Review ArticleExperimental Study on the Influence of Aging on MechanicalProperties of Geogrids and Bearing Capacity of ReinforcedSand Cushion

Hui Yuan12 Xiaohong Bai 1 Hehui Zhao1 and Jingren Wang1

1College of Architecture and Civil Engineering Taiyuan University of Technology Taiyuan 030024 China2Taiyuan City Vocational College Taiyuan 030027 China

Correspondence should be addressed to Xiaohong Bai bxhongtyuteducn

Received 9 July 2020 Revised 13 August 2020 Accepted 12 September 2020 Published 8 October 2020

Academic Editor Qiang Tang

Copyright copy 2020 Hui Yuan 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

Geogrids are widely used in foundation engineering for reinforcing foundations due to their light weight high strength andexcellent performance In this study two kinds of polypropylene biaxial geogrids were used and indoor thermal oxygen andphotooxygen aging tests were carried out +e residual mechanical stability of the exposed materials was determined by tensiletesting +e results of both accelerated test methods are discussed and compared in detail After aging of the geogrid the trend oftensile strength and fracture elongation change with aging time is obtained +e gray prediction model was used to predict thevariation in the retention rate of tensile strength in the geogrid with photooxygen aging time Model tests of cushions were carriedout in a large geogroove to compare the load bearing characteristics of pure sand and the unaged and aged geogrid-reinforced sandcushions +e results show that ultraviolet radiation illuminance plays a decisive role in the aging degree of the polypropylenegeogrid +e influence of photooxygen aging on the tensile strength and fracture elongation of a polypropylene biaxial geogrid isgreater than that of thermal oxygen aging Different types of polypropylene biaxial geogrids with photooxygen aging showeddifferent retention rates of tensile strength and the aging resistance of the geogrid with higher tensile strength was significantlyhigher than that of the geogrid with lower tensile strength+e tensile strength of the geogrid has an effect on the bearing capacityof reinforced sand cushions Under proper elongation the bearing capacity of the reinforced sand cushion is clearly improvedcompared with that of the unreinforced cushion +e aging behavior of the two geogrids reduces the load bearing capacity of thereinforced cushion by influencing the property of the interface between the geogrid and sand

1 Introduction

Geogrids which are made of polymeric materials have beenused as a construction material for many applications suchas walls slopes roads and building foundations [1] Geogridproducts which have been used in several civil engineeringapplications for nearly 40 years are mostly based onpolyolefins such as polyethylene (PE) or polypropylene(PP) Given that repair or exchange of these products isnearly impossible and the consequences of failure are severeseveral applications require service times between 50 and100 years Carbon black and antioxidantsstabilizers are usedto retard polymer degradation depending on photooxidationand thermal oxidation [2] However this is inevitable in the

process of production storage transportation and con-struction +e polymer will be exposed to sunlight whichcauses photooxidation aging Geogrids are expected toeventually experience chemical degradation leading to areduction in their mechanical properties and change in theirchemical and physical properties [3]

+e long-term durability of geogrids depends on the onehand on their resistance to ultraviolet light and thermaloxidation and on the other hand on the loss of stabilizerscaused by the specific natural environment and the oxidativeconsumption of the stabilizer +e long-term durability of ageogrid is governed by its resistance to thermal oxidation andloss of stabilizer by extraction owing to the particular naturalenvironment and oxidative consumption of the stabilizer [4]

HindawiAdvances in Civil EngineeringVolume 2020 Article ID 8839919 13 pageshttpsdoiorg10115520208839919

+e time to nominal failure of PP geogrids could bedivided into three stages in assessing lifetime depletion ofantioxidants induction time and time to reach half-life of arelevant engineering property [3] Although tests can beconducted at the temperature of specific interest if thetemperature is low these tests may take many years beforethere is sufficient change in antioxidants to allow a reliableestimate of the time to depletion +us in many casesaccelerated aging tests are conducted at temperatures abovethe target temperature [5ndash12] Estimation of the expectedservice lifetimes is then based on the estimation of anti-oxidant depletion times

As a plane reinforcing material a geogrid can providelateral constraint improving and reinforcing a soft foun-dation +e biaxial geogrid has been well understood inimproving the bearing capacity and bearing direction offoundations Many researchers have published scientificresearch results on bearing capacity and settlement ofgeogrid reinforced soil Determining the ultimate bearingcapacity of footings resting on reinforced soil is a complexproblem and knowledge of the load-settlement behavior offootings on reinforced soil is limited [13] +e researchershave stated that the parameters identified as critical inimproving the bearing capacity include the embedmentdepth of the reinforcement number of reinforcement layersvertical spacing between reinforcement layers reinforceddepth and width of reinforcement [14] +e engineeringperformance (CBR) for different types of soil subgradesreinforced with geogrids was studied [15] Two oxidativeinduction time (OIT)methods a standard (Std) OIT test anda high pressure (HP) OIT test were used to evaluate theinteraction of CB with two types of antioxidants during theoven aging [4] A fuzzy logic- (FL-) based modeling ap-proach is employed for geogrid-reinforced subgrade soil ofunpaved roads [16]

In this study two different extruded biaxial geogridswere used to test the thermal oxygen aging and photooxi-dation aging using a xenon lamp weathering test chamber+ese two geogrids are biaxial geogrids with the same shapebut different ultimate tensile strength +e changes in me-chanical properties (especially stress and strain at the rup-ture point which is the most sensitive mechanical propertywith respect to oxidative degradation) were monitoredduring the aging +en a laboratory plane load test of thebearing capacity of a series of reinforced soil foundationswas carried out using the aged geogrid +e influence of theaging factors on the bearing capacity of geogrid-reinforcedfoundations is considered

2 Materials and Apparatus

21 Materials

211 Geogrid After surveying the current market of PP-based geogrid products two commercially available biaxialgeogrids were selected and provided by the manufacturer ofTansa in China Table 1 lists the corresponding materialproperties such as geogrid size density and tensileproperties

Basic information about the products such as the typeand amount of the used stabilizer was not disclosed by themanufacturers +e definition of tensile strength is maxi-mum tensile force divided by (unit) cross section area As ahigh concentration of additives might cause problemsduring manufacturing of the fibers (extruding through asmall hole spinneret cooling and stretching) antioxidantsare mixed into fiber resins in the extrusion process essen-tially for the purpose of protecting them from oxidation Inaddition all PP biaxial geogrids are UV-stabilized by carbonblack with content in the range of 05ndash1wt

212 Foundation Soil Natural graded river sand was usedfor the underlying layer In the reinforced concrete geo-groove (9mtimes 4mtimes 3m) the total thickness of sand laying is24m+e sand was layered and filled and each layer was notless than 300mm After each layer had been filled it was fullyconsolidated three times to ensure that each layer of sandhad the same compactness +e improved semiautomaticN635 heavy-duty cone dynamic penetration equipment wasused to test the compactness of sand in cone dynamicpenetration test +e sand is in medium-dense state

Since the main purpose of this test is to explore theinfluence of aging on the bearing characteristics of geogridreinforced soil in order to avoid the influence of the dis-creteness of fill on the accuracy of test data standard sandwas used as foundation soil +e standard sand produced byFujian Nonmetal Mine Co Ltd was used as foundation soil+e shear strength index of physical properties of sand forreinforced cushion is as follows c 0 and φ 1878deg and therelative density is 70 in the test See Table 2 for physicalindexes of soil

22 Testing Apparatus

221 Aging Test Apparatus +e xenon lamp weathering testchamber is of the SN-150 type which consists of a xenonlamp illumination system cooling system air heating deviceair circulation device rain system humidity control systemand sensors (temperature blackboard temperature andhumidity sensors) It can simulate the conditions of lighthumidity rain condensation and high temperature innature and can combine various factors for the weatherresistance test

An important component of the xenon lamp weatheringtest chamber is the long-arc xenon lamp which is air-cooled+e working principle of the xenon lamp is as follows thehelium gas in the lamp tube will radiate light energy close tothe solar spectrum after being energized +e accuracy errorof the three sensors can be controlled at 1

+e specific technical conditions of the xenon lampaging test box are as follows temperature range+10degCndash80degC plusmn2degC humidity range 45ndash90 rainfall cycle1ndash9999 h distance between the sample frame and the xenonlamp 280ndash370mm (adjustable) heating power 25 kW wetpower 15 kW internal dimensions of the test box600times 600times 600mm

2 Advances in Civil Engineering

Universal testing machines are used to carry out tensiletests on geogrids with varying degrees of aging It is mainlyused to test forces in metal and nonmetallic materials andperformance testing +e universal testing machine iscomposed of two parts mechanical transmission system andmicrocomputer control system It has high precision and fastresponse and can achieve real-time recording of test resultsIts technical conditions are as follows load error le05displacement error le05 heavy element test 100 kN ofinterval selection load recognition accuracy 001N movingspeed 0001ndash250mmmin and effective moving distance650mm

222 Model Test Apparatus In order to study the effect ofgeogrid aging on the bearing characteristics of a geogrid-reinforced sand cushion approach the boundary conditionsof a field test and avoid the adverse effects of environmentalfactors the laboratory model test site was located in theLaboratory of the Institute of Taiyuan University of Tech-nology +e size is 9times 4times 31m +e whole test system wascomposed of the foundation soil material static servoloading system test elements and data acquisition systemAs shown in Figure 1 it was loaded by a hydraulic jackBecause of the large amount of sand used in the test and thelimited conditions of the test site local river sand was used asthe experimental soil Before testing the sand was sieved toremove large particles and impurities +e total thickness ofthe sandy soil laid in the groove was 24m In order to ensurethe compactness of the sandy soil to meet the test re-quirements the sandy soil was layered and tamped with aflat-plate rammer+e thickness of each layer was 03m andthe compaction was fully tamped 3 times to ensure that thecompactness of the sandy soil in each layer was the same Asteel rigid plate measuring 200mmtimes 200mm in plane with25mm thickness was used as a square footing to apply theload Four dial gauges were set up to measure the settlementof the footing during application of the load A hydraulicjack loading system was used to apply the load on thegeogrid-reinforced sand cushion through the footing plate

+e load was applied through the high-precision statichydraulic servo table produced by Chengdu Servo HydraulicEquipment Co Ltd and the load application level wascontrolled by the static strain gauge connected with the jackstrain sensor+e strain sensor on the top of the jack and thestrain gauge adopt the full bridge connection method to

control the application of each level of load in the testprocess according to the calibrated strain load relationship+e hydraulic high precision thrust jack used in verticalloading has a rated load of 100 kN

Increasing static loads were applied by using a hydraulicjack +e load and corresponding footing settlement weremeasured by a calibrated pressure gauge and four dialgauges respectively +e testing procedure was performedaccording to GB50007-2012 [17] where increments of loadper unit area were applied and maintained until the rate ofsettlement was less than 01mmh over two consecutivehours +e test was terminated when the total settlementreached 006 B that is 12mm in this program

3 Experimental Procedures

31 (ermal Oxygen Aging +e thermal oxygen aging testwhich is also known as the hot air exposure test artificiallyaccelerates the plastic product or other high molecularpolymers using a gravity convection or forced ventilationheat aging test chamber +e principle of the thermal agingtest is to expose the sample to a high-temperature envi-ronment for a long time by using the high-temperaturestability of the oven +e high temperature can shorten theinduction period of the oxidatively degradable polymermaterial With further increase of time the crystallinity ofthe sample increases and the tensile strength elongationmodulus and impact strength will change +en thechanges in the properties before and after aging are com-pared to determine the degree of aging of the sample Factorsaffecting the test data include the temperature control in thetest chamber humidity in the heat aging test chamber airflow velocity on the surface of the sample and whether it isaffected by periodic rain It can be seen that many factorsaffect the test results All these factors must be preciselycontrolled to ensure that the error is within acceptable limits

For thermal oxygen aging the specimens were aged in athermostatically regulated xenon lamp weathering testchamber +e dimensions of the working chamber were600mmtimes 600mmtimes 600mm +e aging temperature wasachieved and maintained by controlled heating of theworking chamber walls An air ventilation fan in the ceilingof the working chamber forced internal circulationAccording to the technical data there were 10 air changesper hour and a fresh air supply of approximately 10m3h

Table 2 Physical indicators of sand for testing

Layer Cu Cc ρdmax (gcm3) ρdmin (gcm3) d60 (mm) d30 (mm) d10 (mm)

Underlying layer 556 113 170 136 O62 041 022Bearing layer 179 091 171 133 052 037 029

Table 1 Physical property indexes of biaxial geogrids

Samplecode

Grid size(mmtimesmm)

Tensile strength(kNm)

Strength at 2 strain(kNm)

Strength at 5 strain(kNm)

Mass per unit area(kgm2)

Elongation atbreak ()

BG1 35times 33 152 62 114 041 125BG2 33times 32 255 85 165 053 143

Advances in Civil Engineering 3

Square specimens of 50times 50 cm were cut out of the geogridrolls in the middle at randomly distributed locations

After comparing the appearance of the geogrid samplesaged by thermal oxygen with that of the original geogridsamples it was found that the color depth of the geogridsamples aged by thermal oxygen for a long time was slightlylighter

32 Photooxygen Aging In the whole life cycle of geo-synthetics photooxidative aging occurs once they are ex-posed to sunlight for a long time Many studies have shownthat light is the most important factor causing photooxi-dation aging +e outdoor exposure test and the indoor lightsource aging test are the two most important procedures toevaluate the photooxidative aging resistance of geosyntheticmaterials +e test period required for the outdoor exposuretest is long and the external conditions are complicated andnot easy to control +e indoor light source aging test periodis short and it is less interfered by external environmentalconditions It can accurately control the light intensitytemperature humidity oxygen concentration pH andother conditions but it does not match the actual engi-neering environment +e aging time under natural climaticconditions should be found through multiple tests to findout the relationship between the two and establish a con-version formula to predict the service life of the material

+e photooxidation aging test adopts the xenon lampweathering test chamber +e aging test of geogrid wascarried out according to ASTM G154-16 standard [18] +etest box has a built-in rotating sample rack and a whole gridsample is fixed on the sample rack +e rotation speed is setto 3 cyclesmin to ensure uniform illumination and the test isconducted In the aging process the cycle illuminationmodeis adopted +e specific steps are performed for 8 h underlight conditions and then condensed for 4 h without lightand the cumulative aging time is 700 h +e irradiationintensity was set to 600Wm2 the humidity in the testchamber was 70plusmn 2 and the blackboard temperature was70degC

+rough observation of the grid sample after fracturethere was a very obvious phenomenon +ere was a slight

difference in the fracture mode of the grid sample during thetensile test Figure 2 shows the fracture diagram of theoriginal geogrid and the fracture diagram of the test pieceafter the photooxygen aging for 700 h

33 Tensile Tests For the tensile tests universal testingmachines are employed Multirib and single-rib testmethods can be used and this study uses the single-rib testmethod +e first step is to determine the tensile rate duringthe geogrid tensile test In the case of a fixture clamping thetwo ends of the geogrid specimen the distance between thetwo ends of the fixture is set from 11 to 14 cm +e tensilerate selected in this test is 24mmmin Before the tensile testbegins it is necessary to apply 1 prestress to calibrate thetensile strength +e measured data includes the tensilestrength at peak tension fracture elongation and 2 and 5elongation+e longitudinal and transverse ribs are differentin mechanical properties +erefore the longitudinal andtransverse ribs are tested separately tomeasure the tension of10 ribs to obtain an average value f

+e formula for calculating the geogrid tensile strength is

F f times N

L times n (1)

where F is the tensile strength of the geogrid (kNm) f is theaverage of the pulling force (kN) N is the number of ribs onthe sample width n is the number of ribs of the specimenand L is the width of the sample (m)

Statistical analysis of tensile test data of geogrid is inTable 3 It can be seen from Table 3 that the systematic errorof the test results of geogrid tensile test is small and thereliability of the test results is high

34 Model Test In this test a replacement test pit with adesign size of 600mmtimes 600mmtimes 600mm was excavated+e standard dry sand was laid into the pit layer by layer andthe relative compactness of each layer of each group isensured to be the same by compacting the constant qualitysand to the set height Calculate the weight of standard drysand required for each layer with 70 relative density and it

Hydraulic servo system

Counterforce beam

Strain gauge

ComputerGeogrid

Figure 1 Schematic diagram of loading device for the model test

4 Advances in Civil Engineering

(a) (b)

Figure 2 Tensile fracture diagram of test piece (a) Original geogrid (b) Aging geogrid

Table 3 Statistical analysis of tensile test data of geogrid

Geogrid Testing unitSample number Average

valueStandarddeviation

Coefficient ofvariation1 2 3 4 5 6 7 8 9 10

BG1

Tensilestrength 1521 1532 1533 1521 1522 1523 1491 1522 1523 1531 1522 01126 00074

Elongation atbreak 1241 1253 1262 1251 1241 1262 1251 1272 1252 1233 1252 01098 00088

BG2

Tensilestrength 2543 2542 2561 2563 2562 2571 2542 2533 2541 2562 2552 01235 00048

Elongation atbreak 1434 1425 1442 1421 1443 1454 1433 1432 1421 1423 1433 01037 00073

(a)

11 13(12) 15(14)

6 8(7) 10(9)

1 3(2) 5(4)

600

100

100

200

600

200

Loading plate

(b)

Figure 3 +e picture of the model test site and the layout of the Earth pressure box (a) Picture of the model test site (b) Layout of soilpressure box

Advances in Civil Engineering 5

is laied in the soil pit and compacted to the thickness of50mm for each layer+e geogrid is laid at a depth of 70mmunder the loading plate and fixed with U-nails+e picture ofthe model test site and the layout of the Earth pressure boxare shown in Figure 3

4 Results and Discussion

+e thermal oxygen and photooxygen aging will result in thereduction in mechanical properties of the geogrid +emechanical properties of the geogrid with aging parametersare described by the tensile strength retention rate andfracture elongation retention rate +e retention rate oftensile strength is the ratio between the tensile strength of

the aged geogrid and the tensile strength of the geogridbefore aging expressed as a percentage +e fracture elon-gation retention rate is the ratio of the aged fracture elon-gation to the preaging fracture elongation expressed as apercentage

41 Geogrid Aging

411 (ermal Oxygen Aging Test +e results of the thermaloxygen aging test for samples BG1 and BG2 of the biaxialgeogrid of PP are shown in Figures 4 and 5 In Figure 4 theresults show relatively small differences for different aging time(at least for temperatures of 60 and 70 degrees) and aging time

Tens

ile st

reng

th re

tent

ion

()

60degC70degC80degC

0 100 200 300 400 500 600 700 800Thermal oxygen aging time (h)

92

94

96

98

100

102

104

(a)

Tens

ile st

reng

th re

tent

ion

()

60degC70degC80degC

Thermal oxygen aging time (h)0 100 200 300 400 500 600 700 800

93

94

95

96

97

98

99

100

101

102

(b)

Figure 4 Relationship curves of tensile strength retention rate-thermal aging time (a) BG1 (b) BG2

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

60degC70degC80degC

0 100 200 300 400 500 600 700 800Thermal oxygen aging time (h)

828486889092949698

100102104106

(a)

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

60degC70degC80degC

Thermal oxygen aging time (h)0 100 200 300 400 500 600 700 800

86889092949698

100102104

(b)

Figure 5 Relationship curves of elongation at break-thermal aging time (a) BG1 (b) BG2

6 Advances in Civil Engineering

up to 400 hours In Figure 5(a) the results show that theelongation at break of the PP biaxial geogrid increases slightlyin the early stage of thermal oxygen aging and the higher thetemperature is the shorter the increase period is At 60degC and70degC the influence of thermal oxygen aging on tensile strengthand fracture elongation is not obvious +e tensile test oflongitudinal and transverse ribs has been carried out by theauthors +e test results show that the tensile test results oflongitudinal and transverse ribs are not different

Comparing samples BG1 and BG2 of the two grids itcan be found that the temperature is 80degC and the tensilestrength retention rates of samples BG1 and BG2 are 928and 943 respectively when aging is of 700 h +e re-tention rates of BG1 and BG2 fracture elongation were 84

and 8741 respectively when aging is of 700 h +e ex-perimental results show that the tensile strength andfracture elongation of BG1 and BG2 which are both biaxialgeogrids of PP have similar trends with thermal oxygenaging time

412 Photooxygen Aging Test +e relationship betweentensile strength retention rate and photooxygen agingtime of two kinds of geogrid is obtained by the indoorultraviolet photooxygen aging test (Figure 6) It can beseen in Figure 6 that the two curves have the same trendthat is with the increase in photooxygen aging time thetensile strength of the two kinds of PP biaxial geogridsshows a significant trend of decrease After 300 h agingtime the BG1 starts to decline at greater rate than BG1and the difference at the end of the test is gt10 Figure 7shows the relationship between the fracture elongationretention rate and photooxygen aging time of the twogeogrids It can be seen from Figure 7 that within 400 hthe fracture elongation retention rate of the two geogridsalternately declines with the photooxygen aging time Atthe photooxygen aging time of 700 h the fracture elon-gation retention rates of samples BG1 and BG2 are notsignificantly different

