar the diameter of grout bulb was suitable and the soil doesn't collapse. Therefore, the appropriate used pressure of the grout is 1.5 bars during all test series. The elements of pressurized grout chamber included a compressor with a pressurized air valve, tank under pressure, motor gear for flipping the grout, funnel for pouring the grout, two pieces of pressure valve and pressure gauge to control the pressure inside the tank and prevent the internal pressure from exceeding the gage limit. Two-pieces of pressure hose one of them for transporting the air from compressor to the tank and another piece to transport the grout from the tank to the pile. Fig 2 shows the grout pump system.

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Model of screw pile Mild steel pipes of 21/16 mm outer/inner diameter with total length of 700 mm are used as

model piles in this study. In order to form the helixes, two steel circular plates 2.0 mm thickness with 60 mm diameter, spacing ratio of dh/dp equal 3 are welded to the hollow steel pipe at specified locations and directions. Fig. 3 shows the model pile.

Vertical ribs Inverter

Soil tank

Steel wire

Moving cart

Lever

Load hanger

Compresso

Funnel for pouring grout

Motor Gear for

Tank under pressure

Pressure

Pressure

Pressure

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Figure 1: The experimental set up and the grout pump station.

Tested sand The sand used in this paper is rated as poorly graded (SP) according to the Unified Soil

Classification System. The specific gravity of the soil particles was determined by the jar method. Three tests were carried out, producing an average value of 2.63. The maximum and minimum dry unit weights of the sand were found to be 18.63 and 14.70 kN/m3 respectively, and corresponding values of minimum an d maximum void ratios were 0.4117 and 0.789 respectively. The particle size distribution was determined using the dry sieving method according to ASTM D 422 – 63 (2007), and the results are shown in Fig. 4. The geotechnical properties of the used sand are given in Table 1.

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Figure 3: Model of tested screw piles before and after injection

Figure 4: Grain size distribution of the tested sand in the present investigation

Table 1: The physical and mechanical properties of the used sand Properties Value

Maximum unit weight, kN/m3 18.63

Mimimum unit weight, kN/m3 14.70

Specific gravity , Gs 2.63

The effective size ( D10), mm 0.35

Uniformity coefficient (Cu) 2.29

Coefficient of curvature (Cc) 0.89

0

10

20

30

40

50

60

70

80

90

100

0.00010.0010.010.1110100

Per

cent

age

of p

assi

ng b

y W

t%

Diameter in mm

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Maximum angle of internal friction, φ̊ 42.9

Mimimum angle of internal friction, φ̊ 33.7

Dense sand properties

Unit weight, kN/m3 17.68

Relative density, 80%

Angle of internal friction, φ̊ 40.2

Medium sand properties


Relative density, 50 %


Loose sand properties


Relative density, 30 %


Grouting mix properties

The grout was delivered through the model helical piles into dry sand. The water is mixed with the cement to form the grout. The adopted water cement ratio (Ww/C) is 0.40. Water reducing additive is superplasticizer (SIKAMENT 163 M) which is approximately 1% by weight of cement. This ratio was taken from different experimental mixes, which provided acceptable grouting viscosity and strength. The chosen of water cement ratio (W/C) and superplasticizer make a grout has a low bleeding and having a viscosity permitting the grout easy to penetrate. The ratio of the exit hole diameter (De) in the central shaft to the mean grain size (D50) of the ultrafine cement (De/D50) is 143. The larger this ratio is the less likely clogging at the exit hole will occur Bian et al (2006). A series of compression strength tests were conducted on samples at ages 3, 7 and 28 days. Three cubes of each group were prepared by mixing 1% of superplasticizer (SIKAMENT 163 M). The friction angle (φ) of the grout-sand interface for tested grout after 7 days of casting were determined from direct shear box testes at different normal stresses. The chemical analysis of the tested cement and the physical properties of the used grouting are summarized in Table 2(A&B).