413 Comparative Analysis Figure 8 shows the relationshipof tensile strength retention rate between the thermal oxygenaging and the photooxygen aging specimens with aging timeat 70degC

Figure 9 shows the relationship of retention rate ofelongation at break between the thermal oxygen aging andthe photooxygen aging specimens with aging time at 70degCIt can be seen that under the conditions of thermal oxygenaging the curves of tensile strength and fracture elongationretention rates with aging time of samples BG1 and BG2 arerelatively gentle However the tensile strength and fractureelongation retention curves with aging time are steeperunder photooxygen aging conditions +e thermal oxygenaging intensity is reduced by no more than 5 while thephotooxygen aging intensity is reduced by about 35 andthus the impact of photooxygen aging is much greater thanthat of thermal oxygen aging It is shown that the effect oflight on the geogrid tensile strength during aging is muchgreater than that of temperature

414 Prediction Model +e gray system theory proposed byProfessor Deng (China) in 1982 has been widely used ineconomic social and engineering fields It has advantages ofsmooth degree of data and a good precision [19] Based onthe gray prediction model (GM) by using a small amount ofraw data and simple modeling calculation the future datacan be predicted and the accuracy meets the research needsIn this study eight groups of data of tensile strength andelongation at rupture obtained from the photooxidation

0 100 200 300 400 500 600 700 800Photooxidative aging time (h)

BG1BG2

60

65

70

75

80

85

90

95

100Te

nsile

stre

ngth

rete

ntio

n (

)

Figure 6 Relationship curves of tensile strength retention rate-photooxidation aging time

BG1BG2

0 100 200 300 400 500 600 700 800Aging time (h)

50

60

70

80

90

100

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

Figure 7 Relationship curves of retention rate of elongation atbreak-photooxidation aging time

Advances in Civil Engineering 7

aging test of PP biaxial geogrids were provided +e GM wasused to fit the photooxidative aging test results of PP biaxialgeogrids and the failure rule of PP biaxial geogrids underindoor photooxidative aging is inferred

Gray model is a differential equation established bygenerating new data series from the original data series GM(1 N) represents the differential equation of N variables oforder 1 +e modeling process is as follows

Sequence based on more than four known pieces ofdata is

X(0)

(k)1113966 1113967 k 1 2 3 n (2)

Generating sequence of accumulated data with givendata sequence is as follows

X(1)

(k)1113966 1113967 k 1 2 3 n (3)

Approximating cumulative data series values with con-tinuous smooth values of exponential curves is as follows

1113957X(1)

(k) 1113957X(1)

(1) minusu

a1113876 1113877bulle

minus a(kminus 1)+

u

a k 1 2 3 n

(4)

+e smoothing approximation value of the original datasequence is reduced

1113957X(0)

(k) 1113957X(1)

(k) minus 1113957X(1)

(k minus 1) k 1 2 3 n (5)

In the above formula

ermal oxygen agingPhotooxidation aging

Tens

ile st

reng

th re

tent

ion

rate

()

65

70

75

80

85

90

95

100

100 200 300 400 500 600 700 8000Aging time (h)

(a)

ermal oxygen agingPhotooxidation aging

Tens

ile st

reng

th re

tent

ion

()

70

75

80

85

90

95

100

100 200 300 400 500 600 700 8000Aging time (h)

(b)

Figure 8 Relationship curves of tensile strength retention rate-aging time (a) BG1 (b) BG2

ermal oxygen agingPhotooxygen aging

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

50

60

70

80

90

100

110

100 200 300 400 500 600 700 8000Aging time (h)

(a)

ermal oxygen agingPhotooxidation aging

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

50556065707580859095

100105

100 200 300 400 500 600 700 8000Aging time (h)

(b)

Figure 9 Relationship curves of retention rate of elongation at break-aging time (a) BG1 (b) BG2

8 Advances in Civil Engineering

a

u

⎡⎢⎢⎢⎣ ⎤⎥⎥⎥⎦ BTB1113872 1113873

minus 1B

minus 1B

TYn

B

b1 1

b2 1

middot middot middot 1

bnminus1 1

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦

Yn

y1

y2

middot middot middot

ynminus1

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦

m (n minus 1) 1113944nminus1

i1bi( 1113857

2minus 1113944

nminus 1

i1bi

⎛⎝ ⎞⎠

2

a (n minus 1)1113944

nminus1i1 biyi minus 1113944

nminus1i1 bi1113944

nminus1i1 yi1113876 1113877

m

u minus1113944

nminus1i1 bi1113944

nminus1i1 biyi + 1113944

nminus1i1 bi( 1113857

21113944

nminus1i1 yi1113876 1113877

m

(6)

Based on the experimental data of tensile strength andelongation at rupture of PP biaxial geogrids under photo-oxidation aging a GMwas established to predict the changesin tensile strength and elongation at rupture of PP biaxialgeogrids under long-term photooxidation aging +e pre-diction results are listed in Table 4

It can be seen from Table 3 that the GM (1 1) model isestablished for the tensile strength of samples BG1 and BG2under photooxidation aging for 0ndash600 h +e maximumrelative error of fitting the photooxidation aging test resultsof BG1 is only 248 the average error is only 149 and thetensile strength retention rate of BG1 and BG2 under 700 h isonly 662 +e GM is used to predict the tensile strength ofBG1 and BG2 +e calculated predicted value is 646 andthe relative error of data recovery is only 242 +emaximum relative error of fitting BG2 is only 165 theaverage error is only 030 and the relative error ofchecking data with 700 h is only 565 +is clearly showsthat the GM (1 1) model has better fitting effect

Prediction curves of tensile strength and photooxidationaging time are shown in Figure 10 It can be seen fromFigure 10(a) that the tensile strength retention rate of BG1 is5198 When the photooxidation aging time is 1000 h thetensile strength retention rate of BG1 is 4832 When thephotooxidation aging time is 1100 h the tensile strengthretention rate of BG1 is less than 50 so the PP biaxialgeogrid losees efficacy From Figure 10(b) it can be seen thatthe tensile strength retention rate of BG2 is 5188 when thephotooxidation aging time is 1700 h and 4983 when it is1800 h less than 50 and thus the PP biaxial geogrid isinvalid

+e aging test curve changes greatly in the early stage ofaging and is relatively unstable +e prediction curve is notvery accurate in the early stage of aging but rather accurate indescribing the long-term performance of aging

42 Test of Load Bearing Capacity of Reinforced Sand Foun-dation considering theEffects ofAging +e tensile strength ofthe geogrid before and after aging is listed in Table 5

Load-settlement curves of the cushion are obtainedaccording to load at all levels as shown in Figure 10Compared with the nonreinforced sand cushion the set-tlement value of the reinforced sand cushion under the sameload decreases and the bearing capacity of the foundationincreases the settlement value of the reinforced sandcushion of BG2 geogrid was smaller than that of the rein-forced sand cushion of BG1 geogrid and the bearing ca-pacity of the cushion was increased the difference in theload-settlement curves of the reinforced sand cushion andthe nonreinforced sand cushion at the early stage was smallIt shows that the reinforcement effect was not obvious at theinitial stage of loading With the increase in load the spacingbetween the load-settlement curves of the pure sand BG1reinforced sand and BG2 reinforced sand cushions grad-ually increased+us the reinforcement effect is increasinglyobvious

+e P-S curve of geogrid-reinforced sand consideringthe aging effect is shown in Figure 11 Figures 11(a) and11(b) show that the P-S curves of aging geogrids coincidewith those of nonaging geogrids when the load P was lessthan 125 kPa With the increase in load the two P-S curvesbegin to separate and under the same load the settlementratio of the aging-reinforced sand cushion of the BG1geogrids to that of the non-aging-reinforced sand cushionwas less than that of the aging-reinforced sand cushion Ingeneral the bearing capacity of the aging-reinforcedcushion was slightly lower than that of the non-aging-

Table 4 +e results of gray model prediction

Aging time (h) 0 100 200 300 400 500 600

BG1 tensile strength retention rateTest value () 100 986 944 888 7980 737 697

Predicted value () 100 1001 931 866 805 748 695Relative error () mdash 152 138 248 088 149 029

BG2 tensile strength retention rateTest value () 100 978 964 908 865 853 801

Predicted value () 100 9870 9481 9108 8749 8404 8073Relative error () mdash 092 165 030 114 148 078

Advances in Civil Engineering 9

reinforced cushion but the effect is not obvious +ebearing capacity of reinforced soil decreases with the in-crease of aging time

+e effect of aging of geogrids in the early stage ofloading on the bearing capacity of reinforced soil is

negligible +e authors define an aging effect on the initialload When the load value is less than the initial load of agingeffect the aging effect is small on the settlement of foun-dation When the load value exceeds the initial load of agingeffect the aging effect on the settlement of foundation

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

Photooxidative aging time (h)

Test valuePredicted value

Tens

ile st

reng

th re

tent

ion

rate

()

40

50

60

70

80

90

100

(a)

0

200

400

600

800

1000

1200

1400

1600

1800

Photooxidative aging time (h)

Test valuePredicted value

Tens

ile st

reng

th re

tent

ion

()

404550556065707580859095

100

(b)

Figure 10 Prediction curves of tensile strength and photooxidation aging time (a) BG1 (b) BG2

Table 5 +e tensile strength of geogrid before and after aging

Geogrid Tensile strength of geogrid before aging(kNm)

Radiation intensity (Wm2)

Tensile strength of geogrid after aging of700 h

Aging time(h)

BG1 158 600 108 700BG2 255 600 1835 700

0 100 200 300 400 500p (kPa)

Pure sand Unaged BG1

Aging 500h BG1Aging 600h BG1

25

20

15

10

5

0

s (m

m)

(a)

p (kPa)

Pure sand Unaged BG2

Aging 500h BG2Aging 600h BG2

0 100 200 300 400 500 600

25

20

15

10

5

0

s (m

m)

(b)

Figure 11 Load-settlement (P-S) curve of geogrid reinforced sand considering aging effect (a) BG1 (b) BG2

10 Advances in Civil Engineering

gradually appears For BG1 the aging load has an initialvalue of 125 kPa Similar to BG1 the initial aging load ofBG2 bars is 175 kPa which is much larger than that of BG1bars (Figure 11(b)) +e aging effect of BG2 with highertensile strength on the initial load is greater than that of theBG1 geogrid with lower tensile strength It indicates that theaging effect of BG2 with higher tensile strength on thebearing capacity of the reinforced cushion foundation issmaller than that of the BG1 geogrid with lower tensilestrength

After the model test it was found that the defor-mation of biaxial geogrid was significant (see Figure 12)+e deformed geogrid was measured and the centralpoint of the biaxial geogrid shall be concave about17 mm

+e beneficial effect of reinforcement for increasing thebearing capacity is conveniently described by the bearingcapacity ratio (BCR) according to [20]

BCR qR

q0 (7)

In order to compare the strengths of geogrids underdifferent load levels the ultimate bearing capacity and ul-timate bearing capacity ratio are listed in Table 6 It can beseen that the bearing capacity of the geogrid-reinforcedcushion decreases with aging For BG1 the bearing capacityof the foundation decreases by 1333 and 20 respectivelyafter 500 h and 600 h of aging For BG2 aged 500 h and 600 hthe bearing capacity of the foundation decreases by 625and 1875 respectively

+e BCR of the geogrid-reinforced sand is listed inTable 6 It can be seen that the aging effect of the geogrid

leads to the decline of the reinforced foundation BCR whichreduces the bearing capacity of the geogrid-reinforcedfoundation For different types of geogrids the degree ofdecline in BCR owing to aging is different and the value ofdecline in BCR owing to aging is slightly smaller for thehigher-strength BG2 than for the lower-strength BG1

5 MechanismAnalysis of PhotooxygenAging inPP Biaxial Geogrid

+e energy of the ultraviolet light accelerates the agingprocess of the grid Photooxidation causes degradation andaging of the PP germanium geogrid decomposes the PPpolymer and destroys its internal structure [21]+e tensilestrength and fracture elongation of two kinds of PP biaxialgeogrids were reduced in different degrees after differentaging modes temperature aging time and aging condi-tions After thermal aging there is a decrease in cross-linking degree and in binding and friction betweenmolecular chains +e effects of photooxygen aging aregreater than the effects of thermal oxygen aging +esunlight ultraviolet light provides the energy required forchemical crosslinking of the PP biaxial geogrid +e tensilestrength and fracture elongation of the geogrid temporarilyincrease in a short period of time (100ndash200 h) with theextension of aging time

6 Conclusion

In this study thermal oxygen test is carried out at 60degC 70degCand 80degC and ultraviolet photooxygen aging tests are carriedout at 70degC for two kinds of PP biaxial geogrids of different

(a) (b)

Figure 12 Geogrid-reinforced foundation damage

Table 6 BCR of geogrid-reinforced sand

Group Aging time Ultimate bearing capacity (kPa) BCR Percentage reduction in strength due to agingUnreinforced mdash 250 1 mdashBG1 0 375 15 mdashBG1 500 325 13 1333BG1 600 300 12 20BG2 0 400 16 mdashBG2 500 375 15 625BG2 600 325 13 1875

Advances in Civil Engineering 11

specifications +e influence of aging factors on the bearingcapacity of geogrid-reinforced foundations is considered

After investigating the effects of thermal oxygen andphotooxygen aging tests on tensile strength and fractureelongation of geogrids the following conclusions are drawn

(1) +e tensile strength of the geogrid decreases withthe increase in aging time and there is an increasein properties in aging period up to 100ndash200 hours+e comparison with the same thermal oxygenaging time shows that the tensile strength of thegeogrid decreases with the increase in thermaloxygen aging temperature With the increase inphotooxygen aging time the tensile strength of thegeogrid showed a significant trend of decrease +eeffect of ultraviolet light on the geogrid tensilestrength during aging is much greater than that oftemperature +e effect of photooxygen aging ontensile strength is greater than that of thermaloxygen aging for two different types of PP biaxialgeogrid Under the same temperature and agingtime the tensile strength of geogrid is reduced by nomore than 5 while the strength of geogrid isreduced by about 35

(2) +e retention rate of fracture elongation decreaseswith the increase in thermal oxygen aging tempera-ture and aging time+e elongation at break decreaseswith the photooxygen aging time and the influence ofultraviolet light on geogrid fracture elongation ismuch greater than that of temperature +e tensilestrength retention rate of different types of PP biaxialgrids showed different photooxygen aging charac-teristics+e aging resistance performance of BG2wassignificantly higher than that of BG1

(3) +e effect against tensile strength retention rate isbetter with the GM +e following conclusion camefrom prediction models with a blackboard tem-perature of 70degCplusmn 2degC irradiation of 8 h nonirra-diated condensation of 4 h relative humidity of70plusmn 2 and ultraviolet irradiance of 600Wm2the lower-intensity BG1 fails at approximately1100 h +e higher-strength BG2 fails at approxi-mately 1800 h

(4) Geogrid reinforcement significantly changes thebearing characteristics of a sand cushion by in-creasing the bearing capacity of the reinforcedcushion+e reinforcement effect of the geogrid withhigher tensile strength is more significant Agingchanges the interface characteristics between thegeogrid and soil In this test the bearing properties ofthe geogrid cushion after aging have certain changes+e aging behavior of the two geogrids reduces theload bearing capacity of the reinforced cushion by20 and 1875 respectively

Data Availability

No data were used to support this study

Conflicts of Interest

+e authors declare that they have no conflicts of interestrelated to this work

Acknowledgments

+e authors would like to acknowledge the National NaturalScience Foundation of China for the financial support forthis study (no 51578359)

References

[1] Y-L Dong J Han and X-H Bai ldquoNumerical analysis oftensile behavior of geogrids with rectangular and triangularaperturesrdquo Geotextiles and Geomembranes vol 29 no 2pp 83ndash91 2011

[2] A M R Ewais R K Rowe and J Scheirs ldquoDegradationbehaviour of HDPE geomembranes with high and low initialhigh-pressure oxidative induction timerdquo Geotextiles andGeomembranes vol 42 no 2 pp 111ndash126 2014

[3] Y G Hsuan and R M Koerner ldquoAntioxidant depletionlifetime in high density polyethylene geomembranesrdquo Journalof Geotechnical and Geoenvironmental Engineering vol 124no 6 pp 532ndash541 1998

[4] W-K Wong and Y G Hsuan ldquoInteraction of antioxidantswith carbon black in polyethylene using oxidative inductiontime methodsrdquo Geotextiles and Geomembranes vol 42 no 6pp 641ndash647 2014

[5] R K Rowe and H P Sangam ldquoDurability of HDPE geo-membranesrdquo Geotextiles and Geomembranes vol 20 no 2pp 77ndash95 2002

[6] H P Sangam and R K Rowe ldquoEffects of exposure conditionson the depletion of antioxidants from high-density polyeth-ylene (HDPE) geomembranesrdquo Canadian GeotechnicalJournal vol 39 no 6 pp 1221ndash1230 2002

[7] W Muller and I Jacob ldquoOxidative resistance of high densitypolyethylene geomembranesrdquo Polymer Degradation andStability vol 79 pp 161ndash172 2003

[8] S B Gulec T B Edil and C H Benson ldquoEffect of acidic minedrainage on the polymer properties of an HDPE geo-membranerdquo Geosynthetics International vol 11 no 2pp 60ndash72 2004

[9] R K Rowe and S Rimal ldquoDepletion of antioxidants from aHDPE geomembrane in a composite linerrdquo Journal of Geo-technical and Geoenvironmental Engineering vol 134 no 1pp 68ndash78 2008

[10] R K Rowe S Rimal and H P Sangam ldquoAgeing of HDPEgeomembrane exposed to air water and leachate at differenttemperaturesrdquo Geotextiles and Geomembranes vol 27pp 131ndash151 2009

[11] R K RoweM Z Islam and Y G Hsuan ldquoEffects of thicknesson the aging of HDPE geomembranesrdquo Journal of Geotech-nical and Geoenvironmental Engineering vol 136 no 2pp 299ndash309 2010

[12] F B Abdelaal and R K Rowe ldquoEffect of high temperatures onantioxidant depletion from different HDPE geomembranesrdquoGeotextiles and Geomembranes vol 42 no 4 pp 284ndash3012014

[13] X-H Bai X-Z Huang and W Zhang ldquoBearing capacity ofsquare footing supported by a geobelt-reinforced crushedstone cushion on soft soilrdquo Geotextiles and Geomembranesvol 38 pp 37ndash42 2013

12 Advances in Civil Engineering

Universal testing machines are used to carry out tensiletests on geogrids with varying degrees of aging It is mainlyused to test forces in metal and nonmetallic materials andperformance testing +e universal testing machine iscomposed of two parts mechanical transmission system andmicrocomputer control system It has high precision and fastresponse and can achieve real-time recording of test resultsIts technical conditions are as follows load error le05displacement error le05 heavy element test 100 kN ofinterval selection load recognition accuracy 001N movingspeed 0001ndash250mmmin and effective moving distance650mm

222 Model Test Apparatus In order to study the effect ofgeogrid aging on the bearing characteristics of a geogrid-reinforced sand cushion approach the boundary conditionsof a field test and avoid the adverse effects of environmentalfactors the laboratory model test site was located in theLaboratory of the Institute of Taiyuan University of Tech-nology +e size is 9times 4times 31m +e whole test system wascomposed of the foundation soil material static servoloading system test elements and data acquisition systemAs shown in Figure 1 it was loaded by a hydraulic jackBecause of the large amount of sand used in the test and thelimited conditions of the test site local river sand was used asthe experimental soil Before testing the sand was sieved toremove large particles and impurities +e total thickness ofthe sandy soil laid in the groove was 24m In order to ensurethe compactness of the sandy soil to meet the test re-quirements the sandy soil was layered and tamped with aflat-plate rammer+e thickness of each layer was 03m andthe compaction was fully tamped 3 times to ensure that thecompactness of the sandy soil in each layer was the same Asteel rigid plate measuring 200mmtimes 200mm in plane with25mm thickness was used as a square footing to apply theload Four dial gauges were set up to measure the settlementof the footing during application of the load A hydraulicjack loading system was used to apply the load on thegeogrid-reinforced sand cushion through the footing plate

+e load was applied through the high-precision statichydraulic servo table produced by Chengdu Servo HydraulicEquipment Co Ltd and the load application level wascontrolled by the static strain gauge connected with the jackstrain sensor+e strain sensor on the top of the jack and thestrain gauge adopt the full bridge connection method to

control the application of each level of load in the testprocess according to the calibrated strain load relationship+e hydraulic high precision thrust jack used in verticalloading has a rated load of 100 kN

Increasing static loads were applied by using a hydraulicjack +e load and corresponding footing settlement weremeasured by a calibrated pressure gauge and four dialgauges respectively +e testing procedure was performedaccording to GB50007-2012 [17] where increments of loadper unit area were applied and maintained until the rate ofsettlement was less than 01mmh over two consecutivehours +e test was terminated when the total settlementreached 006 B that is 12mm in this program

3 Experimental Procedures

31 (ermal Oxygen Aging +e thermal oxygen aging testwhich is also known as the hot air exposure test artificiallyaccelerates the plastic product or other high molecularpolymers using a gravity convection or forced ventilationheat aging test chamber +e principle of the thermal agingtest is to expose the sample to a high-temperature envi-ronment for a long time by using the high-temperaturestability of the oven +e high temperature can shorten theinduction period of the oxidatively degradable polymermaterial With further increase of time the crystallinity ofthe sample increases and the tensile strength elongationmodulus and impact strength will change +en thechanges in the properties before and after aging are com-pared to determine the degree of aging of the sample Factorsaffecting the test data include the temperature control in thetest chamber humidity in the heat aging test chamber airflow velocity on the surface of the sample and whether it isaffected by periodic rain It can be seen that many factorsaffect the test results All these factors must be preciselycontrolled to ensure that the error is within acceptable limits

For thermal oxygen aging the specimens were aged in athermostatically regulated xenon lamp weathering testchamber +e dimensions of the working chamber were600mmtimes 600mmtimes 600mm +e aging temperature wasachieved and maintained by controlled heating of theworking chamber walls An air ventilation fan in the ceilingof the working chamber forced internal circulationAccording to the technical data there were 10 air changesper hour and a fresh air supply of approximately 10m3h

Table 2 Physical indicators of sand for testing

Layer Cu Cc ρdmax (gcm3) ρdmin (gcm3) d60 (mm) d30 (mm) d10 (mm)

Underlying layer 556 113 170 136 O62 041 022Bearing layer 179 091 171 133 052 037 029

Table 1 Physical property indexes of biaxial geogrids

Samplecode

Grid size(mmtimesmm)

Tensile strength(kNm)

Strength at 2 strain(kNm)

Strength at 5 strain(kNm)

Mass per unit area(kgm2)

Elongation atbreak ()

BG1 35times 33 152 62 114 041 125BG2 33times 32 255 85 165 053 143

Advances in Civil Engineering 3

Square specimens of 50times 50 cm were cut out of the geogridrolls in the middle at randomly distributed locations

After comparing the appearance of the geogrid samplesaged by thermal oxygen with that of the original geogridsamples it was found that the color depth of the geogridsamples aged by thermal oxygen for a long time was slightlylighter

32 Photooxygen Aging In the whole life cycle of geo-synthetics photooxidative aging occurs once they are ex-posed to sunlight for a long time Many studies have shownthat light is the most important factor causing photooxi-dation aging +e outdoor exposure test and the indoor lightsource aging test are the two most important procedures toevaluate the photooxidative aging resistance of geosyntheticmaterials +e test period required for the outdoor exposuretest is long and the external conditions are complicated andnot easy to control +e indoor light source aging test periodis short and it is less interfered by external environmentalconditions It can accurately control the light intensitytemperature humidity oxygen concentration pH andother conditions but it does not match the actual engi-neering environment +e aging time under natural climaticconditions should be found through multiple tests to findout the relationship between the two and establish a con-version formula to predict the service life of the material

+e photooxidation aging test adopts the xenon lampweathering test chamber +e aging test of geogrid wascarried out according to ASTM G154-16 standard [18] +etest box has a built-in rotating sample rack and a whole gridsample is fixed on the sample rack +e rotation speed is setto 3 cyclesmin to ensure uniform illumination and the test isconducted In the aging process the cycle illuminationmodeis adopted +e specific steps are performed for 8 h underlight conditions and then condensed for 4 h without lightand the cumulative aging time is 700 h +e irradiationintensity was set to 600Wm2 the humidity in the testchamber was 70plusmn 2 and the blackboard temperature was70degC

+rough observation of the grid sample after fracturethere was a very obvious phenomenon +ere was a slight

difference in the fracture mode of the grid sample during thetensile test Figure 2 shows the fracture diagram of theoriginal geogrid and the fracture diagram of the test pieceafter the photooxygen aging for 700 h

33 Tensile Tests For the tensile tests universal testingmachines are employed Multirib and single-rib testmethods can be used and this study uses the single-rib testmethod +e first step is to determine the tensile rate duringthe geogrid tensile test In the case of a fixture clamping thetwo ends of the geogrid specimen the distance between thetwo ends of the fixture is set from 11 to 14 cm +e tensilerate selected in this test is 24mmmin Before the tensile testbegins it is necessary to apply 1 prestress to calibrate thetensile strength +e measured data includes the tensilestrength at peak tension fracture elongation and 2 and 5elongation+e longitudinal and transverse ribs are differentin mechanical properties +erefore the longitudinal andtransverse ribs are tested separately tomeasure the tension of10 ribs to obtain an average value f

+e formula for calculating the geogrid tensile strength is

F f times N

L times n (1)

where F is the tensile strength of the geogrid (kNm) f is theaverage of the pulling force (kN) N is the number of ribs onthe sample width n is the number of ribs of the specimenand L is the width of the sample (m)

Statistical analysis of tensile test data of geogrid is inTable 3 It can be seen from Table 3 that the systematic errorof the test results of geogrid tensile test is small and thereliability of the test results is high

34 Model Test In this test a replacement test pit with adesign size of 600mmtimes 600mmtimes 600mm was excavated+e standard dry sand was laid into the pit layer by layer andthe relative compactness of each layer of each group isensured to be the same by compacting the constant qualitysand to the set height Calculate the weight of standard drysand required for each layer with 70 relative density and it

Hydraulic servo system

Counterforce beam

Strain gauge

ComputerGeogrid

Figure 1 Schematic diagram of loading device for the model test

4 Advances in Civil Engineering

(a) (b)

Figure 2 Tensile fracture diagram of test piece (a) Original geogrid (b) Aging geogrid

Table 3 Statistical analysis of tensile test data of geogrid

Geogrid Testing unitSample number Average

valueStandarddeviation

Coefficient ofvariation1 2 3 4 5 6 7 8 9 10

BG1

Tensilestrength 1521 1532 1533 1521 1522 1523 1491 1522 1523 1531 1522 01126 00074

Elongation atbreak 1241 1253 1262 1251 1241 1262 1251 1272 1252 1233 1252 01098 00088

BG2

Tensilestrength 2543 2542 2561 2563 2562 2571 2542 2533 2541 2562 2552 01235 00048

Elongation atbreak 1434 1425 1442 1421 1443 1454 1433 1432 1421 1423 1433 01037 00073

(a)

11 13(12) 15(14)

6 8(7) 10(9)

1 3(2) 5(4)

600

100

100

200

600

200

Loading plate

(b)

Figure 3 +e picture of the model test site and the layout of the Earth pressure box (a) Picture of the model test site (b) Layout of soilpressure box

Advances in Civil Engineering 5

is laied in the soil pit and compacted to the thickness of50mm for each layer+e geogrid is laid at a depth of 70mmunder the loading plate and fixed with U-nails+e picture ofthe model test site and the layout of the Earth pressure boxare shown in Figure 3

4 Results and Discussion

+e thermal oxygen and photooxygen aging will result in thereduction in mechanical properties of the geogrid +emechanical properties of the geogrid with aging parametersare described by the tensile strength retention rate andfracture elongation retention rate +e retention rate oftensile strength is the ratio between the tensile strength of

the aged geogrid and the tensile strength of the geogridbefore aging expressed as a percentage +e fracture elon-gation retention rate is the ratio of the aged fracture elon-gation to the preaging fracture elongation expressed as apercentage

41 Geogrid Aging

411 (ermal Oxygen Aging Test +e results of the thermaloxygen aging test for samples BG1 and BG2 of the biaxialgeogrid of PP are shown in Figures 4 and 5 In Figure 4 theresults show relatively small differences for different aging time(at least for temperatures of 60 and 70 degrees) and aging time

Tens

ile st

reng

th re

tent

ion

()

60degC70degC80degC

0 100 200 300 400 500 600 700 800Thermal oxygen aging time (h)

92

94

96

98

100

102

104

(a)

Tens

ile st

reng

th re

tent

ion

()

60degC70degC80degC

Thermal oxygen aging time (h)0 100 200 300 400 500 600 700 800

93

94

95

96

97

98

99

100

101

102

(b)

Figure 4 Relationship curves of tensile strength retention rate-thermal aging time (a) BG1 (b) BG2

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

60degC70degC80degC

0 100 200 300 400 500 600 700 800Thermal oxygen aging time (h)

828486889092949698

100102104106

(a)

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

60degC70degC80degC

Thermal oxygen aging time (h)0 100 200 300 400 500 600 700 800

86889092949698

100102104

(b)

Figure 5 Relationship curves of elongation at break-thermal aging time (a) BG1 (b) BG2

6 Advances in Civil Engineering

up to 400 hours In Figure 5(a) the results show that theelongation at break of the PP biaxial geogrid increases slightlyin the early stage of thermal oxygen aging and the higher thetemperature is the shorter the increase period is At 60degC and70degC the influence of thermal oxygen aging on tensile strengthand fracture elongation is not obvious +e tensile test oflongitudinal and transverse ribs has been carried out by theauthors +e test results show that the tensile test results oflongitudinal and transverse ribs are not different

Comparing samples BG1 and BG2 of the two grids itcan be found that the temperature is 80degC and the tensilestrength retention rates of samples BG1 and BG2 are 928and 943 respectively when aging is of 700 h +e re-tention rates of BG1 and BG2 fracture elongation were 84

and 8741 respectively when aging is of 700 h +e ex-perimental results show that the tensile strength andfracture elongation of BG1 and BG2 which are both biaxialgeogrids of PP have similar trends with thermal oxygenaging time

412 Photooxygen Aging Test +e relationship betweentensile strength retention rate and photooxygen agingtime of two kinds of geogrid is obtained by the indoorultraviolet photooxygen aging test (Figure 6) It can beseen in Figure 6 that the two curves have the same trendthat is with the increase in photooxygen aging time thetensile strength of the two kinds of PP biaxial geogridsshows a significant trend of decrease After 300 h agingtime the BG1 starts to decline at greater rate than BG1and the difference at the end of the test is gt10 Figure 7shows the relationship between the fracture elongationretention rate and photooxygen aging time of the twogeogrids It can be seen from Figure 7 that within 400 hthe fracture elongation retention rate of the two geogridsalternately declines with the photooxygen aging time Atthe photooxygen aging time of 700 h the fracture elon-gation retention rates of samples BG1 and BG2 are notsignificantly different

413 Comparative Analysis Figure 8 shows the relationshipof tensile strength retention rate between the thermal oxygenaging and the photooxygen aging specimens with aging timeat 70degC

Figure 9 shows the relationship of retention rate ofelongation at break between the thermal oxygen aging andthe photooxygen aging specimens with aging time at 70degCIt can be seen that under the conditions of thermal oxygenaging the curves of tensile strength and fracture elongationretention rates with aging time of samples BG1 and BG2 arerelatively gentle However the tensile strength and fractureelongation retention curves with aging time are steeperunder photooxygen aging conditions +e thermal oxygenaging intensity is reduced by no more than 5 while thephotooxygen aging intensity is reduced by about 35 andthus the impact of photooxygen aging is much greater thanthat of thermal oxygen aging It is shown that the effect oflight on the geogrid tensile strength during aging is muchgreater than that of temperature

414 Prediction Model +e gray system theory proposed byProfessor Deng (China) in 1982 has been widely used ineconomic social and engineering fields It has advantages ofsmooth degree of data and a good precision [19] Based onthe gray prediction model (GM) by using a small amount ofraw data and simple modeling calculation the future datacan be predicted and the accuracy meets the research needsIn this study eight groups of data of tensile strength andelongation at rupture obtained from the photooxidation

0 100 200 300 400 500 600 700 800Photooxidative aging time (h)

BG1BG2

60

65

70

75

80

85

90

95

100Te

nsile

stre

ngth

rete

ntio

n (

)

Figure 6 Relationship curves of tensile strength retention rate-photooxidation aging time

BG1BG2

0 100 200 300 400 500 600 700 800Aging time (h)

50

60

70

80

90

100

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

Figure 7 Relationship curves of retention rate of elongation atbreak-photooxidation aging time

Advances in Civil Engineering 7

aging test of PP biaxial geogrids were provided +e GM wasused to fit the photooxidative aging test results of PP biaxialgeogrids and the failure rule of PP biaxial geogrids underindoor photooxidative aging is inferred

Gray model is a differential equation established bygenerating new data series from the original data series GM(1 N) represents the differential equation of N variables oforder 1 +e modeling process is as follows

Sequence based on more than four known pieces ofdata is

X(0)

(k)1113966 1113967 k 1 2 3 n (2)

Generating sequence of accumulated data with givendata sequence is as follows

X(1)

(k)1113966 1113967 k 1 2 3 n (3)

Approximating cumulative data series values with con-tinuous smooth values of exponential curves is as follows

1113957X(1)

(k) 1113957X(1)

(1) minusu

a1113876 1113877bulle

minus a(kminus 1)+

u

a k 1 2 3 n

(4)

+e smoothing approximation value of the original datasequence is reduced

1113957X(0)

(k) 1113957X(1)

(k) minus 1113957X(1)

(k minus 1) k 1 2 3 n (5)

In the above formula

ermal oxygen agingPhotooxidation aging

Tens

ile st

reng

th re

tent

ion

rate

()

65

70

75

80

85

90

95

100

100 200 300 400 500 600 700 8000Aging time (h)

(a)

ermal oxygen agingPhotooxidation aging

Tens

ile st

reng

th re

tent

ion

()

70

75

80

85

90

95

100

100 200 300 400 500 600 700 8000Aging time (h)

(b)

Figure 8 Relationship curves of tensile strength retention rate-aging time (a) BG1 (b) BG2

ermal oxygen agingPhotooxygen aging

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

50

60

70

80

90

100

110

100 200 300 400 500 600 700 8000Aging time (h)

(a)

ermal oxygen agingPhotooxidation aging

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

50556065707580859095

100105

100 200 300 400 500 600 700 8000Aging time (h)

(b)

Figure 9 Relationship curves of retention rate of elongation at break-aging time (a) BG1 (b) BG2

8 Advances in Civil Engineering

a

u

⎡⎢⎢⎢⎣ ⎤⎥⎥⎥⎦ BTB1113872 1113873

minus 1B

minus 1B

TYn

B

b1 1

b2 1

middot middot middot 1

bnminus1 1

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦

Yn

y1

y2

middot middot middot

ynminus1

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦

m (n minus 1) 1113944nminus1

i1bi( 1113857

2minus 1113944

nminus 1

i1bi

⎛⎝ ⎞⎠

2

a (n minus 1)1113944

nminus1i1 biyi minus 1113944

nminus1i1 bi1113944

nminus1i1 yi1113876 1113877

m

u minus1113944

nminus1i1 bi1113944

nminus1i1 biyi + 1113944

nminus1i1 bi( 1113857

21113944

nminus1i1 yi1113876 1113877

m

(6)

Based on the experimental data of tensile strength andelongation at rupture of PP biaxial geogrids under photo-oxidation aging a GMwas established to predict the changesin tensile strength and elongation at rupture of PP biaxialgeogrids under long-term photooxidation aging +e pre-diction results are listed in Table 4

It can be seen from Table 3 that the GM (1 1) model isestablished for the tensile strength of samples BG1 and BG2under photooxidation aging for 0ndash600 h +e maximumrelative error of fitting the photooxidation aging test resultsof BG1 is only 248 the average error is only 149 and thetensile strength retention rate of BG1 and BG2 under 700 h isonly 662 +e GM is used to predict the tensile strength ofBG1 and BG2 +e calculated predicted value is 646 andthe relative error of data recovery is only 242 +emaximum relative error of fitting BG2 is only 165 theaverage error is only 030 and the relative error ofchecking data with 700 h is only 565 +is clearly showsthat the GM (1 1) model has better fitting effect

Prediction curves of tensile strength and photooxidationaging time are shown in Figure 10 It can be seen fromFigure 10(a) that the tensile strength retention rate of BG1 is5198 When the photooxidation aging time is 1000 h thetensile strength retention rate of BG1 is 4832 When thephotooxidation aging time is 1100 h the tensile strengthretention rate of BG1 is less than 50 so the PP biaxialgeogrid losees efficacy From Figure 10(b) it can be seen thatthe tensile strength retention rate of BG2 is 5188 when thephotooxidation aging time is 1700 h and 4983 when it is1800 h less than 50 and thus the PP biaxial geogrid isinvalid

+e aging test curve changes greatly in the early stage ofaging and is relatively unstable +e prediction curve is notvery accurate in the early stage of aging but rather accurate indescribing the long-term performance of aging

42 Test of Load Bearing Capacity of Reinforced Sand Foun-dation considering theEffects ofAging +e tensile strength ofthe geogrid before and after aging is listed in Table 5

Load-settlement curves of the cushion are obtainedaccording to load at all levels as shown in Figure 10Compared with the nonreinforced sand cushion the set-tlement value of the reinforced sand cushion under the sameload decreases and the bearing capacity of the foundationincreases the settlement value of the reinforced sandcushion of BG2 geogrid was smaller than that of the rein-forced sand cushion of BG1 geogrid and the bearing ca-pacity of the cushion was increased the difference in theload-settlement curves of the reinforced sand cushion andthe nonreinforced sand cushion at the early stage was smallIt shows that the reinforcement effect was not obvious at theinitial stage of loading With the increase in load the spacingbetween the load-settlement curves of the pure sand BG1reinforced sand and BG2 reinforced sand cushions grad-ually increased+us the reinforcement effect is increasinglyobvious

+e P-S curve of geogrid-reinforced sand consideringthe aging effect is shown in Figure 11 Figures 11(a) and11(b) show that the P-S curves of aging geogrids coincidewith those of nonaging geogrids when the load P was lessthan 125 kPa With the increase in load the two P-S curvesbegin to separate and under the same load the settlementratio of the aging-reinforced sand cushion of the BG1geogrids to that of the non-aging-reinforced sand cushionwas less than that of the aging-reinforced sand cushion Ingeneral the bearing capacity of the aging-reinforcedcushion was slightly lower than that of the non-aging-

Table 4 +e results of gray model prediction

Aging time (h) 0 100 200 300 400 500 600

BG1 tensile strength retention rateTest value () 100 986 944 888 7980 737 697

Predicted value () 100 1001 931 866 805 748 695Relative error () mdash 152 138 248 088 149 029

BG2 tensile strength retention rateTest value () 100 978 964 908 865 853 801

Predicted value () 100 9870 9481 9108 8749 8404 8073Relative error () mdash 092 165 030 114 148 078

Advances in Civil Engineering 9

reinforced cushion but the effect is not obvious +ebearing capacity of reinforced soil decreases with the in-crease of aging time

+e effect of aging of geogrids in the early stage ofloading on the bearing capacity of reinforced soil is

negligible +e authors define an aging effect on the initialload When the load value is less than the initial load of agingeffect the aging effect is small on the settlement of foun-dation When the load value exceeds the initial load of agingeffect the aging effect on the settlement of foundation

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

Photooxidative aging time (h)

Test valuePredicted value

Tens

ile st

reng

th re

tent

ion

rate

()

40

50

60

70

80

90

100

(a)

0

200

400

600

800

1000

1200

1400

1600

1800

Photooxidative aging time (h)

Test valuePredicted value

Tens

ile st

reng

th re

tent

ion

()

404550556065707580859095

100

(b)

Figure 10 Prediction curves of tensile strength and photooxidation aging time (a) BG1 (b) BG2

Table 5 +e tensile strength of geogrid before and after aging

Geogrid Tensile strength of geogrid before aging(kNm)

Radiation intensity (Wm2)

Tensile strength of geogrid after aging of700 h

Aging time(h)

BG1 158 600 108 700BG2 255 600 1835 700

0 100 200 300 400 500p (kPa)

Pure sand Unaged BG1

Aging 500h BG1Aging 600h BG1

25

20

15

10

5

0

s (m

m)

(a)

p (kPa)

Pure sand Unaged BG2

Aging 500h BG2Aging 600h BG2

0 100 200 300 400 500 600

25

20

15

10

5

0

s (m

m)

(b)

Figure 11 Load-settlement (P-S) curve of geogrid reinforced sand considering aging effect (a) BG1 (b) BG2

10 Advances in Civil Engineering

gradually appears For BG1 the aging load has an initialvalue of 125 kPa Similar to BG1 the initial aging load ofBG2 bars is 175 kPa which is much larger than that of BG1bars (Figure 11(b)) +e aging effect of BG2 with highertensile strength on the initial load is greater than that of theBG1 geogrid with lower tensile strength It indicates that theaging effect of BG2 with higher tensile strength on thebearing capacity of the reinforced cushion foundation issmaller than that of the BG1 geogrid with lower tensilestrength