Table 2A: The chemical analysis of the used cement in this research

SiO2 AL2O3 Fe2O3 CaO MgO Na2O K2O CL SO3 Loss Total

0.8 .26 66 3.22 6 44 34 .001 29 .39 00

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Table 2B: The compressive and the shear strength results of the used grouting Test type Cube 1 Cube 2 Cube 3 Mean

Strength Compression Strength after 3 days (MPa) 53.0 50.9 51 51.6

Compression Strength after 7 days (MPa) 69.3 66.2 66.6 67.4

Compression Strength after 28 days (MPa) 79.1 76.7 78.5 78.1

Friction angle (φ) between Grout- Dense sand 46.5º

Friction angle (φ) between Grout- Medium sand 41.8º

Friction angle (φ) between grout- Loose sand 38.5º

EXPERIMENTAL TESTING PROGRAM An experimental testing program was carried out to study the behavior of grouted screw piles

under inclined tensile loads constructed in dry sand have different relative density. The sand is filled in layers of 50 mm thickness and compacted to the pre-required density with total height of 800 mm. The total height of sand was chosen in such a way that the sand bed extends to 100 mm under the model screw pile. The model piles were screwed to the required pre-determined level using the mechanical motor and adjusted to be in exact vertical position. Loading frame consisted of horizontal steel angle attached to a strong vertical fixed frame. Two pulleys were attached to the horizontal angle and vertical fixed frame. Steel wire running over the pulleys was used to apply inclined tensile loads by placing the cast iron weights over the load hanger as shown in Fig. 5. The load was applied in small increments. Each load increment was maintained at a constant value until the pile deformation has been stabilized. The deformation of the pile head was measured using 25 mm travel digital dial gauge with accuracy of 0.01 mm. Two dial gauges were used to measure the axial vertical and horizontal displacement. A total number of sixty tests were conducted on screw anchor piles at different grout embedment ratios (Z/dp, where Z is grout embedment depth and dp is the diameter of pile shaft).The tested values of Z/dp are 4, 6, 8 and 10 which shown in Fig. 6. The tests were carried out at different sand relative densities of 30, 50 and 80 %. The inclination angles measured from the horizontal level are equal to 0º , 30º ,60º , and 90.0º.The eccentricity is taken as a ratio (E/dp), (where E the vertical distance of point of application to the top surface of the soil) . E is constant during all tests and taken E = 4dp.

For comparison twelve tests were conducted on normal screw piles without grouting to study the influence of grouted screw piles on the inclined tensile capacity under different parameters. These parameters are, sand relative density (Dr), grout embedment ratio (Z/dp) and inclination angle (θ). All tests were done at constant depth ratio; (L/dp) is equal 30. Table 3 shows details of the experimental testing program and the test results are summarized in Table 4.

In order to study the effect of the time to start loading after injection of grout, a series of uplift load were done after 2, 3, 7 and 28 days from injection. It is noticed that ultimate capacity of screw grouted piles remain almost constant for all tested times. Therefore the time to start the test after injection is insignificant. Because the grout soil interaction is gained a hole strength initially after setting time of 7hours. So, in all tested groups the piles were loaded after 3 days of injection.

Based on experimental tests, it is found that at Z/dp =2 ,the injection of the grout is insignificant because the overburden pressure at this depth is not enough to cause soil stability . Pressurized grout when installed at depth of Z/dp =2 , the soil particle is disturbed and the grout

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Steel wire

Protractor

Pulley

Pulley

bulb is very small comparing with the grout bulb at Z/dp =4.So the suitable depth for starting tests is at Z/dp =4. This depth can significantly help in formation of grout bulb at the top surface of the ground.

Figure 5: The displacement measurements for inclined load

L shape plate

Vertical dial gauge

Horizontal dial gauge

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Figure 6: The model tested pile

Table 3: Experimental testing program

Series Constant parameters Variable parameters 1 Dr = 30% , θ =90º Z/dp= 0.0(no grout), 4, 6 , 8 and 10

2 Dr = 50% , θ =90º Z/dp = 0.0(no grout), 4, 6 , 8 and 10




6 Dr = 80%, , θ =60º Z/dp = 0.0(no grout), 4, 6 , 8 and 10



9 Dr = 80%, θ =30º Z/dp = 0.0(no grout), 4, 6 , 8 and 10

10 Dr = 30%, , θ =0.0º Z/dp = 0.0(no grout), 4, 6 , 8 and 10



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Notes: All tests were conducted at constant of depth ratio, (L/dp) is equal 30, No. of helix=2 , H/dh=7.5 , S/dh=1.5, E/dp =4 . Dr relative density of sand, H embedded depth of pile up to the top helix, dp diameter of pile shaft, dh helix diameter, S helix spacing , θ angle of applied load measured from the horizontal level and E the vertical distance of point of application to the top surface of soil.

Table 4: Summary of the ultimate inclined load (Qult) for grouted screw anchor pile with diameter of helical plate, dh= 60 mm

Dr % Ultimate inclined load Qult (N) Angle= 0.0º Lateral tensile loads Angle =30.0º Inclined tensile loads Grout embedment ratio (Z/dp) Grout embedment ratio (Z/dp) 0 4 6 8 10 0 4 6 8 10

Loose=30% 200 400 350 300 250 212.5 375 325 300 262.5 Medium=50% 250 650 500 450 350 350 750 650 550 450 Dense =80% 300 900 750 600 450 425 1050 900 750 575 Dr % Ultimate inclined load Qult (N)

Angle=60.0º Inclined tensile loads Angle =90.0º Uplift loads Grout embedment ratio (Z/dp) Grout embedment ratio (Z/dp) 0 4 6 8 10 0 4 6 8 10

Loose=30% 225 412.5 337.5 312.5 275 225 400 375 300 250 Medium=50% 400 775 700 625 525 462.5 850 775 675 575 Dense =80% 500 1150 1000 825 675 650 1275 1150 1000 850

TEST RESULTS AND DISCUSSION

Load- displacement curves.

The behavior of grouted screw piles can better be assessed with the help of the results obtained from the Load-displacement curves for screw anchor piles at different grout embedment ratio. Due to the limit space, some of the load -displacement curves are exhibited as shown in Figs.(7 to 10). The displacement (S) of the helical pile is expressed in non-dimensional form in terms of screw pile diameter (dp) as percentage ratio (S/dp, %).

First, the definition of the failure load to obtain the ultimate lateral capacity of the pile is considered as the point which the load- displacement curve becomes linear (Satyendra, et- al 2010 and Prasad, et al 1996).

The existence of grouted bulb can significantly modify the load displacement behavior of screw pile and improve the ultimate load capacity in accordance of embedment depth and sand density. Fig.7 presents typical load- displacement curves for screw anchor piles at relative density 80% and inclination angle measured from the horizontal level is 0.0º. The corresponding capacities were 900, 750, 600 and 450 N for grouted screw pile at (Z/dp) ratio of 4, 6 ,8 and 10 respectively. While, this value is 300 N for normal screw pile without grout. It has been clearly observed that the ultimate lateral capacity of the pile increases with the decrease of the grout embedment ratios (Z/d), however the normal screw pile without grout has the least lateral capacity. For grouted screw piles, at normalized displacement of 20 % the values of lateral capacities were found to be 359, 313, 280 and 258 N for embedment ratio (Z/dp) ratio = 4 , 6 , 8

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and 10 respectively. While, this value is observed around 210N at the same normalized displacement. It is noticed that the maximum normalized displacement at the pile’s failures is increased as the sand relative density increased. This trend is observed for all test results at different values of inclination angle (θ). For the same lateral load, the horizontal displacement is decreased significantly as the grout embedment ratio (Z/dp) decreased. For example, at lateral load of 100 N, the normalized displacement is decreased from 7.14 % for normal screw pile without grout to, 2.81 , 4.29 5.81 , and 6.19 % with displacement reduction of 60.6 , 40 , 18.6 and 13.3 % for Z/dp ratio equal 4, 6 , 8 and 10 respectively.

Fig. 8 shows the typical load-displacement curves for screw anchor pile at sand relative density of 30%, and inclination angle 30.0º. The corresponding capacities were 375, 325, 300 and 262.5 N for grouted screw pile at (Z/dp) ratio = 4, 6, 8 and 10 respectively. However, this value is 212.5 N for normal screw pile without grout. On the other hand, the load- displacement curves for screw anchor pile at relative density 80%, and inclination angle of 60.0º is illustrated in Fig. 9. It is also noticed that, the corresponding capacities were 1150, 1000, 825 and 675 N for grouted screw pile at (Z/dp) ratio = 4 , 6 , 8 and 10 respectively. But, this value is 500 N for normal screw pile without grout. At the same time as shown in Fig.10 for screw anchor pile at relative density 50%, and inclination angle of 90.0º. The corresponding capacities were found to be 850, 775, 675 and 575 N for grouted screw pile at (Z/dp) ratio = 4, 6, 8 and 10 respectively. While, this value is 462.5 N for normal screw pile without grout. It can be concluded that the ultimate capacity of screw grouted piles were reached to maximum value when the embedment ratio within the range of Z/dp = 4 for all test series. But its ultimate load capacity is related to inclination load angle and sand relative density.