After the model test it was found that the defor-mation of biaxial geogrid was significant (see Figure 12)+e deformed geogrid was measured and the centralpoint of the biaxial geogrid shall be concave about17 mm

+e beneficial effect of reinforcement for increasing thebearing capacity is conveniently described by the bearingcapacity ratio (BCR) according to [20]

BCR qR

q0 (7)

In order to compare the strengths of geogrids underdifferent load levels the ultimate bearing capacity and ul-timate bearing capacity ratio are listed in Table 6 It can beseen that the bearing capacity of the geogrid-reinforcedcushion decreases with aging For BG1 the bearing capacityof the foundation decreases by 1333 and 20 respectivelyafter 500 h and 600 h of aging For BG2 aged 500 h and 600 hthe bearing capacity of the foundation decreases by 625and 1875 respectively

+e BCR of the geogrid-reinforced sand is listed inTable 6 It can be seen that the aging effect of the geogrid

leads to the decline of the reinforced foundation BCR whichreduces the bearing capacity of the geogrid-reinforcedfoundation For different types of geogrids the degree ofdecline in BCR owing to aging is different and the value ofdecline in BCR owing to aging is slightly smaller for thehigher-strength BG2 than for the lower-strength BG1

5 MechanismAnalysis of PhotooxygenAging inPP Biaxial Geogrid

+e energy of the ultraviolet light accelerates the agingprocess of the grid Photooxidation causes degradation andaging of the PP germanium geogrid decomposes the PPpolymer and destroys its internal structure [21]+e tensilestrength and fracture elongation of two kinds of PP biaxialgeogrids were reduced in different degrees after differentaging modes temperature aging time and aging condi-tions After thermal aging there is a decrease in cross-linking degree and in binding and friction betweenmolecular chains +e effects of photooxygen aging aregreater than the effects of thermal oxygen aging +esunlight ultraviolet light provides the energy required forchemical crosslinking of the PP biaxial geogrid +e tensilestrength and fracture elongation of the geogrid temporarilyincrease in a short period of time (100ndash200 h) with theextension of aging time

6 Conclusion

In this study thermal oxygen test is carried out at 60degC 70degCand 80degC and ultraviolet photooxygen aging tests are carriedout at 70degC for two kinds of PP biaxial geogrids of different

(a) (b)

Figure 12 Geogrid-reinforced foundation damage

Table 6 BCR of geogrid-reinforced sand

Group Aging time Ultimate bearing capacity (kPa) BCR Percentage reduction in strength due to agingUnreinforced mdash 250 1 mdashBG1 0 375 15 mdashBG1 500 325 13 1333BG1 600 300 12 20BG2 0 400 16 mdashBG2 500 375 15 625BG2 600 325 13 1875

Advances in Civil Engineering 11

specifications +e influence of aging factors on the bearingcapacity of geogrid-reinforced foundations is considered

After investigating the effects of thermal oxygen andphotooxygen aging tests on tensile strength and fractureelongation of geogrids the following conclusions are drawn

(1) +e tensile strength of the geogrid decreases withthe increase in aging time and there is an increasein properties in aging period up to 100ndash200 hours+e comparison with the same thermal oxygenaging time shows that the tensile strength of thegeogrid decreases with the increase in thermaloxygen aging temperature With the increase inphotooxygen aging time the tensile strength of thegeogrid showed a significant trend of decrease +eeffect of ultraviolet light on the geogrid tensilestrength during aging is much greater than that oftemperature +e effect of photooxygen aging ontensile strength is greater than that of thermaloxygen aging for two different types of PP biaxialgeogrid Under the same temperature and agingtime the tensile strength of geogrid is reduced by nomore than 5 while the strength of geogrid isreduced by about 35

(2) +e retention rate of fracture elongation decreaseswith the increase in thermal oxygen aging tempera-ture and aging time+e elongation at break decreaseswith the photooxygen aging time and the influence ofultraviolet light on geogrid fracture elongation ismuch greater than that of temperature +e tensilestrength retention rate of different types of PP biaxialgrids showed different photooxygen aging charac-teristics+e aging resistance performance of BG2wassignificantly higher than that of BG1

(3) +e effect against tensile strength retention rate isbetter with the GM +e following conclusion camefrom prediction models with a blackboard tem-perature of 70degCplusmn 2degC irradiation of 8 h nonirra-diated condensation of 4 h relative humidity of70plusmn 2 and ultraviolet irradiance of 600Wm2the lower-intensity BG1 fails at approximately1100 h +e higher-strength BG2 fails at approxi-mately 1800 h

(4) Geogrid reinforcement significantly changes thebearing characteristics of a sand cushion by in-creasing the bearing capacity of the reinforcedcushion+e reinforcement effect of the geogrid withhigher tensile strength is more significant Agingchanges the interface characteristics between thegeogrid and soil In this test the bearing properties ofthe geogrid cushion after aging have certain changes+e aging behavior of the two geogrids reduces theload bearing capacity of the reinforced cushion by20 and 1875 respectively

Data Availability

No data were used to support this study

Conflicts of Interest

+e authors declare that they have no conflicts of interestrelated to this work

Acknowledgments

+e authors would like to acknowledge the National NaturalScience Foundation of China for the financial support forthis study (no 51578359)

References

[1] Y-L Dong J Han and X-H Bai ldquoNumerical analysis oftensile behavior of geogrids with rectangular and triangularaperturesrdquo Geotextiles and Geomembranes vol 29 no 2pp 83ndash91 2011

[2] A M R Ewais R K Rowe and J Scheirs ldquoDegradationbehaviour of HDPE geomembranes with high and low initialhigh-pressure oxidative induction timerdquo Geotextiles andGeomembranes vol 42 no 2 pp 111ndash126 2014

[3] Y G Hsuan and R M Koerner ldquoAntioxidant depletionlifetime in high density polyethylene geomembranesrdquo Journalof Geotechnical and Geoenvironmental Engineering vol 124no 6 pp 532ndash541 1998

[4] W-K Wong and Y G Hsuan ldquoInteraction of antioxidantswith carbon black in polyethylene using oxidative inductiontime methodsrdquo Geotextiles and Geomembranes vol 42 no 6pp 641ndash647 2014

[5] R K Rowe and H P Sangam ldquoDurability of HDPE geo-membranesrdquo Geotextiles and Geomembranes vol 20 no 2pp 77ndash95 2002

[6] H P Sangam and R K Rowe ldquoEffects of exposure conditionson the depletion of antioxidants from high-density polyeth-ylene (HDPE) geomembranesrdquo Canadian GeotechnicalJournal vol 39 no 6 pp 1221ndash1230 2002

[7] W Muller and I Jacob ldquoOxidative resistance of high densitypolyethylene geomembranesrdquo Polymer Degradation andStability vol 79 pp 161ndash172 2003

[8] S B Gulec T B Edil and C H Benson ldquoEffect of acidic minedrainage on the polymer properties of an HDPE geo-membranerdquo Geosynthetics International vol 11 no 2pp 60ndash72 2004

[9] R K Rowe and S Rimal ldquoDepletion of antioxidants from aHDPE geomembrane in a composite linerrdquo Journal of Geo-technical and Geoenvironmental Engineering vol 134 no 1pp 68ndash78 2008

[10] R K Rowe S Rimal and H P Sangam ldquoAgeing of HDPEgeomembrane exposed to air water and leachate at differenttemperaturesrdquo Geotextiles and Geomembranes vol 27pp 131ndash151 2009

[11] R K RoweM Z Islam and Y G Hsuan ldquoEffects of thicknesson the aging of HDPE geomembranesrdquo Journal of Geotech-nical and Geoenvironmental Engineering vol 136 no 2pp 299ndash309 2010

[12] F B Abdelaal and R K Rowe ldquoEffect of high temperatures onantioxidant depletion from different HDPE geomembranesrdquoGeotextiles and Geomembranes vol 42 no 4 pp 284ndash3012014

[13] X-H Bai X-Z Huang and W Zhang ldquoBearing capacity ofsquare footing supported by a geobelt-reinforced crushedstone cushion on soft soilrdquo Geotextiles and Geomembranesvol 38 pp 37ndash42 2013

12 Advances in Civil Engineering

Square specimens of 50times 50 cm were cut out of the geogridrolls in the middle at randomly distributed locations

After comparing the appearance of the geogrid samplesaged by thermal oxygen with that of the original geogridsamples it was found that the color depth of the geogridsamples aged by thermal oxygen for a long time was slightlylighter

32 Photooxygen Aging In the whole life cycle of geo-synthetics photooxidative aging occurs once they are ex-posed to sunlight for a long time Many studies have shownthat light is the most important factor causing photooxi-dation aging +e outdoor exposure test and the indoor lightsource aging test are the two most important procedures toevaluate the photooxidative aging resistance of geosyntheticmaterials +e test period required for the outdoor exposuretest is long and the external conditions are complicated andnot easy to control +e indoor light source aging test periodis short and it is less interfered by external environmentalconditions It can accurately control the light intensitytemperature humidity oxygen concentration pH andother conditions but it does not match the actual engi-neering environment +e aging time under natural climaticconditions should be found through multiple tests to findout the relationship between the two and establish a con-version formula to predict the service life of the material

+e photooxidation aging test adopts the xenon lampweathering test chamber +e aging test of geogrid wascarried out according to ASTM G154-16 standard [18] +etest box has a built-in rotating sample rack and a whole gridsample is fixed on the sample rack +e rotation speed is setto 3 cyclesmin to ensure uniform illumination and the test isconducted In the aging process the cycle illuminationmodeis adopted +e specific steps are performed for 8 h underlight conditions and then condensed for 4 h without lightand the cumulative aging time is 700 h +e irradiationintensity was set to 600Wm2 the humidity in the testchamber was 70plusmn 2 and the blackboard temperature was70degC

+rough observation of the grid sample after fracturethere was a very obvious phenomenon +ere was a slight

difference in the fracture mode of the grid sample during thetensile test Figure 2 shows the fracture diagram of theoriginal geogrid and the fracture diagram of the test pieceafter the photooxygen aging for 700 h

33 Tensile Tests For the tensile tests universal testingmachines are employed Multirib and single-rib testmethods can be used and this study uses the single-rib testmethod +e first step is to determine the tensile rate duringthe geogrid tensile test In the case of a fixture clamping thetwo ends of the geogrid specimen the distance between thetwo ends of the fixture is set from 11 to 14 cm +e tensilerate selected in this test is 24mmmin Before the tensile testbegins it is necessary to apply 1 prestress to calibrate thetensile strength +e measured data includes the tensilestrength at peak tension fracture elongation and 2 and 5elongation+e longitudinal and transverse ribs are differentin mechanical properties +erefore the longitudinal andtransverse ribs are tested separately tomeasure the tension of10 ribs to obtain an average value f

+e formula for calculating the geogrid tensile strength is

F f times N

L times n (1)

where F is the tensile strength of the geogrid (kNm) f is theaverage of the pulling force (kN) N is the number of ribs onthe sample width n is the number of ribs of the specimenand L is the width of the sample (m)

Statistical analysis of tensile test data of geogrid is inTable 3 It can be seen from Table 3 that the systematic errorof the test results of geogrid tensile test is small and thereliability of the test results is high

34 Model Test In this test a replacement test pit with adesign size of 600mmtimes 600mmtimes 600mm was excavated+e standard dry sand was laid into the pit layer by layer andthe relative compactness of each layer of each group isensured to be the same by compacting the constant qualitysand to the set height Calculate the weight of standard drysand required for each layer with 70 relative density and it

Hydraulic servo system

Counterforce beam

Strain gauge

ComputerGeogrid

Figure 1 Schematic diagram of loading device for the model test

4 Advances in Civil Engineering

(a) (b)

Figure 2 Tensile fracture diagram of test piece (a) Original geogrid (b) Aging geogrid

Table 3 Statistical analysis of tensile test data of geogrid

Geogrid Testing unitSample number Average

valueStandarddeviation

Coefficient ofvariation1 2 3 4 5 6 7 8 9 10

BG1

Tensilestrength 1521 1532 1533 1521 1522 1523 1491 1522 1523 1531 1522 01126 00074

Elongation atbreak 1241 1253 1262 1251 1241 1262 1251 1272 1252 1233 1252 01098 00088

BG2

Tensilestrength 2543 2542 2561 2563 2562 2571 2542 2533 2541 2562 2552 01235 00048

Elongation atbreak 1434 1425 1442 1421 1443 1454 1433 1432 1421 1423 1433 01037 00073

(a)

11 13(12) 15(14)

6 8(7) 10(9)

1 3(2) 5(4)

600

100

100

200

600

200

Loading plate

(b)

Figure 3 +e picture of the model test site and the layout of the Earth pressure box (a) Picture of the model test site (b) Layout of soilpressure box

Advances in Civil Engineering 5

is laied in the soil pit and compacted to the thickness of50mm for each layer+e geogrid is laid at a depth of 70mmunder the loading plate and fixed with U-nails+e picture ofthe model test site and the layout of the Earth pressure boxare shown in Figure 3

4 Results and Discussion

+e thermal oxygen and photooxygen aging will result in thereduction in mechanical properties of the geogrid +emechanical properties of the geogrid with aging parametersare described by the tensile strength retention rate andfracture elongation retention rate +e retention rate oftensile strength is the ratio between the tensile strength of

the aged geogrid and the tensile strength of the geogridbefore aging expressed as a percentage +e fracture elon-gation retention rate is the ratio of the aged fracture elon-gation to the preaging fracture elongation expressed as apercentage

41 Geogrid Aging

411 (ermal Oxygen Aging Test +e results of the thermaloxygen aging test for samples BG1 and BG2 of the biaxialgeogrid of PP are shown in Figures 4 and 5 In Figure 4 theresults show relatively small differences for different aging time(at least for temperatures of 60 and 70 degrees) and aging time

Tens

ile st

reng

th re

tent

ion

()

60degC70degC80degC

0 100 200 300 400 500 600 700 800Thermal oxygen aging time (h)

92

94

96

98

100

102

104

(a)

Tens

ile st

reng

th re

tent

ion

()

60degC70degC80degC

Thermal oxygen aging time (h)0 100 200 300 400 500 600 700 800

93

94

95

96

97

98

99

100

101

102

(b)

Figure 4 Relationship curves of tensile strength retention rate-thermal aging time (a) BG1 (b) BG2

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

60degC70degC80degC

0 100 200 300 400 500 600 700 800Thermal oxygen aging time (h)

828486889092949698

100102104106

(a)

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

60degC70degC80degC

Thermal oxygen aging time (h)0 100 200 300 400 500 600 700 800

86889092949698

100102104

(b)

Figure 5 Relationship curves of elongation at break-thermal aging time (a) BG1 (b) BG2

6 Advances in Civil Engineering

up to 400 hours In Figure 5(a) the results show that theelongation at break of the PP biaxial geogrid increases slightlyin the early stage of thermal oxygen aging and the higher thetemperature is the shorter the increase period is At 60degC and70degC the influence of thermal oxygen aging on tensile strengthand fracture elongation is not obvious +e tensile test oflongitudinal and transverse ribs has been carried out by theauthors +e test results show that the tensile test results oflongitudinal and transverse ribs are not different

Comparing samples BG1 and BG2 of the two grids itcan be found that the temperature is 80degC and the tensilestrength retention rates of samples BG1 and BG2 are 928and 943 respectively when aging is of 700 h +e re-tention rates of BG1 and BG2 fracture elongation were 84

and 8741 respectively when aging is of 700 h +e ex-perimental results show that the tensile strength andfracture elongation of BG1 and BG2 which are both biaxialgeogrids of PP have similar trends with thermal oxygenaging time

412 Photooxygen Aging Test +e relationship betweentensile strength retention rate and photooxygen agingtime of two kinds of geogrid is obtained by the indoorultraviolet photooxygen aging test (Figure 6) It can beseen in Figure 6 that the two curves have the same trendthat is with the increase in photooxygen aging time thetensile strength of the two kinds of PP biaxial geogridsshows a significant trend of decrease After 300 h agingtime the BG1 starts to decline at greater rate than BG1and the difference at the end of the test is gt10 Figure 7shows the relationship between the fracture elongationretention rate and photooxygen aging time of the twogeogrids It can be seen from Figure 7 that within 400 hthe fracture elongation retention rate of the two geogridsalternately declines with the photooxygen aging time Atthe photooxygen aging time of 700 h the fracture elon-gation retention rates of samples BG1 and BG2 are notsignificantly different

413 Comparative Analysis Figure 8 shows the relationshipof tensile strength retention rate between the thermal oxygenaging and the photooxygen aging specimens with aging timeat 70degC

Figure 9 shows the relationship of retention rate ofelongation at break between the thermal oxygen aging andthe photooxygen aging specimens with aging time at 70degCIt can be seen that under the conditions of thermal oxygenaging the curves of tensile strength and fracture elongationretention rates with aging time of samples BG1 and BG2 arerelatively gentle However the tensile strength and fractureelongation retention curves with aging time are steeperunder photooxygen aging conditions +e thermal oxygenaging intensity is reduced by no more than 5 while thephotooxygen aging intensity is reduced by about 35 andthus the impact of photooxygen aging is much greater thanthat of thermal oxygen aging It is shown that the effect oflight on the geogrid tensile strength during aging is muchgreater than that of temperature

414 Prediction Model +e gray system theory proposed byProfessor Deng (China) in 1982 has been widely used ineconomic social and engineering fields It has advantages ofsmooth degree of data and a good precision [19] Based onthe gray prediction model (GM) by using a small amount ofraw data and simple modeling calculation the future datacan be predicted and the accuracy meets the research needsIn this study eight groups of data of tensile strength andelongation at rupture obtained from the photooxidation

0 100 200 300 400 500 600 700 800Photooxidative aging time (h)

BG1BG2

60

65

70

75

80

85

90

95

100Te

nsile

stre

ngth

rete

ntio

n (

)

Figure 6 Relationship curves of tensile strength retention rate-photooxidation aging time

BG1BG2

0 100 200 300 400 500 600 700 800Aging time (h)

50

60

70

80

90

100

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

Figure 7 Relationship curves of retention rate of elongation atbreak-photooxidation aging time

Advances in Civil Engineering 7

aging test of PP biaxial geogrids were provided +e GM wasused to fit the photooxidative aging test results of PP biaxialgeogrids and the failure rule of PP biaxial geogrids underindoor photooxidative aging is inferred

Gray model is a differential equation established bygenerating new data series from the original data series GM(1 N) represents the differential equation of N variables oforder 1 +e modeling process is as follows

Sequence based on more than four known pieces ofdata is

X(0)

(k)1113966 1113967 k 1 2 3 n (2)

Generating sequence of accumulated data with givendata sequence is as follows

X(1)

(k)1113966 1113967 k 1 2 3 n (3)

Approximating cumulative data series values with con-tinuous smooth values of exponential curves is as follows

1113957X(1)

(k) 1113957X(1)

(1) minusu

a1113876 1113877bulle

minus a(kminus 1)+

u

a k 1 2 3 n

(4)

+e smoothing approximation value of the original datasequence is reduced

1113957X(0)

(k) 1113957X(1)

(k) minus 1113957X(1)

(k minus 1) k 1 2 3 n (5)

In the above formula

ermal oxygen agingPhotooxidation aging

Tens

ile st

reng

th re

tent

ion

rate

()

65

70

75

80

85

90

95

100

100 200 300 400 500 600 700 8000Aging time (h)

(a)

ermal oxygen agingPhotooxidation aging

Tens

ile st

reng

th re

tent

ion

()

70

75

80

85

90

95

100

100 200 300 400 500 600 700 8000Aging time (h)

(b)

Figure 8 Relationship curves of tensile strength retention rate-aging time (a) BG1 (b) BG2

ermal oxygen agingPhotooxygen aging

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

50

60

70

80

90

100

110

100 200 300 400 500 600 700 8000Aging time (h)

(a)

ermal oxygen agingPhotooxidation aging

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

50556065707580859095

100105

100 200 300 400 500 600 700 8000Aging time (h)

(b)

Figure 9 Relationship curves of retention rate of elongation at break-aging time (a) BG1 (b) BG2

8 Advances in Civil Engineering

a

u

⎡⎢⎢⎢⎣ ⎤⎥⎥⎥⎦ BTB1113872 1113873

minus 1B

minus 1B

TYn

B

b1 1

b2 1

middot middot middot 1

bnminus1 1

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦

Yn

y1

y2

middot middot middot

ynminus1

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦

m (n minus 1) 1113944nminus1

i1bi( 1113857

2minus 1113944

nminus 1

i1bi

⎛⎝ ⎞⎠

2

a (n minus 1)1113944

nminus1i1 biyi minus 1113944

nminus1i1 bi1113944

nminus1i1 yi1113876 1113877

m

u minus1113944

nminus1i1 bi1113944

nminus1i1 biyi + 1113944

nminus1i1 bi( 1113857

21113944

nminus1i1 yi1113876 1113877

m

(6)