Figure 7: Variation of lateral load with normalized displacement for Dr =80%, with

angle = 0.0º in different grout embedment ratio

0

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80

100

120

0 100 200 300 400 500 600 700 800 900 1000

Nor

mal

ized

disp

lace

men

t ( S

/dp

) %

Lateral load , (N)

Z =0.0(No grout)Z = 4 dpZ = 6 dpZ =8 dpZ = 10 dp

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Figure 8: Variation of inclined load with normalized displacement for Dr =30%, with angle = 30º in different grout embedment ratio

Fig. 9: Variation of inclined load with normalized displacement for Dr =80%, with angle = 60º in different grout embedment ratio

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90

0 50 100 150 200 250 300 350 400 450

Nor

mal

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disp

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men

t ( S

/dp

) %

Inclined Load , (N)

Z= 0.0 (No grout )Z =4 dpZ =6 dpZ =8 dpZ =10 dp

0

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0 100 200 300 400 500 600 700 800 900 1000 1100 1200

Nor

mal

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disp

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t ( S

/dp

) %

Inclined Load (N)

Z= 0.0 (No Grout )

Z= 4 dp

Z= 6 dp

Z= 8 dp

Z= 10 dp

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Figure 10: Variation of uplift load with normalized displacement for Dr =50%, with angle = 90º in different grout embedment ratio

Influence of grout embedment ratio

The effect of grout embedment ratio, (Z/dp) on the ultimate capacities of screw piles at different sand relative density was studied. Figs. 11 to 14 show the significant effect of grout embedment ratio (Z/dp) on the efficiency of improvement for the ultimate load capacity in the form of dimensionless factor. This load factor can be expressed as the ratio of Qult /Qulto where Qult is the ultimate capacity for grouted screw piles under different inclination angles and Qulto is the ultimate capacity for screw pile without grout. It is observed that the load factor for ultimate capacities increases as the ratio (Z/dp) decreases .However, for the same grout embedment ratio (Z/dp), the load factor for ultimate capacities is increased as the sand relative density increased. Fig.11 confirms and shows that the ultimate capacity of grouted screw pile under horizontal loads was reached to three times of ultimate capacity of normal screw pile without grout at dense sand. It is noticed that, the percentage of increase in the load factor for lateral capacities in cases of medium and dense sand compared with loose sand is found to be 30 and 50% respectively at Z/dp = 4. These percentages were found to be 25 & 42.9 % for Z/dp =6, 20 and 33.33 % for Z/dp = 8. Finally these values were found to be 12 & 20% for Z/dp =10. On the other hand Fig. 11 again justified that over the range of Z/dP > 4 a sharp decrease in the load factor is obtained. This can be confirmed the optimum depth of injection is considered at embedment depth Z of four times of pile diameter. This trend is valid for all sand density and inclination load angle.

Whereas Fig. 12 ,13 show the ratio of the ultimate capacity of grouted screw pile under inclined load against grout embedment ratio for different sand density. It has been found that, in dense sand, the load factor of grouted screw piles were attained to (2.5 and 2.3) for inclination angle of (30.0º, 60.0º) respectively.

To study the effect of sand density, for the percentage of increase in the load factor for inclined capacities in medium and dense sand compared with loose sand is found to be (21.59 & 40.34)% respectively at (Z/dp = 4 and inclination angle 30º). These values were dropped to (6 & 25.68) % at inclination angle of 60º. Fig. 14 shows the variation of load ratio or pile efficiency against embedment depth ratio for inclination angle of 90o. The ultimate capacity of grouted

0

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30

40

50

60

0 100 200 300 400 500 600 700 800 900

Nor

mal

ized

disp

lace

men

t (S/

dp) %

Uplift load ,(N)

Z= 0.0 (No grout )

Z= 4 dp

Z= 6 dp

Z= 8 dp

Z= 10 dp

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screw pile under uplift load was found to be two times of normal screw pile without grout at (Dr =80% and Z/dp = 4). It is also found that the percentage of increase in the load factor for uplift capacities in medium and dense sand compared with loose sand are 3.37 & 10.11% respectively. it can be concluded that as the load angle increased the load factor is decreased.