Based on the experimental data of tensile strength andelongation at rupture of PP biaxial geogrids under photo-oxidation aging a GMwas established to predict the changesin tensile strength and elongation at rupture of PP biaxialgeogrids under long-term photooxidation aging +e pre-diction results are listed in Table 4

It can be seen from Table 3 that the GM (1 1) model isestablished for the tensile strength of samples BG1 and BG2under photooxidation aging for 0ndash600 h +e maximumrelative error of fitting the photooxidation aging test resultsof BG1 is only 248 the average error is only 149 and thetensile strength retention rate of BG1 and BG2 under 700 h isonly 662 +e GM is used to predict the tensile strength ofBG1 and BG2 +e calculated predicted value is 646 andthe relative error of data recovery is only 242 +emaximum relative error of fitting BG2 is only 165 theaverage error is only 030 and the relative error ofchecking data with 700 h is only 565 +is clearly showsthat the GM (1 1) model has better fitting effect

Prediction curves of tensile strength and photooxidationaging time are shown in Figure 10 It can be seen fromFigure 10(a) that the tensile strength retention rate of BG1 is5198 When the photooxidation aging time is 1000 h thetensile strength retention rate of BG1 is 4832 When thephotooxidation aging time is 1100 h the tensile strengthretention rate of BG1 is less than 50 so the PP biaxialgeogrid losees efficacy From Figure 10(b) it can be seen thatthe tensile strength retention rate of BG2 is 5188 when thephotooxidation aging time is 1700 h and 4983 when it is1800 h less than 50 and thus the PP biaxial geogrid isinvalid

+e aging test curve changes greatly in the early stage ofaging and is relatively unstable +e prediction curve is notvery accurate in the early stage of aging but rather accurate indescribing the long-term performance of aging

42 Test of Load Bearing Capacity of Reinforced Sand Foun-dation considering theEffects ofAging +e tensile strength ofthe geogrid before and after aging is listed in Table 5

Load-settlement curves of the cushion are obtainedaccording to load at all levels as shown in Figure 10Compared with the nonreinforced sand cushion the set-tlement value of the reinforced sand cushion under the sameload decreases and the bearing capacity of the foundationincreases the settlement value of the reinforced sandcushion of BG2 geogrid was smaller than that of the rein-forced sand cushion of BG1 geogrid and the bearing ca-pacity of the cushion was increased the difference in theload-settlement curves of the reinforced sand cushion andthe nonreinforced sand cushion at the early stage was smallIt shows that the reinforcement effect was not obvious at theinitial stage of loading With the increase in load the spacingbetween the load-settlement curves of the pure sand BG1reinforced sand and BG2 reinforced sand cushions grad-ually increased+us the reinforcement effect is increasinglyobvious

+e P-S curve of geogrid-reinforced sand consideringthe aging effect is shown in Figure 11 Figures 11(a) and11(b) show that the P-S curves of aging geogrids coincidewith those of nonaging geogrids when the load P was lessthan 125 kPa With the increase in load the two P-S curvesbegin to separate and under the same load the settlementratio of the aging-reinforced sand cushion of the BG1geogrids to that of the non-aging-reinforced sand cushionwas less than that of the aging-reinforced sand cushion Ingeneral the bearing capacity of the aging-reinforcedcushion was slightly lower than that of the non-aging-

Table 4 +e results of gray model prediction

Aging time (h) 0 100 200 300 400 500 600

BG1 tensile strength retention rateTest value () 100 986 944 888 7980 737 697

Predicted value () 100 1001 931 866 805 748 695Relative error () mdash 152 138 248 088 149 029

BG2 tensile strength retention rateTest value () 100 978 964 908 865 853 801

Predicted value () 100 9870 9481 9108 8749 8404 8073Relative error () mdash 092 165 030 114 148 078

Advances in Civil Engineering 9

reinforced cushion but the effect is not obvious +ebearing capacity of reinforced soil decreases with the in-crease of aging time

+e effect of aging of geogrids in the early stage ofloading on the bearing capacity of reinforced soil is

negligible +e authors define an aging effect on the initialload When the load value is less than the initial load of agingeffect the aging effect is small on the settlement of foun-dation When the load value exceeds the initial load of agingeffect the aging effect on the settlement of foundation

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

Photooxidative aging time (h)

Test valuePredicted value

Tens

ile st

reng

th re

tent

ion

rate

()

40

50

60

70

80

90

100

(a)

0

200

400

600

800

1000

1200

1400

1600

1800

Photooxidative aging time (h)

Test valuePredicted value

Tens

ile st

reng

th re

tent

ion

()

404550556065707580859095

100

(b)

Figure 10 Prediction curves of tensile strength and photooxidation aging time (a) BG1 (b) BG2

Table 5 +e tensile strength of geogrid before and after aging

Geogrid Tensile strength of geogrid before aging(kNm)

Radiation intensity (Wm2)

Tensile strength of geogrid after aging of700 h

Aging time(h)

BG1 158 600 108 700BG2 255 600 1835 700

0 100 200 300 400 500p (kPa)

Pure sand Unaged BG1

Aging 500h BG1Aging 600h BG1

25

20

15

10

5

0

s (m

m)

(a)

p (kPa)

Pure sand Unaged BG2

Aging 500h BG2Aging 600h BG2

0 100 200 300 400 500 600

25

20

15

10

5

0

s (m

m)

(b)

Figure 11 Load-settlement (P-S) curve of geogrid reinforced sand considering aging effect (a) BG1 (b) BG2

10 Advances in Civil Engineering

gradually appears For BG1 the aging load has an initialvalue of 125 kPa Similar to BG1 the initial aging load ofBG2 bars is 175 kPa which is much larger than that of BG1bars (Figure 11(b)) +e aging effect of BG2 with highertensile strength on the initial load is greater than that of theBG1 geogrid with lower tensile strength It indicates that theaging effect of BG2 with higher tensile strength on thebearing capacity of the reinforced cushion foundation issmaller than that of the BG1 geogrid with lower tensilestrength

After the model test it was found that the defor-mation of biaxial geogrid was significant (see Figure 12)+e deformed geogrid was measured and the centralpoint of the biaxial geogrid shall be concave about17 mm

+e beneficial effect of reinforcement for increasing thebearing capacity is conveniently described by the bearingcapacity ratio (BCR) according to [20]

BCR qR

q0 (7)

In order to compare the strengths of geogrids underdifferent load levels the ultimate bearing capacity and ul-timate bearing capacity ratio are listed in Table 6 It can beseen that the bearing capacity of the geogrid-reinforcedcushion decreases with aging For BG1 the bearing capacityof the foundation decreases by 1333 and 20 respectivelyafter 500 h and 600 h of aging For BG2 aged 500 h and 600 hthe bearing capacity of the foundation decreases by 625and 1875 respectively

+e BCR of the geogrid-reinforced sand is listed inTable 6 It can be seen that the aging effect of the geogrid

leads to the decline of the reinforced foundation BCR whichreduces the bearing capacity of the geogrid-reinforcedfoundation For different types of geogrids the degree ofdecline in BCR owing to aging is different and the value ofdecline in BCR owing to aging is slightly smaller for thehigher-strength BG2 than for the lower-strength BG1

5 MechanismAnalysis of PhotooxygenAging inPP Biaxial Geogrid

+e energy of the ultraviolet light accelerates the agingprocess of the grid Photooxidation causes degradation andaging of the PP germanium geogrid decomposes the PPpolymer and destroys its internal structure [21]+e tensilestrength and fracture elongation of two kinds of PP biaxialgeogrids were reduced in different degrees after differentaging modes temperature aging time and aging condi-tions After thermal aging there is a decrease in cross-linking degree and in binding and friction betweenmolecular chains +e effects of photooxygen aging aregreater than the effects of thermal oxygen aging +esunlight ultraviolet light provides the energy required forchemical crosslinking of the PP biaxial geogrid +e tensilestrength and fracture elongation of the geogrid temporarilyincrease in a short period of time (100ndash200 h) with theextension of aging time

6 Conclusion

In this study thermal oxygen test is carried out at 60degC 70degCand 80degC and ultraviolet photooxygen aging tests are carriedout at 70degC for two kinds of PP biaxial geogrids of different

(a) (b)

Figure 12 Geogrid-reinforced foundation damage

Table 6 BCR of geogrid-reinforced sand

Group Aging time Ultimate bearing capacity (kPa) BCR Percentage reduction in strength due to agingUnreinforced mdash 250 1 mdashBG1 0 375 15 mdashBG1 500 325 13 1333BG1 600 300 12 20BG2 0 400 16 mdashBG2 500 375 15 625BG2 600 325 13 1875

Advances in Civil Engineering 11

specifications +e influence of aging factors on the bearingcapacity of geogrid-reinforced foundations is considered

After investigating the effects of thermal oxygen andphotooxygen aging tests on tensile strength and fractureelongation of geogrids the following conclusions are drawn

(1) +e tensile strength of the geogrid decreases withthe increase in aging time and there is an increasein properties in aging period up to 100ndash200 hours+e comparison with the same thermal oxygenaging time shows that the tensile strength of thegeogrid decreases with the increase in thermaloxygen aging temperature With the increase inphotooxygen aging time the tensile strength of thegeogrid showed a significant trend of decrease +eeffect of ultraviolet light on the geogrid tensilestrength during aging is much greater than that oftemperature +e effect of photooxygen aging ontensile strength is greater than that of thermaloxygen aging for two different types of PP biaxialgeogrid Under the same temperature and agingtime the tensile strength of geogrid is reduced by nomore than 5 while the strength of geogrid isreduced by about 35

(2) +e retention rate of fracture elongation decreaseswith the increase in thermal oxygen aging tempera-ture and aging time+e elongation at break decreaseswith the photooxygen aging time and the influence ofultraviolet light on geogrid fracture elongation ismuch greater than that of temperature +e tensilestrength retention rate of different types of PP biaxialgrids showed different photooxygen aging charac-teristics+e aging resistance performance of BG2wassignificantly higher than that of BG1

(3) +e effect against tensile strength retention rate isbetter with the GM +e following conclusion camefrom prediction models with a blackboard tem-perature of 70degCplusmn 2degC irradiation of 8 h nonirra-diated condensation of 4 h relative humidity of70plusmn 2 and ultraviolet irradiance of 600Wm2the lower-intensity BG1 fails at approximately1100 h +e higher-strength BG2 fails at approxi-mately 1800 h

(4) Geogrid reinforcement significantly changes thebearing characteristics of a sand cushion by in-creasing the bearing capacity of the reinforcedcushion+e reinforcement effect of the geogrid withhigher tensile strength is more significant Agingchanges the interface characteristics between thegeogrid and soil In this test the bearing properties ofthe geogrid cushion after aging have certain changes+e aging behavior of the two geogrids reduces theload bearing capacity of the reinforced cushion by20 and 1875 respectively

Data Availability

No data were used to support this study

Conflicts of Interest

+e authors declare that they have no conflicts of interestrelated to this work

Acknowledgments

+e authors would like to acknowledge the National NaturalScience Foundation of China for the financial support forthis study (no 51578359)

References

[1] Y-L Dong J Han and X-H Bai ldquoNumerical analysis oftensile behavior of geogrids with rectangular and triangularaperturesrdquo Geotextiles and Geomembranes vol 29 no 2pp 83ndash91 2011

[2] A M R Ewais R K Rowe and J Scheirs ldquoDegradationbehaviour of HDPE geomembranes with high and low initialhigh-pressure oxidative induction timerdquo Geotextiles andGeomembranes vol 42 no 2 pp 111ndash126 2014

[3] Y G Hsuan and R M Koerner ldquoAntioxidant depletionlifetime in high density polyethylene geomembranesrdquo Journalof Geotechnical and Geoenvironmental Engineering vol 124no 6 pp 532ndash541 1998

[4] W-K Wong and Y G Hsuan ldquoInteraction of antioxidantswith carbon black in polyethylene using oxidative inductiontime methodsrdquo Geotextiles and Geomembranes vol 42 no 6pp 641ndash647 2014

[5] R K Rowe and H P Sangam ldquoDurability of HDPE geo-membranesrdquo Geotextiles and Geomembranes vol 20 no 2pp 77ndash95 2002

[6] H P Sangam and R K Rowe ldquoEffects of exposure conditionson the depletion of antioxidants from high-density polyeth-ylene (HDPE) geomembranesrdquo Canadian GeotechnicalJournal vol 39 no 6 pp 1221ndash1230 2002

[7] W Muller and I Jacob ldquoOxidative resistance of high densitypolyethylene geomembranesrdquo Polymer Degradation andStability vol 79 pp 161ndash172 2003

[8] S B Gulec T B Edil and C H Benson ldquoEffect of acidic minedrainage on the polymer properties of an HDPE geo-membranerdquo Geosynthetics International vol 11 no 2pp 60ndash72 2004

[9] R K Rowe and S Rimal ldquoDepletion of antioxidants from aHDPE geomembrane in a composite linerrdquo Journal of Geo-technical and Geoenvironmental Engineering vol 134 no 1pp 68ndash78 2008

[10] R K Rowe S Rimal and H P Sangam ldquoAgeing of HDPEgeomembrane exposed to air water and leachate at differenttemperaturesrdquo Geotextiles and Geomembranes vol 27pp 131ndash151 2009

[11] R K RoweM Z Islam and Y G Hsuan ldquoEffects of thicknesson the aging of HDPE geomembranesrdquo Journal of Geotech-nical and Geoenvironmental Engineering vol 136 no 2pp 299ndash309 2010

[12] F B Abdelaal and R K Rowe ldquoEffect of high temperatures onantioxidant depletion from different HDPE geomembranesrdquoGeotextiles and Geomembranes vol 42 no 4 pp 284ndash3012014

[13] X-H Bai X-Z Huang and W Zhang ldquoBearing capacity ofsquare footing supported by a geobelt-reinforced crushedstone cushion on soft soilrdquo Geotextiles and Geomembranesvol 38 pp 37ndash42 2013

12 Advances in Civil Engineering

(a) (b)

Figure 2 Tensile fracture diagram of test piece (a) Original geogrid (b) Aging geogrid

Table 3 Statistical analysis of tensile test data of geogrid

Geogrid Testing unitSample number Average

valueStandarddeviation

Coefficient ofvariation1 2 3 4 5 6 7 8 9 10

BG1

Tensilestrength 1521 1532 1533 1521 1522 1523 1491 1522 1523 1531 1522 01126 00074

Elongation atbreak 1241 1253 1262 1251 1241 1262 1251 1272 1252 1233 1252 01098 00088

BG2

Tensilestrength 2543 2542 2561 2563 2562 2571 2542 2533 2541 2562 2552 01235 00048

Elongation atbreak 1434 1425 1442 1421 1443 1454 1433 1432 1421 1423 1433 01037 00073

(a)

11 13(12) 15(14)

6 8(7) 10(9)

1 3(2) 5(4)

600

100

100

200

600

200

Loading plate

(b)

Figure 3 +e picture of the model test site and the layout of the Earth pressure box (a) Picture of the model test site (b) Layout of soilpressure box

Advances in Civil Engineering 5

is laied in the soil pit and compacted to the thickness of50mm for each layer+e geogrid is laid at a depth of 70mmunder the loading plate and fixed with U-nails+e picture ofthe model test site and the layout of the Earth pressure boxare shown in Figure 3

4 Results and Discussion

+e thermal oxygen and photooxygen aging will result in thereduction in mechanical properties of the geogrid +emechanical properties of the geogrid with aging parametersare described by the tensile strength retention rate andfracture elongation retention rate +e retention rate oftensile strength is the ratio between the tensile strength of

the aged geogrid and the tensile strength of the geogridbefore aging expressed as a percentage +e fracture elon-gation retention rate is the ratio of the aged fracture elon-gation to the preaging fracture elongation expressed as apercentage

41 Geogrid Aging

411 (ermal Oxygen Aging Test +e results of the thermaloxygen aging test for samples BG1 and BG2 of the biaxialgeogrid of PP are shown in Figures 4 and 5 In Figure 4 theresults show relatively small differences for different aging time(at least for temperatures of 60 and 70 degrees) and aging time

Tens

ile st

reng

th re

tent

ion

()

60degC70degC80degC

0 100 200 300 400 500 600 700 800Thermal oxygen aging time (h)

92

94

96

98

100

102

104

(a)

Tens

ile st

reng

th re

tent

ion

()

60degC70degC80degC

Thermal oxygen aging time (h)0 100 200 300 400 500 600 700 800

93

94

95

96

97

98

99

100

101

102

(b)

Figure 4 Relationship curves of tensile strength retention rate-thermal aging time (a) BG1 (b) BG2

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

60degC70degC80degC

0 100 200 300 400 500 600 700 800Thermal oxygen aging time (h)

828486889092949698

100102104106

(a)

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

60degC70degC80degC

Thermal oxygen aging time (h)0 100 200 300 400 500 600 700 800

86889092949698

100102104

(b)

Figure 5 Relationship curves of elongation at break-thermal aging time (a) BG1 (b) BG2

6 Advances in Civil Engineering

up to 400 hours In Figure 5(a) the results show that theelongation at break of the PP biaxial geogrid increases slightlyin the early stage of thermal oxygen aging and the higher thetemperature is the shorter the increase period is At 60degC and70degC the influence of thermal oxygen aging on tensile strengthand fracture elongation is not obvious +e tensile test oflongitudinal and transverse ribs has been carried out by theauthors +e test results show that the tensile test results oflongitudinal and transverse ribs are not different

Comparing samples BG1 and BG2 of the two grids itcan be found that the temperature is 80degC and the tensilestrength retention rates of samples BG1 and BG2 are 928and 943 respectively when aging is of 700 h +e re-tention rates of BG1 and BG2 fracture elongation were 84

and 8741 respectively when aging is of 700 h +e ex-perimental results show that the tensile strength andfracture elongation of BG1 and BG2 which are both biaxialgeogrids of PP have similar trends with thermal oxygenaging time

412 Photooxygen Aging Test +e relationship betweentensile strength retention rate and photooxygen agingtime of two kinds of geogrid is obtained by the indoorultraviolet photooxygen aging test (Figure 6) It can beseen in Figure 6 that the two curves have the same trendthat is with the increase in photooxygen aging time thetensile strength of the two kinds of PP biaxial geogridsshows a significant trend of decrease After 300 h agingtime the BG1 starts to decline at greater rate than BG1and the difference at the end of the test is gt10 Figure 7shows the relationship between the fracture elongationretention rate and photooxygen aging time of the twogeogrids It can be seen from Figure 7 that within 400 hthe fracture elongation retention rate of the two geogridsalternately declines with the photooxygen aging time Atthe photooxygen aging time of 700 h the fracture elon-gation retention rates of samples BG1 and BG2 are notsignificantly different

413 Comparative Analysis Figure 8 shows the relationshipof tensile strength retention rate between the thermal oxygenaging and the photooxygen aging specimens with aging timeat 70degC

Figure 9 shows the relationship of retention rate ofelongation at break between the thermal oxygen aging andthe photooxygen aging specimens with aging time at 70degCIt can be seen that under the conditions of thermal oxygenaging the curves of tensile strength and fracture elongationretention rates with aging time of samples BG1 and BG2 arerelatively gentle However the tensile strength and fractureelongation retention curves with aging time are steeperunder photooxygen aging conditions +e thermal oxygenaging intensity is reduced by no more than 5 while thephotooxygen aging intensity is reduced by about 35 andthus the impact of photooxygen aging is much greater thanthat of thermal oxygen aging It is shown that the effect oflight on the geogrid tensile strength during aging is muchgreater than that of temperature

414 Prediction Model +e gray system theory proposed byProfessor Deng (China) in 1982 has been widely used ineconomic social and engineering fields It has advantages ofsmooth degree of data and a good precision [19] Based onthe gray prediction model (GM) by using a small amount ofraw data and simple modeling calculation the future datacan be predicted and the accuracy meets the research needsIn this study eight groups of data of tensile strength andelongation at rupture obtained from the photooxidation

0 100 200 300 400 500 600 700 800Photooxidative aging time (h)

BG1BG2

60

65

70

75

80

85

90

95

100Te

nsile

stre

ngth

rete

ntio

n (

)

Figure 6 Relationship curves of tensile strength retention rate-photooxidation aging time

BG1BG2

0 100 200 300 400 500 600 700 800Aging time (h)

50

60

70

80

90

100

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

Figure 7 Relationship curves of retention rate of elongation atbreak-photooxidation aging time

Advances in Civil Engineering 7

aging test of PP biaxial geogrids were provided +e GM wasused to fit the photooxidative aging test results of PP biaxialgeogrids and the failure rule of PP biaxial geogrids underindoor photooxidative aging is inferred

Gray model is a differential equation established bygenerating new data series from the original data series GM(1 N) represents the differential equation of N variables oforder 1 +e modeling process is as follows

Sequence based on more than four known pieces ofdata is

X(0)

(k)1113966 1113967 k 1 2 3 n (2)

Generating sequence of accumulated data with givendata sequence is as follows

X(1)

(k)1113966 1113967 k 1 2 3 n (3)

Approximating cumulative data series values with con-tinuous smooth values of exponential curves is as follows

1113957X(1)

(k) 1113957X(1)

(1) minusu

a1113876 1113877bulle

minus a(kminus 1)+

u

a k 1 2 3 n

(4)

+e smoothing approximation value of the original datasequence is reduced

1113957X(0)

(k) 1113957X(1)

(k) minus 1113957X(1)

(k minus 1) k 1 2 3 n (5)

In the above formula

ermal oxygen agingPhotooxidation aging

Tens

ile st

reng

th re

tent

ion

rate

()

65

70

75

80

85

90

95

100

100 200 300 400 500 600 700 8000Aging time (h)

(a)

ermal oxygen agingPhotooxidation aging

Tens

ile st

reng

th re

tent

ion

()

70

75

80

85

90

95

100

100 200 300 400 500 600 700 8000Aging time (h)

(b)

Figure 8 Relationship curves of tensile strength retention rate-aging time (a) BG1 (b) BG2

ermal oxygen agingPhotooxygen aging

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

50

60

70

80

90

100

110

100 200 300 400 500 600 700 8000Aging time (h)

(a)

ermal oxygen agingPhotooxidation aging

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

50556065707580859095

100105

100 200 300 400 500 600 700 8000Aging time (h)

(b)

Figure 9 Relationship curves of retention rate of elongation at break-aging time (a) BG1 (b) BG2

8 Advances in Civil Engineering

a

u

⎡⎢⎢⎢⎣ ⎤⎥⎥⎥⎦ BTB1113872 1113873

minus 1B

minus 1B

TYn

B

b1 1

b2 1

middot middot middot 1

bnminus1 1

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦

Yn

y1

y2

middot middot middot

ynminus1

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦

m (n minus 1) 1113944nminus1

i1bi( 1113857

2minus 1113944

nminus 1

i1bi

⎛⎝ ⎞⎠

2

a (n minus 1)1113944

nminus1i1 biyi minus 1113944

nminus1i1 bi1113944

nminus1i1 yi1113876 1113877

m

u minus1113944

nminus1i1 bi1113944

nminus1i1 biyi + 1113944

nminus1i1 bi( 1113857

21113944

nminus1i1 yi1113876 1113877

m

(6)

Based on the experimental data of tensile strength andelongation at rupture of PP biaxial geogrids under photo-oxidation aging a GMwas established to predict the changesin tensile strength and elongation at rupture of PP biaxialgeogrids under long-term photooxidation aging +e pre-diction results are listed in Table 4

It can be seen from Table 3 that the GM (1 1) model isestablished for the tensile strength of samples BG1 and BG2under photooxidation aging for 0ndash600 h +e maximumrelative error of fitting the photooxidation aging test resultsof BG1 is only 248 the average error is only 149 and thetensile strength retention rate of BG1 and BG2 under 700 h isonly 662 +e GM is used to predict the tensile strength ofBG1 and BG2 +e calculated predicted value is 646 andthe relative error of data recovery is only 242 +emaximum relative error of fitting BG2 is only 165 theaverage error is only 030 and the relative error ofchecking data with 700 h is only 565 +is clearly showsthat the GM (1 1) model has better fitting effect

Prediction curves of tensile strength and photooxidationaging time are shown in Figure 10 It can be seen fromFigure 10(a) that the tensile strength retention rate of BG1 is5198 When the photooxidation aging time is 1000 h thetensile strength retention rate of BG1 is 4832 When thephotooxidation aging time is 1100 h the tensile strengthretention rate of BG1 is less than 50 so the PP biaxialgeogrid losees efficacy From Figure 10(b) it can be seen thatthe tensile strength retention rate of BG2 is 5188 when thephotooxidation aging time is 1700 h and 4983 when it is1800 h less than 50 and thus the PP biaxial geogrid isinvalid

+e aging test curve changes greatly in the early stage ofaging and is relatively unstable +e prediction curve is notvery accurate in the early stage of aging but rather accurate indescribing the long-term performance of aging

42 Test of Load Bearing Capacity of Reinforced Sand Foun-dation considering theEffects ofAging +e tensile strength ofthe geogrid before and after aging is listed in Table 5

Load-settlement curves of the cushion are obtainedaccording to load at all levels as shown in Figure 10Compared with the nonreinforced sand cushion the set-tlement value of the reinforced sand cushion under the sameload decreases and the bearing capacity of the foundationincreases the settlement value of the reinforced sandcushion of BG2 geogrid was smaller than that of the rein-forced sand cushion of BG1 geogrid and the bearing ca-pacity of the cushion was increased the difference in theload-settlement curves of the reinforced sand cushion andthe nonreinforced sand cushion at the early stage was smallIt shows that the reinforcement effect was not obvious at theinitial stage of loading With the increase in load the spacingbetween the load-settlement curves of the pure sand BG1reinforced sand and BG2 reinforced sand cushions grad-ually increased+us the reinforcement effect is increasinglyobvious

+e P-S curve of geogrid-reinforced sand consideringthe aging effect is shown in Figure 11 Figures 11(a) and11(b) show that the P-S curves of aging geogrids coincidewith those of nonaging geogrids when the load P was lessthan 125 kPa With the increase in load the two P-S curvesbegin to separate and under the same load the settlementratio of the aging-reinforced sand cushion of the BG1geogrids to that of the non-aging-reinforced sand cushionwas less than that of the aging-reinforced sand cushion Ingeneral the bearing capacity of the aging-reinforcedcushion was slightly lower than that of the non-aging-

Table 4 +e results of gray model prediction

Aging time (h) 0 100 200 300 400 500 600

BG1 tensile strength retention rateTest value () 100 986 944 888 7980 737 697

Predicted value () 100 1001 931 866 805 748 695Relative error () mdash 152 138 248 088 149 029

BG2 tensile strength retention rateTest value () 100 978 964 908 865 853 801

Predicted value () 100 9870 9481 9108 8749 8404 8073Relative error () mdash 092 165 030 114 148 078

Advances in Civil Engineering 9

reinforced cushion but the effect is not obvious +ebearing capacity of reinforced soil decreases with the in-crease of aging time

+e effect of aging of geogrids in the early stage ofloading on the bearing capacity of reinforced soil is

negligible +e authors define an aging effect on the initialload When the load value is less than the initial load of agingeffect the aging effect is small on the settlement of foun-dation When the load value exceeds the initial load of agingeffect the aging effect on the settlement of foundation

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

Photooxidative aging time (h)

Test valuePredicted value

Tens

ile st

reng

th re

tent

ion

rate

()

40

50

60

70

80

90

100

(a)

0

200

400

600

800

1000

1200

1400

1600

1800

Photooxidative aging time (h)

Test valuePredicted value

Tens

ile st

reng

th re

tent

ion

()

404550556065707580859095

100

(b)

Figure 10 Prediction curves of tensile strength and photooxidation aging time (a) BG1 (b) BG2

Table 5 +e tensile strength of geogrid before and after aging

Geogrid Tensile strength of geogrid before aging(kNm)

Radiation intensity (Wm2)

Tensile strength of geogrid after aging of700 h

Aging time(h)

BG1 158 600 108 700BG2 255 600 1835 700

0 100 200 300 400 500p (kPa)

Pure sand Unaged BG1

Aging 500h BG1Aging 600h BG1

25

20

15

10

5

0

s (m

m)

(a)

p (kPa)

Pure sand Unaged BG2

Aging 500h BG2Aging 600h BG2

0 100 200 300 400 500 600

25

20

15

10

5

0

s (m

m)

(b)

Figure 11 Load-settlement (P-S) curve of geogrid reinforced sand considering aging effect (a) BG1 (b) BG2

10 Advances in Civil Engineering

gradually appears For BG1 the aging load has an initialvalue of 125 kPa Similar to BG1 the initial aging load ofBG2 bars is 175 kPa which is much larger than that of BG1bars (Figure 11(b)) +e aging effect of BG2 with highertensile strength on the initial load is greater than that of theBG1 geogrid with lower tensile strength It indicates that theaging effect of BG2 with higher tensile strength on thebearing capacity of the reinforced cushion foundation issmaller than that of the BG1 geogrid with lower tensilestrength

After the model test it was found that the defor-mation of biaxial geogrid was significant (see Figure 12)+e deformed geogrid was measured and the centralpoint of the biaxial geogrid shall be concave about17 mm

+e beneficial effect of reinforcement for increasing thebearing capacity is conveniently described by the bearingcapacity ratio (BCR) according to [20]

BCR qR

q0 (7)

In order to compare the strengths of geogrids underdifferent load levels the ultimate bearing capacity and ul-timate bearing capacity ratio are listed in Table 6 It can beseen that the bearing capacity of the geogrid-reinforcedcushion decreases with aging For BG1 the bearing capacityof the foundation decreases by 1333 and 20 respectivelyafter 500 h and 600 h of aging For BG2 aged 500 h and 600 hthe bearing capacity of the foundation decreases by 625and 1875 respectively

+e BCR of the geogrid-reinforced sand is listed inTable 6 It can be seen that the aging effect of the geogrid

leads to the decline of the reinforced foundation BCR whichreduces the bearing capacity of the geogrid-reinforcedfoundation For different types of geogrids the degree ofdecline in BCR owing to aging is different and the value ofdecline in BCR owing to aging is slightly smaller for thehigher-strength BG2 than for the lower-strength BG1

5 MechanismAnalysis of PhotooxygenAging inPP Biaxial Geogrid

+e energy of the ultraviolet light accelerates the agingprocess of the grid Photooxidation causes degradation andaging of the PP germanium geogrid decomposes the PPpolymer and destroys its internal structure [21]+e tensilestrength and fracture elongation of two kinds of PP biaxialgeogrids were reduced in different degrees after differentaging modes temperature aging time and aging condi-tions After thermal aging there is a decrease in cross-linking degree and in binding and friction betweenmolecular chains +e effects of photooxygen aging aregreater than the effects of thermal oxygen aging +esunlight ultraviolet light provides the energy required forchemical crosslinking of the PP biaxial geogrid +e tensilestrength and fracture elongation of the geogrid temporarilyincrease in a short period of time (100ndash200 h) with theextension of aging time

6 Conclusion

In this study thermal oxygen test is carried out at 60degC 70degCand 80degC and ultraviolet photooxygen aging tests are carriedout at 70degC for two kinds of PP biaxial geogrids of different

(a) (b)

Figure 12 Geogrid-reinforced foundation damage

Table 6 BCR of geogrid-reinforced sand

Group Aging time Ultimate bearing capacity (kPa) BCR Percentage reduction in strength due to agingUnreinforced mdash 250 1 mdashBG1 0 375 15 mdashBG1 500 325 13 1333BG1 600 300 12 20BG2 0 400 16 mdashBG2 500 375 15 625BG2 600 325 13 1875

Advances in Civil Engineering 11

specifications +e influence of aging factors on the bearingcapacity of geogrid-reinforced foundations is considered

After investigating the effects of thermal oxygen andphotooxygen aging tests on tensile strength and fractureelongation of geogrids the following conclusions are drawn

(1) +e tensile strength of the geogrid decreases withthe increase in aging time and there is an increasein properties in aging period up to 100ndash200 hours+e comparison with the same thermal oxygenaging time shows that the tensile strength of thegeogrid decreases with the increase in thermaloxygen aging temperature With the increase inphotooxygen aging time the tensile strength of thegeogrid showed a significant trend of decrease +eeffect of ultraviolet light on the geogrid tensilestrength during aging is much greater than that oftemperature +e effect of photooxygen aging ontensile strength is greater than that of thermaloxygen aging for two different types of PP biaxialgeogrid Under the same temperature and agingtime the tensile strength of geogrid is reduced by nomore than 5 while the strength of geogrid isreduced by about 35

(2) +e retention rate of fracture elongation decreaseswith the increase in thermal oxygen aging tempera-ture and aging time+e elongation at break decreaseswith the photooxygen aging time and the influence ofultraviolet light on geogrid fracture elongation ismuch greater than that of temperature +e tensilestrength retention rate of different types of PP biaxialgrids showed different photooxygen aging charac-teristics+e aging resistance performance of BG2wassignificantly higher than that of BG1

(3) +e effect against tensile strength retention rate isbetter with the GM +e following conclusion camefrom prediction models with a blackboard tem-perature of 70degCplusmn 2degC irradiation of 8 h nonirra-diated condensation of 4 h relative humidity of70plusmn 2 and ultraviolet irradiance of 600Wm2the lower-intensity BG1 fails at approximately1100 h +e higher-strength BG2 fails at approxi-mately 1800 h

(4) Geogrid reinforcement significantly changes thebearing characteristics of a sand cushion by in-creasing the bearing capacity of the reinforcedcushion+e reinforcement effect of the geogrid withhigher tensile strength is more significant Agingchanges the interface characteristics between thegeogrid and soil In this test the bearing properties ofthe geogrid cushion after aging have certain changes+e aging behavior of the two geogrids reduces theload bearing capacity of the reinforced cushion by20 and 1875 respectively

Data Availability

No data were used to support this study

Conflicts of Interest

+e authors declare that they have no conflicts of interestrelated to this work

Acknowledgments

+e authors would like to acknowledge the National NaturalScience Foundation of China for the financial support forthis study (no 51578359)

References

[1] Y-L Dong J Han and X-H Bai ldquoNumerical analysis oftensile behavior of geogrids with rectangular and triangularaperturesrdquo Geotextiles and Geomembranes vol 29 no 2pp 83ndash91 2011

[2] A M R Ewais R K Rowe and J Scheirs ldquoDegradationbehaviour of HDPE geomembranes with high and low initialhigh-pressure oxidative induction timerdquo Geotextiles andGeomembranes vol 42 no 2 pp 111ndash126 2014

[3] Y G Hsuan and R M Koerner ldquoAntioxidant depletionlifetime in high density polyethylene geomembranesrdquo Journalof Geotechnical and Geoenvironmental Engineering vol 124no 6 pp 532ndash541 1998

[4] W-K Wong and Y G Hsuan ldquoInteraction of antioxidantswith carbon black in polyethylene using oxidative inductiontime methodsrdquo Geotextiles and Geomembranes vol 42 no 6pp 641ndash647 2014

[5] R K Rowe and H P Sangam ldquoDurability of HDPE geo-membranesrdquo Geotextiles and Geomembranes vol 20 no 2pp 77ndash95 2002

[6] H P Sangam and R K Rowe ldquoEffects of exposure conditionson the depletion of antioxidants from high-density polyeth-ylene (HDPE) geomembranesrdquo Canadian GeotechnicalJournal vol 39 no 6 pp 1221ndash1230 2002

[7] W Muller and I Jacob ldquoOxidative resistance of high densitypolyethylene geomembranesrdquo Polymer Degradation andStability vol 79 pp 161ndash172 2003

[8] S B Gulec T B Edil and C H Benson ldquoEffect of acidic minedrainage on the polymer properties of an HDPE geo-membranerdquo Geosynthetics International vol 11 no 2pp 60ndash72 2004

[9] R K Rowe and S Rimal ldquoDepletion of antioxidants from aHDPE geomembrane in a composite linerrdquo Journal of Geo-technical and Geoenvironmental Engineering vol 134 no 1pp 68ndash78 2008

[10] R K Rowe S Rimal and H P Sangam ldquoAgeing of HDPEgeomembrane exposed to air water and leachate at differenttemperaturesrdquo Geotextiles and Geomembranes vol 27pp 131ndash151 2009

[11] R K RoweM Z Islam and Y G Hsuan ldquoEffects of thicknesson the aging of HDPE geomembranesrdquo Journal of Geotech-nical and Geoenvironmental Engineering vol 136 no 2pp 299ndash309 2010

[12] F B Abdelaal and R K Rowe ldquoEffect of high temperatures onantioxidant depletion from different HDPE geomembranesrdquoGeotextiles and Geomembranes vol 42 no 4 pp 284ndash3012014

[13] X-H Bai X-Z Huang and W Zhang ldquoBearing capacity ofsquare footing supported by a geobelt-reinforced crushedstone cushion on soft soilrdquo Geotextiles and Geomembranesvol 38 pp 37ndash42 2013

12 Advances in Civil Engineering

is laied in the soil pit and compacted to the thickness of50mm for each layer+e geogrid is laid at a depth of 70mmunder the loading plate and fixed with U-nails+e picture ofthe model test site and the layout of the Earth pressure boxare shown in Figure 3

4 Results and Discussion

+e thermal oxygen and photooxygen aging will result in thereduction in mechanical properties of the geogrid +emechanical properties of the geogrid with aging parametersare described by the tensile strength retention rate andfracture elongation retention rate +e retention rate oftensile strength is the ratio between the tensile strength of

the aged geogrid and the tensile strength of the geogridbefore aging expressed as a percentage +e fracture elon-gation retention rate is the ratio of the aged fracture elon-gation to the preaging fracture elongation expressed as apercentage

41 Geogrid Aging

411 (ermal Oxygen Aging Test +e results of the thermaloxygen aging test for samples BG1 and BG2 of the biaxialgeogrid of PP are shown in Figures 4 and 5 In Figure 4 theresults show relatively small differences for different aging time(at least for temperatures of 60 and 70 degrees) and aging time

Tens

ile st

reng

th re

tent

ion

()

60degC70degC80degC

0 100 200 300 400 500 600 700 800Thermal oxygen aging time (h)

92

94

96

98

100

102

104

(a)

Tens

ile st

reng

th re

tent

ion

()

60degC70degC80degC

Thermal oxygen aging time (h)0 100 200 300 400 500 600 700 800

93

94

95

96

97

98

99

100

101

102

(b)

Figure 4 Relationship curves of tensile strength retention rate-thermal aging time (a) BG1 (b) BG2

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

60degC70degC80degC

0 100 200 300 400 500 600 700 800Thermal oxygen aging time (h)

828486889092949698

100102104106

(a)

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

60degC70degC80degC

Thermal oxygen aging time (h)0 100 200 300 400 500 600 700 800

86889092949698

100102104

(b)

Figure 5 Relationship curves of elongation at break-thermal aging time (a) BG1 (b) BG2

6 Advances in Civil Engineering

up to 400 hours In Figure 5(a) the results show that theelongation at break of the PP biaxial geogrid increases slightlyin the early stage of thermal oxygen aging and the higher thetemperature is the shorter the increase period is At 60degC and70degC the influence of thermal oxygen aging on tensile strengthand fracture elongation is not obvious +e tensile test oflongitudinal and transverse ribs has been carried out by theauthors +e test results show that the tensile test results oflongitudinal and transverse ribs are not different

Comparing samples BG1 and BG2 of the two grids itcan be found that the temperature is 80degC and the tensilestrength retention rates of samples BG1 and BG2 are 928and 943 respectively when aging is of 700 h +e re-tention rates of BG1 and BG2 fracture elongation were 84

and 8741 respectively when aging is of 700 h +e ex-perimental results show that the tensile strength andfracture elongation of BG1 and BG2 which are both biaxialgeogrids of PP have similar trends with thermal oxygenaging time

412 Photooxygen Aging Test +e relationship betweentensile strength retention rate and photooxygen agingtime of two kinds of geogrid is obtained by the indoorultraviolet photooxygen aging test (Figure 6) It can beseen in Figure 6 that the two curves have the same trendthat is with the increase in photooxygen aging time thetensile strength of the two kinds of PP biaxial geogridsshows a significant trend of decrease After 300 h agingtime the BG1 starts to decline at greater rate than BG1and the difference at the end of the test is gt10 Figure 7shows the relationship between the fracture elongationretention rate and photooxygen aging time of the twogeogrids It can be seen from Figure 7 that within 400 hthe fracture elongation retention rate of the two geogridsalternately declines with the photooxygen aging time Atthe photooxygen aging time of 700 h the fracture elon-gation retention rates of samples BG1 and BG2 are notsignificantly different

413 Comparative Analysis Figure 8 shows the relationshipof tensile strength retention rate between the thermal oxygenaging and the photooxygen aging specimens with aging timeat 70degC

Figure 9 shows the relationship of retention rate ofelongation at break between the thermal oxygen aging andthe photooxygen aging specimens with aging time at 70degCIt can be seen that under the conditions of thermal oxygenaging the curves of tensile strength and fracture elongationretention rates with aging time of samples BG1 and BG2 arerelatively gentle However the tensile strength and fractureelongation retention curves with aging time are steeperunder photooxygen aging conditions +e thermal oxygenaging intensity is reduced by no more than 5 while thephotooxygen aging intensity is reduced by about 35 andthus the impact of photooxygen aging is much greater thanthat of thermal oxygen aging It is shown that the effect oflight on the geogrid tensile strength during aging is muchgreater than that of temperature