It is noticed that the optimum grout embedment ratio at different inclination angles was found to be at (Z/dp =4), over the range of Z/dp >4 the ultimate capacity of grouted screw pile was sharply decreased. This is attributed to at Z/dp =4 the section modulus of pipe shaft is increased at the top surface. Because the grout bulb is enlarged the pile top as a result the soil pile-grout bulb interaction is took place. Therefore the horizontal and inclined capacities were increased (Vickers and Clemence (2000), Abdelghany and El Naggar (2014). It is proposed that, the installation of the grout bulb was intensified the soil below and around the bulb. This bulb can also act as a rigid stiff element with higher density compared with surrounded soil. However, the soil which is above the bulb collapsed and heaved. The improvement in the ultimate load capacity of grouted screw piles is related to modified failure pattern under loading conditions.

According to Ghaly et al 1991, Fig. 15, indicated that the height h0 of the closed volume bounded by the failure surface is 4dh for loose sand, 5dh for sand of medium density, and 6dh for dense sand. In the present research, the soil between the grouted bulb and the first helix was intensified and behaved like an embedded cylindrical stiff shaft one unit. This can significantly improve the resistance of screw piles under different loading conditions due to induced friction resistance in the circumference area of the obtained imaginary shaft as stated by Azzam, (2014). It is also noticed that the closed volume overlapped with the intensified soil, which is located below the grout bulb. The suggested mechanism of cylindrical and tapered shear for sandy soils and screw piles with grouted bulb and two or more blades is illustrated in Fig. 16.

0

0.5

1

1.5

2

2.5

3

3.5

0 2 4 6 8 10 12

Load

fact

or ,(

Qul

t /Q

ulto

)

Grout embedment ratio (Z/dp)

Dense sand, Dr =80 %

medium sand, Dr =50 %

Loose sand, Dr =30 %

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Figure 11: Variation of load factor ratio of lateral capacity with grout embedment ratio, (Z/dp), for different sand relative densities, inclination angle =0.0º

Figure 12: Variation of load factor ratio of inclined capacity with grout embedment ratio, (Z/dp),

for different sand relative densities, inclination angle =30.0º

Figure 13: Variation of load factor ratio of inclined capacity with grout embedment ratio, (Z/dp),

for different sand relative densities, inclination angle =60.0º

0

0.5

1

1.5

2

2.5

3

0 2 4 6 8 10 12

Load

fact

or ,(

Qul

t /Q

ulto

)


Dense sand , Dr =80%

medium sand , Dr =50%

loose sand , Dr =30%

0

0.5

1

1.5

2

2.5

0 2 4 6 8 10 12

Load

fact

or ,(

Qul

t /Q

ulto

)


Dense sand ,Dr =80 %

Medium sand ,Dr =50 %

Loose sand ,Dr =30 %

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Figure 14: Variation of efficiency of improvement of uplift capacity with grout embedment ratio, (Z/dp), for different sand relative densities, inclination angle =90.0º

0

0.5

1

1.5

2

2.5

0 2 4 6 8 10 12

Load

fact

or ,(

Qul

t /Q

ulto

)


Dense sand , Dr =80%medium sand , Dr =50%loose sand , Dr =30%

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Figure 15: Failure surface of deep screw anchor after Ghaly et al. 1991

Figure 16: Mechanism of cylindrical and tapered shear for sandy soils and screw piles with

grouted bulb

Effect of inclination angle and relative density

The effect of inclination angle on the load factor in ultimate capacities at different sand relative densities was investigated. Fig. 17 (a to d), shows that the load factor for ultimate

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0

0.5

1

1.5

2

2.5

3

3.5

0 30 60 90

Inclination angle ,(θ)

Load

fact

or ,(

Qul

t /Q

ulto

) Dense sand ,Dr =80 %



Z/ dp =4 a)

capacity increases as both of the grout embedment ratio (Z/dp) and inclination angle decrease. It has been noticed that, at the same inclination angle, the load factor for ultimate capacities is increased as the sand relative density increased. For Z/dp ratio = 4, the percentage of increase in the load factor for ultimate capacities values were found to be (3.37 & 10.11)% for inclination angle =90º as a lower improvement in medium and dense condition respectively. But, these values were found to be (6 & 25.7) % for inclination angle = 60º and (21.6 & 40.34) % for inclination angle =30º. Finally the maximum improvement was obtained at inclination angle of 0.0º , these values were found to be (30 & 50)% for sand relative density of 50% and 80% .