414 Prediction Model +e gray system theory proposed byProfessor Deng (China) in 1982 has been widely used ineconomic social and engineering fields It has advantages ofsmooth degree of data and a good precision [19] Based onthe gray prediction model (GM) by using a small amount ofraw data and simple modeling calculation the future datacan be predicted and the accuracy meets the research needsIn this study eight groups of data of tensile strength andelongation at rupture obtained from the photooxidation

0 100 200 300 400 500 600 700 800Photooxidative aging time (h)

BG1BG2

60

65

70

75

80

85

90

95

100Te

nsile

stre

ngth

rete

ntio

n (

)

Figure 6 Relationship curves of tensile strength retention rate-photooxidation aging time

BG1BG2

0 100 200 300 400 500 600 700 800Aging time (h)

50

60

70

80

90

100

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

Figure 7 Relationship curves of retention rate of elongation atbreak-photooxidation aging time

Advances in Civil Engineering 7

aging test of PP biaxial geogrids were provided +e GM wasused to fit the photooxidative aging test results of PP biaxialgeogrids and the failure rule of PP biaxial geogrids underindoor photooxidative aging is inferred

Gray model is a differential equation established bygenerating new data series from the original data series GM(1 N) represents the differential equation of N variables oforder 1 +e modeling process is as follows

Sequence based on more than four known pieces ofdata is

X(0)

(k)1113966 1113967 k 1 2 3 n (2)

Generating sequence of accumulated data with givendata sequence is as follows

X(1)

(k)1113966 1113967 k 1 2 3 n (3)

Approximating cumulative data series values with con-tinuous smooth values of exponential curves is as follows

1113957X(1)

(k) 1113957X(1)

(1) minusu

a1113876 1113877bulle

minus a(kminus 1)+

u

a k 1 2 3 n

(4)

+e smoothing approximation value of the original datasequence is reduced

1113957X(0)

(k) 1113957X(1)

(k) minus 1113957X(1)

(k minus 1) k 1 2 3 n (5)

In the above formula

ermal oxygen agingPhotooxidation aging

Tens

ile st

reng

th re

tent

ion

rate

()

65

70

75

80

85

90

95

100

100 200 300 400 500 600 700 8000Aging time (h)

(a)

ermal oxygen agingPhotooxidation aging

Tens

ile st

reng

th re

tent

ion

()

70

75

80

85

90

95

100

100 200 300 400 500 600 700 8000Aging time (h)

(b)

Figure 8 Relationship curves of tensile strength retention rate-aging time (a) BG1 (b) BG2

ermal oxygen agingPhotooxygen aging

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

50

60

70

80

90

100

110

100 200 300 400 500 600 700 8000Aging time (h)

(a)

ermal oxygen agingPhotooxidation aging

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

50556065707580859095

100105

100 200 300 400 500 600 700 8000Aging time (h)

(b)

Figure 9 Relationship curves of retention rate of elongation at break-aging time (a) BG1 (b) BG2

8 Advances in Civil Engineering

a

u

⎡⎢⎢⎢⎣ ⎤⎥⎥⎥⎦ BTB1113872 1113873

minus 1B

minus 1B

TYn

B

b1 1

b2 1

middot middot middot 1

bnminus1 1

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦

Yn

y1

y2

middot middot middot

ynminus1

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦

m (n minus 1) 1113944nminus1

i1bi( 1113857

2minus 1113944

nminus 1

i1bi

⎛⎝ ⎞⎠

2

a (n minus 1)1113944

nminus1i1 biyi minus 1113944

nminus1i1 bi1113944

nminus1i1 yi1113876 1113877

m

u minus1113944

nminus1i1 bi1113944

nminus1i1 biyi + 1113944

nminus1i1 bi( 1113857

21113944

nminus1i1 yi1113876 1113877

m

(6)

Based on the experimental data of tensile strength andelongation at rupture of PP biaxial geogrids under photo-oxidation aging a GMwas established to predict the changesin tensile strength and elongation at rupture of PP biaxialgeogrids under long-term photooxidation aging +e pre-diction results are listed in Table 4

It can be seen from Table 3 that the GM (1 1) model isestablished for the tensile strength of samples BG1 and BG2under photooxidation aging for 0ndash600 h +e maximumrelative error of fitting the photooxidation aging test resultsof BG1 is only 248 the average error is only 149 and thetensile strength retention rate of BG1 and BG2 under 700 h isonly 662 +e GM is used to predict the tensile strength ofBG1 and BG2 +e calculated predicted value is 646 andthe relative error of data recovery is only 242 +emaximum relative error of fitting BG2 is only 165 theaverage error is only 030 and the relative error ofchecking data with 700 h is only 565 +is clearly showsthat the GM (1 1) model has better fitting effect

Prediction curves of tensile strength and photooxidationaging time are shown in Figure 10 It can be seen fromFigure 10(a) that the tensile strength retention rate of BG1 is5198 When the photooxidation aging time is 1000 h thetensile strength retention rate of BG1 is 4832 When thephotooxidation aging time is 1100 h the tensile strengthretention rate of BG1 is less than 50 so the PP biaxialgeogrid losees efficacy From Figure 10(b) it can be seen thatthe tensile strength retention rate of BG2 is 5188 when thephotooxidation aging time is 1700 h and 4983 when it is1800 h less than 50 and thus the PP biaxial geogrid isinvalid

+e aging test curve changes greatly in the early stage ofaging and is relatively unstable +e prediction curve is notvery accurate in the early stage of aging but rather accurate indescribing the long-term performance of aging

42 Test of Load Bearing Capacity of Reinforced Sand Foun-dation considering theEffects ofAging +e tensile strength ofthe geogrid before and after aging is listed in Table 5

Load-settlement curves of the cushion are obtainedaccording to load at all levels as shown in Figure 10Compared with the nonreinforced sand cushion the set-tlement value of the reinforced sand cushion under the sameload decreases and the bearing capacity of the foundationincreases the settlement value of the reinforced sandcushion of BG2 geogrid was smaller than that of the rein-forced sand cushion of BG1 geogrid and the bearing ca-pacity of the cushion was increased the difference in theload-settlement curves of the reinforced sand cushion andthe nonreinforced sand cushion at the early stage was smallIt shows that the reinforcement effect was not obvious at theinitial stage of loading With the increase in load the spacingbetween the load-settlement curves of the pure sand BG1reinforced sand and BG2 reinforced sand cushions grad-ually increased+us the reinforcement effect is increasinglyobvious

+e P-S curve of geogrid-reinforced sand consideringthe aging effect is shown in Figure 11 Figures 11(a) and11(b) show that the P-S curves of aging geogrids coincidewith those of nonaging geogrids when the load P was lessthan 125 kPa With the increase in load the two P-S curvesbegin to separate and under the same load the settlementratio of the aging-reinforced sand cushion of the BG1geogrids to that of the non-aging-reinforced sand cushionwas less than that of the aging-reinforced sand cushion Ingeneral the bearing capacity of the aging-reinforcedcushion was slightly lower than that of the non-aging-

Table 4 +e results of gray model prediction

Aging time (h) 0 100 200 300 400 500 600

BG1 tensile strength retention rateTest value () 100 986 944 888 7980 737 697

Predicted value () 100 1001 931 866 805 748 695Relative error () mdash 152 138 248 088 149 029

BG2 tensile strength retention rateTest value () 100 978 964 908 865 853 801

Predicted value () 100 9870 9481 9108 8749 8404 8073Relative error () mdash 092 165 030 114 148 078

Advances in Civil Engineering 9

reinforced cushion but the effect is not obvious +ebearing capacity of reinforced soil decreases with the in-crease of aging time

+e effect of aging of geogrids in the early stage ofloading on the bearing capacity of reinforced soil is

negligible +e authors define an aging effect on the initialload When the load value is less than the initial load of agingeffect the aging effect is small on the settlement of foun-dation When the load value exceeds the initial load of agingeffect the aging effect on the settlement of foundation

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

Photooxidative aging time (h)

Test valuePredicted value

Tens

ile st

reng

th re

tent

ion

rate

()

40

50

60

70

80

90

100

(a)

0

200

400

600

800

1000

1200

1400

1600

1800

Photooxidative aging time (h)

Test valuePredicted value

Tens

ile st

reng

th re

tent

ion

()

404550556065707580859095

100

(b)

Figure 10 Prediction curves of tensile strength and photooxidation aging time (a) BG1 (b) BG2

Table 5 +e tensile strength of geogrid before and after aging

Geogrid Tensile strength of geogrid before aging(kNm)

Radiation intensity (Wm2)

Tensile strength of geogrid after aging of700 h

Aging time(h)

BG1 158 600 108 700BG2 255 600 1835 700

0 100 200 300 400 500p (kPa)

Pure sand Unaged BG1

Aging 500h BG1Aging 600h BG1

25

20

15

10

5

0

s (m

m)

(a)

p (kPa)

Pure sand Unaged BG2

Aging 500h BG2Aging 600h BG2

0 100 200 300 400 500 600

25

20

15

10

5

0

s (m

m)

(b)

Figure 11 Load-settlement (P-S) curve of geogrid reinforced sand considering aging effect (a) BG1 (b) BG2

10 Advances in Civil Engineering

gradually appears For BG1 the aging load has an initialvalue of 125 kPa Similar to BG1 the initial aging load ofBG2 bars is 175 kPa which is much larger than that of BG1bars (Figure 11(b)) +e aging effect of BG2 with highertensile strength on the initial load is greater than that of theBG1 geogrid with lower tensile strength It indicates that theaging effect of BG2 with higher tensile strength on thebearing capacity of the reinforced cushion foundation issmaller than that of the BG1 geogrid with lower tensilestrength

After the model test it was found that the defor-mation of biaxial geogrid was significant (see Figure 12)+e deformed geogrid was measured and the centralpoint of the biaxial geogrid shall be concave about17 mm

+e beneficial effect of reinforcement for increasing thebearing capacity is conveniently described by the bearingcapacity ratio (BCR) according to [20]

BCR qR

q0 (7)

In order to compare the strengths of geogrids underdifferent load levels the ultimate bearing capacity and ul-timate bearing capacity ratio are listed in Table 6 It can beseen that the bearing capacity of the geogrid-reinforcedcushion decreases with aging For BG1 the bearing capacityof the foundation decreases by 1333 and 20 respectivelyafter 500 h and 600 h of aging For BG2 aged 500 h and 600 hthe bearing capacity of the foundation decreases by 625and 1875 respectively

+e BCR of the geogrid-reinforced sand is listed inTable 6 It can be seen that the aging effect of the geogrid

leads to the decline of the reinforced foundation BCR whichreduces the bearing capacity of the geogrid-reinforcedfoundation For different types of geogrids the degree ofdecline in BCR owing to aging is different and the value ofdecline in BCR owing to aging is slightly smaller for thehigher-strength BG2 than for the lower-strength BG1

5 MechanismAnalysis of PhotooxygenAging inPP Biaxial Geogrid

+e energy of the ultraviolet light accelerates the agingprocess of the grid Photooxidation causes degradation andaging of the PP germanium geogrid decomposes the PPpolymer and destroys its internal structure [21]+e tensilestrength and fracture elongation of two kinds of PP biaxialgeogrids were reduced in different degrees after differentaging modes temperature aging time and aging condi-tions After thermal aging there is a decrease in cross-linking degree and in binding and friction betweenmolecular chains +e effects of photooxygen aging aregreater than the effects of thermal oxygen aging +esunlight ultraviolet light provides the energy required forchemical crosslinking of the PP biaxial geogrid +e tensilestrength and fracture elongation of the geogrid temporarilyincrease in a short period of time (100ndash200 h) with theextension of aging time

6 Conclusion

In this study thermal oxygen test is carried out at 60degC 70degCand 80degC and ultraviolet photooxygen aging tests are carriedout at 70degC for two kinds of PP biaxial geogrids of different

(a) (b)

Figure 12 Geogrid-reinforced foundation damage

Table 6 BCR of geogrid-reinforced sand

Group Aging time Ultimate bearing capacity (kPa) BCR Percentage reduction in strength due to agingUnreinforced mdash 250 1 mdashBG1 0 375 15 mdashBG1 500 325 13 1333BG1 600 300 12 20BG2 0 400 16 mdashBG2 500 375 15 625BG2 600 325 13 1875

Advances in Civil Engineering 11

specifications +e influence of aging factors on the bearingcapacity of geogrid-reinforced foundations is considered

After investigating the effects of thermal oxygen andphotooxygen aging tests on tensile strength and fractureelongation of geogrids the following conclusions are drawn

(1) +e tensile strength of the geogrid decreases withthe increase in aging time and there is an increasein properties in aging period up to 100ndash200 hours+e comparison with the same thermal oxygenaging time shows that the tensile strength of thegeogrid decreases with the increase in thermaloxygen aging temperature With the increase inphotooxygen aging time the tensile strength of thegeogrid showed a significant trend of decrease +eeffect of ultraviolet light on the geogrid tensilestrength during aging is much greater than that oftemperature +e effect of photooxygen aging ontensile strength is greater than that of thermaloxygen aging for two different types of PP biaxialgeogrid Under the same temperature and agingtime the tensile strength of geogrid is reduced by nomore than 5 while the strength of geogrid isreduced by about 35

(2) +e retention rate of fracture elongation decreaseswith the increase in thermal oxygen aging tempera-ture and aging time+e elongation at break decreaseswith the photooxygen aging time and the influence ofultraviolet light on geogrid fracture elongation ismuch greater than that of temperature +e tensilestrength retention rate of different types of PP biaxialgrids showed different photooxygen aging charac-teristics+e aging resistance performance of BG2wassignificantly higher than that of BG1

(3) +e effect against tensile strength retention rate isbetter with the GM +e following conclusion camefrom prediction models with a blackboard tem-perature of 70degCplusmn 2degC irradiation of 8 h nonirra-diated condensation of 4 h relative humidity of70plusmn 2 and ultraviolet irradiance of 600Wm2the lower-intensity BG1 fails at approximately1100 h +e higher-strength BG2 fails at approxi-mately 1800 h

(4) Geogrid reinforcement significantly changes thebearing characteristics of a sand cushion by in-creasing the bearing capacity of the reinforcedcushion+e reinforcement effect of the geogrid withhigher tensile strength is more significant Agingchanges the interface characteristics between thegeogrid and soil In this test the bearing properties ofthe geogrid cushion after aging have certain changes+e aging behavior of the two geogrids reduces theload bearing capacity of the reinforced cushion by20 and 1875 respectively

Data Availability

No data were used to support this study

Conflicts of Interest

+e authors declare that they have no conflicts of interestrelated to this work

Acknowledgments

+e authors would like to acknowledge the National NaturalScience Foundation of China for the financial support forthis study (no 51578359)

References

[1] Y-L Dong J Han and X-H Bai ldquoNumerical analysis oftensile behavior of geogrids with rectangular and triangularaperturesrdquo Geotextiles and Geomembranes vol 29 no 2pp 83ndash91 2011

[2] A M R Ewais R K Rowe and J Scheirs ldquoDegradationbehaviour of HDPE geomembranes with high and low initialhigh-pressure oxidative induction timerdquo Geotextiles andGeomembranes vol 42 no 2 pp 111ndash126 2014

[3] Y G Hsuan and R M Koerner ldquoAntioxidant depletionlifetime in high density polyethylene geomembranesrdquo Journalof Geotechnical and Geoenvironmental Engineering vol 124no 6 pp 532ndash541 1998

[4] W-K Wong and Y G Hsuan ldquoInteraction of antioxidantswith carbon black in polyethylene using oxidative inductiontime methodsrdquo Geotextiles and Geomembranes vol 42 no 6pp 641ndash647 2014

[5] R K Rowe and H P Sangam ldquoDurability of HDPE geo-membranesrdquo Geotextiles and Geomembranes vol 20 no 2pp 77ndash95 2002

[6] H P Sangam and R K Rowe ldquoEffects of exposure conditionson the depletion of antioxidants from high-density polyeth-ylene (HDPE) geomembranesrdquo Canadian GeotechnicalJournal vol 39 no 6 pp 1221ndash1230 2002

[7] W Muller and I Jacob ldquoOxidative resistance of high densitypolyethylene geomembranesrdquo Polymer Degradation andStability vol 79 pp 161ndash172 2003

[8] S B Gulec T B Edil and C H Benson ldquoEffect of acidic minedrainage on the polymer properties of an HDPE geo-membranerdquo Geosynthetics International vol 11 no 2pp 60ndash72 2004

[9] R K Rowe and S Rimal ldquoDepletion of antioxidants from aHDPE geomembrane in a composite linerrdquo Journal of Geo-technical and Geoenvironmental Engineering vol 134 no 1pp 68ndash78 2008

[10] R K Rowe S Rimal and H P Sangam ldquoAgeing of HDPEgeomembrane exposed to air water and leachate at differenttemperaturesrdquo Geotextiles and Geomembranes vol 27pp 131ndash151 2009

[11] R K RoweM Z Islam and Y G Hsuan ldquoEffects of thicknesson the aging of HDPE geomembranesrdquo Journal of Geotech-nical and Geoenvironmental Engineering vol 136 no 2pp 299ndash309 2010

[12] F B Abdelaal and R K Rowe ldquoEffect of high temperatures onantioxidant depletion from different HDPE geomembranesrdquoGeotextiles and Geomembranes vol 42 no 4 pp 284ndash3012014

[13] X-H Bai X-Z Huang and W Zhang ldquoBearing capacity ofsquare footing supported by a geobelt-reinforced crushedstone cushion on soft soilrdquo Geotextiles and Geomembranesvol 38 pp 37ndash42 2013

12 Advances in Civil Engineering

up to 400 hours In Figure 5(a) the results show that theelongation at break of the PP biaxial geogrid increases slightlyin the early stage of thermal oxygen aging and the higher thetemperature is the shorter the increase period is At 60degC and70degC the influence of thermal oxygen aging on tensile strengthand fracture elongation is not obvious +e tensile test oflongitudinal and transverse ribs has been carried out by theauthors +e test results show that the tensile test results oflongitudinal and transverse ribs are not different

Comparing samples BG1 and BG2 of the two grids itcan be found that the temperature is 80degC and the tensilestrength retention rates of samples BG1 and BG2 are 928and 943 respectively when aging is of 700 h +e re-tention rates of BG1 and BG2 fracture elongation were 84

and 8741 respectively when aging is of 700 h +e ex-perimental results show that the tensile strength andfracture elongation of BG1 and BG2 which are both biaxialgeogrids of PP have similar trends with thermal oxygenaging time

412 Photooxygen Aging Test +e relationship betweentensile strength retention rate and photooxygen agingtime of two kinds of geogrid is obtained by the indoorultraviolet photooxygen aging test (Figure 6) It can beseen in Figure 6 that the two curves have the same trendthat is with the increase in photooxygen aging time thetensile strength of the two kinds of PP biaxial geogridsshows a significant trend of decrease After 300 h agingtime the BG1 starts to decline at greater rate than BG1and the difference at the end of the test is gt10 Figure 7shows the relationship between the fracture elongationretention rate and photooxygen aging time of the twogeogrids It can be seen from Figure 7 that within 400 hthe fracture elongation retention rate of the two geogridsalternately declines with the photooxygen aging time Atthe photooxygen aging time of 700 h the fracture elon-gation retention rates of samples BG1 and BG2 are notsignificantly different

413 Comparative Analysis Figure 8 shows the relationshipof tensile strength retention rate between the thermal oxygenaging and the photooxygen aging specimens with aging timeat 70degC

Figure 9 shows the relationship of retention rate ofelongation at break between the thermal oxygen aging andthe photooxygen aging specimens with aging time at 70degCIt can be seen that under the conditions of thermal oxygenaging the curves of tensile strength and fracture elongationretention rates with aging time of samples BG1 and BG2 arerelatively gentle However the tensile strength and fractureelongation retention curves with aging time are steeperunder photooxygen aging conditions +e thermal oxygenaging intensity is reduced by no more than 5 while thephotooxygen aging intensity is reduced by about 35 andthus the impact of photooxygen aging is much greater thanthat of thermal oxygen aging It is shown that the effect oflight on the geogrid tensile strength during aging is muchgreater than that of temperature

414 Prediction Model +e gray system theory proposed byProfessor Deng (China) in 1982 has been widely used ineconomic social and engineering fields It has advantages ofsmooth degree of data and a good precision [19] Based onthe gray prediction model (GM) by using a small amount ofraw data and simple modeling calculation the future datacan be predicted and the accuracy meets the research needsIn this study eight groups of data of tensile strength andelongation at rupture obtained from the photooxidation

0 100 200 300 400 500 600 700 800Photooxidative aging time (h)

BG1BG2

60

65

70

75

80

85

90

95

100Te

nsile

stre

ngth

rete

ntio

n (

)