On the other hand the degree of the improvement in the load factor is decreased with the increase of the embedment ratio (Z/dp) as clearly shown in Fig. 17(a to d). For confirmation Fig. 18 shows the relationship between the relative density and ultimate load factor ratio at different inclination angle for (Z/dp =4). It can be concluded that, as the inclination angle increases, the load factor ratio of grouted screw piles decreases. The higher load improvement is exhibited at higher relative density with zero inclination angles. For dense condition, the improvement values were found to be around (3, 2.5, 2.3 and 2) times of normal screw pile without grouting for inclination angle of ( 0 o, 30 o, 60 o and 90o) respectively. While these values of the improvement were dropped to lower level for loose sand conditions as confirmed by the relevant Fig.18.

SCALE EFFECT

It is well known that due to scale effect and the nature of granular soils, soils may not play the same role in the laboratory model tests as in the case of prototype. These differences occur preliminary because of the differences in stress level between the model tests and the field tests (Vesic, 1973 and Nazir and Nasr 2013). The stress level around the small scale model anchor is much smaller than that around the full scale piles. Despite the involvement of scale effects, the study not only can provide insight into the likely behavior of screw pile that constructed in different sand densities, but also will provide a useful base for further research using full- scale tests or centrifugal model tests and numerical studies leading to understanding of screw pile installed in sand subjected to inclined tension loads.

0.00

0.50

1.00

1.50

2.00

2.50

3.00

0 30 60 90

Inclination angle ,(θ)

Load

fact

or ,(

Qul

t /Q

ulto

)




Z/ dp =6 b)

Vol. 21 [2016], Bund. 2 590

Figure 17: Variation of efficiency of improvement for ultimate capacity with inclination angle , for different sand relative densities (a: Z/dp =4, b: Z/dp = 6, c: Z/dp =8, and d: Z/dp=10).

Figure 18: The relationship between the relative density and load factor for ultimate capacity at inclination angle

CONCLUSIONS An experimental program of small scale laboratory model was undertaken to study the

ultimate inclined load capacity of vertical screw grouted pile. The study primarily focused on determining the optimum embedment depth of injection of grouted bulb. Angle of load inclination was varied from 0º to 90º measured from the horizontal level of the point of applied load. Experimental values of ultimate vertical load carrying capacity, of grouted screw pile embedded in different densities of sand are compared with non grouted screw pile. This technique can be considered as a novel one to improve the lateral screw pile response under different inclination load angle.

Salient conclusions that can be drawn from the present study are as follows:

1. The ultimate load carrying capacities for grouted screw piles embedded in different densities of sand are increased with the decrease of grout embedment ratio (Z/dp).

0

0.5

1

1.5

2

2.5

3

3.5

0.00 20.00 40.00 60.00 80.00 100.00

Load

fact

or ,(

Qul

t /Q

ulto

)

Relative Density ,Dr %

inclination angle =0.deg




0.00

0.50

1.00

1.50

2.00

2.50

0 30 60 90Inclination angle ,(θ)

Load

fact

or ,(

Qul

t /Q

ulto

)Dense sand ,Dr =80 %


Loose sand ,Dr =30 %Z/ dp =8 c)

0.00

0.50

1.00

1.50

2.00

0 30 60 90Inclination angle ,(θ)

Load

fact

or ,(

Qul

t /Q

ulto

)




Z/ dp=10 d)

Vol. 21 [2016], Bund. 2 591

2. The optimum embedment depth ratio which provides a higher load improvement is found to be at depth below the ground surface around four times of pile diameter.

3. As the grout embedment ratio (Z/dp) increases the load factor ratio (Qult/Qulto ) for ultimate capacities of grouted screw piles is decreased.

4. The lateral load carrying capacity of grouted screw pile at zero inclined angle and dense sand is three times of normal screw pile without grout. This value is dropped to two times in loose condition.

5. The uplift load carrying capacity of grouted screw pile at inclination angle of 90º is two times of normal screw pile without grout at dense sand and this value is reached to 1.78 for loose sand.

6. The soil between the grouted bulb and the first helix was intensified and behaved like an embedded cylindrical stiff shaft as a result the friction resistance is increased.

7. The ultimate load carrying capacities for grouted screw piles embedded in different sand densities are decreased with the increase of inclination angle.

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