Figure 6 Relationship curves of tensile strength retention rate-photooxidation aging time

BG1BG2

0 100 200 300 400 500 600 700 800Aging time (h)

50

60

70

80

90

100

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

Figure 7 Relationship curves of retention rate of elongation atbreak-photooxidation aging time

Advances in Civil Engineering 7

aging test of PP biaxial geogrids were provided +e GM wasused to fit the photooxidative aging test results of PP biaxialgeogrids and the failure rule of PP biaxial geogrids underindoor photooxidative aging is inferred

Gray model is a differential equation established bygenerating new data series from the original data series GM(1 N) represents the differential equation of N variables oforder 1 +e modeling process is as follows

Sequence based on more than four known pieces ofdata is

X(0)

(k)1113966 1113967 k 1 2 3 n (2)

Generating sequence of accumulated data with givendata sequence is as follows

X(1)

(k)1113966 1113967 k 1 2 3 n (3)

Approximating cumulative data series values with con-tinuous smooth values of exponential curves is as follows

1113957X(1)

(k) 1113957X(1)

(1) minusu

a1113876 1113877bulle

minus a(kminus 1)+

u

a k 1 2 3 n

(4)

+e smoothing approximation value of the original datasequence is reduced

1113957X(0)

(k) 1113957X(1)

(k) minus 1113957X(1)

(k minus 1) k 1 2 3 n (5)

In the above formula

ermal oxygen agingPhotooxidation aging

Tens

ile st

reng

th re

tent

ion

rate

()

65

70

75

80

85

90

95

100

100 200 300 400 500 600 700 8000Aging time (h)

(a)

ermal oxygen agingPhotooxidation aging

Tens

ile st

reng

th re

tent

ion

()

70

75

80

85

90

95

100

100 200 300 400 500 600 700 8000Aging time (h)

(b)

Figure 8 Relationship curves of tensile strength retention rate-aging time (a) BG1 (b) BG2

ermal oxygen agingPhotooxygen aging

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

50

60

70

80

90

100

110

100 200 300 400 500 600 700 8000Aging time (h)

(a)

ermal oxygen agingPhotooxidation aging

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

50556065707580859095

100105

100 200 300 400 500 600 700 8000Aging time (h)

(b)

Figure 9 Relationship curves of retention rate of elongation at break-aging time (a) BG1 (b) BG2

8 Advances in Civil Engineering

a

u

⎡⎢⎢⎢⎣ ⎤⎥⎥⎥⎦ BTB1113872 1113873

minus 1B

minus 1B

TYn

B

b1 1

b2 1

middot middot middot 1

bnminus1 1

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦

Yn

y1

y2

middot middot middot

ynminus1

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦

m (n minus 1) 1113944nminus1

i1bi( 1113857

2minus 1113944

nminus 1

i1bi

⎛⎝ ⎞⎠

2

a (n minus 1)1113944

nminus1i1 biyi minus 1113944

nminus1i1 bi1113944

nminus1i1 yi1113876 1113877

m

u minus1113944

nminus1i1 bi1113944

nminus1i1 biyi + 1113944

nminus1i1 bi( 1113857

21113944

nminus1i1 yi1113876 1113877

m

(6)

Based on the experimental data of tensile strength andelongation at rupture of PP biaxial geogrids under photo-oxidation aging a GMwas established to predict the changesin tensile strength and elongation at rupture of PP biaxialgeogrids under long-term photooxidation aging +e pre-diction results are listed in Table 4

It can be seen from Table 3 that the GM (1 1) model isestablished for the tensile strength of samples BG1 and BG2under photooxidation aging for 0ndash600 h +e maximumrelative error of fitting the photooxidation aging test resultsof BG1 is only 248 the average error is only 149 and thetensile strength retention rate of BG1 and BG2 under 700 h isonly 662 +e GM is used to predict the tensile strength ofBG1 and BG2 +e calculated predicted value is 646 andthe relative error of data recovery is only 242 +emaximum relative error of fitting BG2 is only 165 theaverage error is only 030 and the relative error ofchecking data with 700 h is only 565 +is clearly showsthat the GM (1 1) model has better fitting effect

Prediction curves of tensile strength and photooxidationaging time are shown in Figure 10 It can be seen fromFigure 10(a) that the tensile strength retention rate of BG1 is5198 When the photooxidation aging time is 1000 h thetensile strength retention rate of BG1 is 4832 When thephotooxidation aging time is 1100 h the tensile strengthretention rate of BG1 is less than 50 so the PP biaxialgeogrid losees efficacy From Figure 10(b) it can be seen thatthe tensile strength retention rate of BG2 is 5188 when thephotooxidation aging time is 1700 h and 4983 when it is1800 h less than 50 and thus the PP biaxial geogrid isinvalid

+e aging test curve changes greatly in the early stage ofaging and is relatively unstable +e prediction curve is notvery accurate in the early stage of aging but rather accurate indescribing the long-term performance of aging

42 Test of Load Bearing Capacity of Reinforced Sand Foun-dation considering theEffects ofAging +e tensile strength ofthe geogrid before and after aging is listed in Table 5

Load-settlement curves of the cushion are obtainedaccording to load at all levels as shown in Figure 10Compared with the nonreinforced sand cushion the set-tlement value of the reinforced sand cushion under the sameload decreases and the bearing capacity of the foundationincreases the settlement value of the reinforced sandcushion of BG2 geogrid was smaller than that of the rein-forced sand cushion of BG1 geogrid and the bearing ca-pacity of the cushion was increased the difference in theload-settlement curves of the reinforced sand cushion andthe nonreinforced sand cushion at the early stage was smallIt shows that the reinforcement effect was not obvious at theinitial stage of loading With the increase in load the spacingbetween the load-settlement curves of the pure sand BG1reinforced sand and BG2 reinforced sand cushions grad-ually increased+us the reinforcement effect is increasinglyobvious

+e P-S curve of geogrid-reinforced sand consideringthe aging effect is shown in Figure 11 Figures 11(a) and11(b) show that the P-S curves of aging geogrids coincidewith those of nonaging geogrids when the load P was lessthan 125 kPa With the increase in load the two P-S curvesbegin to separate and under the same load the settlementratio of the aging-reinforced sand cushion of the BG1geogrids to that of the non-aging-reinforced sand cushionwas less than that of the aging-reinforced sand cushion Ingeneral the bearing capacity of the aging-reinforcedcushion was slightly lower than that of the non-aging-

Table 4 +e results of gray model prediction

Aging time (h) 0 100 200 300 400 500 600

BG1 tensile strength retention rateTest value () 100 986 944 888 7980 737 697

Predicted value () 100 1001 931 866 805 748 695Relative error () mdash 152 138 248 088 149 029

BG2 tensile strength retention rateTest value () 100 978 964 908 865 853 801

Predicted value () 100 9870 9481 9108 8749 8404 8073Relative error () mdash 092 165 030 114 148 078

Advances in Civil Engineering 9

reinforced cushion but the effect is not obvious +ebearing capacity of reinforced soil decreases with the in-crease of aging time

+e effect of aging of geogrids in the early stage ofloading on the bearing capacity of reinforced soil is

negligible +e authors define an aging effect on the initialload When the load value is less than the initial load of agingeffect the aging effect is small on the settlement of foun-dation When the load value exceeds the initial load of agingeffect the aging effect on the settlement of foundation

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

Photooxidative aging time (h)

Test valuePredicted value

Tens

ile st

reng

th re

tent

ion

rate

()

40

50

60

70

80

90

100

(a)

0

200

400

600

800

1000

1200

1400

1600

1800

Photooxidative aging time (h)

Test valuePredicted value

Tens

ile st

reng

th re

tent

ion

()

404550556065707580859095

100

(b)

Figure 10 Prediction curves of tensile strength and photooxidation aging time (a) BG1 (b) BG2

Table 5 +e tensile strength of geogrid before and after aging

Geogrid Tensile strength of geogrid before aging(kNm)

Radiation intensity (Wm2)

Tensile strength of geogrid after aging of700 h

Aging time(h)

BG1 158 600 108 700BG2 255 600 1835 700

0 100 200 300 400 500p (kPa)

Pure sand Unaged BG1

Aging 500h BG1Aging 600h BG1

25

20

15

10

5

0

s (m

m)

(a)

p (kPa)

Pure sand Unaged BG2

Aging 500h BG2Aging 600h BG2

0 100 200 300 400 500 600

25

20

15

10

5

0

s (m

m)

(b)

Figure 11 Load-settlement (P-S) curve of geogrid reinforced sand considering aging effect (a) BG1 (b) BG2

10 Advances in Civil Engineering

gradually appears For BG1 the aging load has an initialvalue of 125 kPa Similar to BG1 the initial aging load ofBG2 bars is 175 kPa which is much larger than that of BG1bars (Figure 11(b)) +e aging effect of BG2 with highertensile strength on the initial load is greater than that of theBG1 geogrid with lower tensile strength It indicates that theaging effect of BG2 with higher tensile strength on thebearing capacity of the reinforced cushion foundation issmaller than that of the BG1 geogrid with lower tensilestrength

After the model test it was found that the defor-mation of biaxial geogrid was significant (see Figure 12)+e deformed geogrid was measured and the centralpoint of the biaxial geogrid shall be concave about17 mm

+e beneficial effect of reinforcement for increasing thebearing capacity is conveniently described by the bearingcapacity ratio (BCR) according to [20]

BCR qR

q0 (7)

In order to compare the strengths of geogrids underdifferent load levels the ultimate bearing capacity and ul-timate bearing capacity ratio are listed in Table 6 It can beseen that the bearing capacity of the geogrid-reinforcedcushion decreases with aging For BG1 the bearing capacityof the foundation decreases by 1333 and 20 respectivelyafter 500 h and 600 h of aging For BG2 aged 500 h and 600 hthe bearing capacity of the foundation decreases by 625and 1875 respectively

+e BCR of the geogrid-reinforced sand is listed inTable 6 It can be seen that the aging effect of the geogrid

leads to the decline of the reinforced foundation BCR whichreduces the bearing capacity of the geogrid-reinforcedfoundation For different types of geogrids the degree ofdecline in BCR owing to aging is different and the value ofdecline in BCR owing to aging is slightly smaller for thehigher-strength BG2 than for the lower-strength BG1

5 MechanismAnalysis of PhotooxygenAging inPP Biaxial Geogrid

+e energy of the ultraviolet light accelerates the agingprocess of the grid Photooxidation causes degradation andaging of the PP germanium geogrid decomposes the PPpolymer and destroys its internal structure [21]+e tensilestrength and fracture elongation of two kinds of PP biaxialgeogrids were reduced in different degrees after differentaging modes temperature aging time and aging condi-tions After thermal aging there is a decrease in cross-linking degree and in binding and friction betweenmolecular chains +e effects of photooxygen aging aregreater than the effects of thermal oxygen aging +esunlight ultraviolet light provides the energy required forchemical crosslinking of the PP biaxial geogrid +e tensilestrength and fracture elongation of the geogrid temporarilyincrease in a short period of time (100ndash200 h) with theextension of aging time

6 Conclusion

In this study thermal oxygen test is carried out at 60degC 70degCand 80degC and ultraviolet photooxygen aging tests are carriedout at 70degC for two kinds of PP biaxial geogrids of different

(a) (b)

Figure 12 Geogrid-reinforced foundation damage

Table 6 BCR of geogrid-reinforced sand

Group Aging time Ultimate bearing capacity (kPa) BCR Percentage reduction in strength due to agingUnreinforced mdash 250 1 mdashBG1 0 375 15 mdashBG1 500 325 13 1333BG1 600 300 12 20BG2 0 400 16 mdashBG2 500 375 15 625BG2 600 325 13 1875

Advances in Civil Engineering 11

specifications +e influence of aging factors on the bearingcapacity of geogrid-reinforced foundations is considered

After investigating the effects of thermal oxygen andphotooxygen aging tests on tensile strength and fractureelongation of geogrids the following conclusions are drawn

(1) +e tensile strength of the geogrid decreases withthe increase in aging time and there is an increasein properties in aging period up to 100ndash200 hours+e comparison with the same thermal oxygenaging time shows that the tensile strength of thegeogrid decreases with the increase in thermaloxygen aging temperature With the increase inphotooxygen aging time the tensile strength of thegeogrid showed a significant trend of decrease +eeffect of ultraviolet light on the geogrid tensilestrength during aging is much greater than that oftemperature +e effect of photooxygen aging ontensile strength is greater than that of thermaloxygen aging for two different types of PP biaxialgeogrid Under the same temperature and agingtime the tensile strength of geogrid is reduced by nomore than 5 while the strength of geogrid isreduced by about 35

(2) +e retention rate of fracture elongation decreaseswith the increase in thermal oxygen aging tempera-ture and aging time+e elongation at break decreaseswith the photooxygen aging time and the influence ofultraviolet light on geogrid fracture elongation ismuch greater than that of temperature +e tensilestrength retention rate of different types of PP biaxialgrids showed different photooxygen aging charac-teristics+e aging resistance performance of BG2wassignificantly higher than that of BG1

(3) +e effect against tensile strength retention rate isbetter with the GM +e following conclusion camefrom prediction models with a blackboard tem-perature of 70degCplusmn 2degC irradiation of 8 h nonirra-diated condensation of 4 h relative humidity of70plusmn 2 and ultraviolet irradiance of 600Wm2the lower-intensity BG1 fails at approximately1100 h +e higher-strength BG2 fails at approxi-mately 1800 h

(4) Geogrid reinforcement significantly changes thebearing characteristics of a sand cushion by in-creasing the bearing capacity of the reinforcedcushion+e reinforcement effect of the geogrid withhigher tensile strength is more significant Agingchanges the interface characteristics between thegeogrid and soil In this test the bearing properties ofthe geogrid cushion after aging have certain changes+e aging behavior of the two geogrids reduces theload bearing capacity of the reinforced cushion by20 and 1875 respectively

Data Availability

No data were used to support this study

Conflicts of Interest

+e authors declare that they have no conflicts of interestrelated to this work

Acknowledgments

+e authors would like to acknowledge the National NaturalScience Foundation of China for the financial support forthis study (no 51578359)

References

[1] Y-L Dong J Han and X-H Bai ldquoNumerical analysis oftensile behavior of geogrids with rectangular and triangularaperturesrdquo Geotextiles and Geomembranes vol 29 no 2pp 83ndash91 2011

[2] A M R Ewais R K Rowe and J Scheirs ldquoDegradationbehaviour of HDPE geomembranes with high and low initialhigh-pressure oxidative induction timerdquo Geotextiles andGeomembranes vol 42 no 2 pp 111ndash126 2014

[3] Y G Hsuan and R M Koerner ldquoAntioxidant depletionlifetime in high density polyethylene geomembranesrdquo Journalof Geotechnical and Geoenvironmental Engineering vol 124no 6 pp 532ndash541 1998

[4] W-K Wong and Y G Hsuan ldquoInteraction of antioxidantswith carbon black in polyethylene using oxidative inductiontime methodsrdquo Geotextiles and Geomembranes vol 42 no 6pp 641ndash647 2014

[5] R K Rowe and H P Sangam ldquoDurability of HDPE geo-membranesrdquo Geotextiles and Geomembranes vol 20 no 2pp 77ndash95 2002

[6] H P Sangam and R K Rowe ldquoEffects of exposure conditionson the depletion of antioxidants from high-density polyeth-ylene (HDPE) geomembranesrdquo Canadian GeotechnicalJournal vol 39 no 6 pp 1221ndash1230 2002

[7] W Muller and I Jacob ldquoOxidative resistance of high densitypolyethylene geomembranesrdquo Polymer Degradation andStability vol 79 pp 161ndash172 2003

[8] S B Gulec T B Edil and C H Benson ldquoEffect of acidic minedrainage on the polymer properties of an HDPE geo-membranerdquo Geosynthetics International vol 11 no 2pp 60ndash72 2004

[9] R K Rowe and S Rimal ldquoDepletion of antioxidants from aHDPE geomembrane in a composite linerrdquo Journal of Geo-technical and Geoenvironmental Engineering vol 134 no 1pp 68ndash78 2008

[10] R K Rowe S Rimal and H P Sangam ldquoAgeing of HDPEgeomembrane exposed to air water and leachate at differenttemperaturesrdquo Geotextiles and Geomembranes vol 27pp 131ndash151 2009

[11] R K RoweM Z Islam and Y G Hsuan ldquoEffects of thicknesson the aging of HDPE geomembranesrdquo Journal of Geotech-nical and Geoenvironmental Engineering vol 136 no 2pp 299ndash309 2010

[12] F B Abdelaal and R K Rowe ldquoEffect of high temperatures onantioxidant depletion from different HDPE geomembranesrdquoGeotextiles and Geomembranes vol 42 no 4 pp 284ndash3012014

[13] X-H Bai X-Z Huang and W Zhang ldquoBearing capacity ofsquare footing supported by a geobelt-reinforced crushedstone cushion on soft soilrdquo Geotextiles and Geomembranesvol 38 pp 37ndash42 2013

12 Advances in Civil Engineering

aging test of PP biaxial geogrids were provided +e GM wasused to fit the photooxidative aging test results of PP biaxialgeogrids and the failure rule of PP biaxial geogrids underindoor photooxidative aging is inferred

Gray model is a differential equation established bygenerating new data series from the original data series GM(1 N) represents the differential equation of N variables oforder 1 +e modeling process is as follows

Sequence based on more than four known pieces ofdata is

X(0)

(k)1113966 1113967 k 1 2 3 n (2)

Generating sequence of accumulated data with givendata sequence is as follows

X(1)

(k)1113966 1113967 k 1 2 3 n (3)

Approximating cumulative data series values with con-tinuous smooth values of exponential curves is as follows

1113957X(1)

(k) 1113957X(1)

(1) minusu

a1113876 1113877bulle

minus a(kminus 1)+

u

a k 1 2 3 n

(4)

+e smoothing approximation value of the original datasequence is reduced

1113957X(0)

(k) 1113957X(1)

(k) minus 1113957X(1)

(k minus 1) k 1 2 3 n (5)

In the above formula

ermal oxygen agingPhotooxidation aging

Tens

ile st

reng

th re

tent

ion

rate

()

65

70

75

80

85

90

95

100

100 200 300 400 500 600 700 8000Aging time (h)

(a)

ermal oxygen agingPhotooxidation aging

Tens

ile st

reng

th re

tent

ion

()

70

75

80

85

90

95

100

100 200 300 400 500 600 700 8000Aging time (h)

(b)

Figure 8 Relationship curves of tensile strength retention rate-aging time (a) BG1 (b) BG2

ermal oxygen agingPhotooxygen aging

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

50

60

70

80

90

100

110

100 200 300 400 500 600 700 8000Aging time (h)

(a)

ermal oxygen agingPhotooxidation aging

Rete

ntio

n ra

te o

f elo

ngat

ion

at b

reak

()

50556065707580859095

100105

100 200 300 400 500 600 700 8000Aging time (h)

(b)

Figure 9 Relationship curves of retention rate of elongation at break-aging time (a) BG1 (b) BG2

8 Advances in Civil Engineering

a

u

⎡⎢⎢⎢⎣ ⎤⎥⎥⎥⎦ BTB1113872 1113873

minus 1B

minus 1B

TYn

B

b1 1

b2 1

middot middot middot 1

bnminus1 1

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦

Yn

y1

y2

middot middot middot

ynminus1

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦

m (n minus 1) 1113944nminus1

i1bi( 1113857

2minus 1113944

nminus 1

i1bi

⎛⎝ ⎞⎠

2

a (n minus 1)1113944

nminus1i1 biyi minus 1113944

nminus1i1 bi1113944

nminus1i1 yi1113876 1113877

m

u minus1113944

nminus1i1 bi1113944

nminus1i1 biyi + 1113944

nminus1i1 bi( 1113857

21113944

nminus1i1 yi1113876 1113877

m

(6)

Based on the experimental data of tensile strength andelongation at rupture of PP biaxial geogrids under photo-oxidation aging a GMwas established to predict the changesin tensile strength and elongation at rupture of PP biaxialgeogrids under long-term photooxidation aging +e pre-diction results are listed in Table 4

It can be seen from Table 3 that the GM (1 1) model isestablished for the tensile strength of samples BG1 and BG2under photooxidation aging for 0ndash600 h +e maximumrelative error of fitting the photooxidation aging test resultsof BG1 is only 248 the average error is only 149 and thetensile strength retention rate of BG1 and BG2 under 700 h isonly 662 +e GM is used to predict the tensile strength ofBG1 and BG2 +e calculated predicted value is 646 andthe relative error of data recovery is only 242 +emaximum relative error of fitting BG2 is only 165 theaverage error is only 030 and the relative error ofchecking data with 700 h is only 565 +is clearly showsthat the GM (1 1) model has better fitting effect

Prediction curves of tensile strength and photooxidationaging time are shown in Figure 10 It can be seen fromFigure 10(a) that the tensile strength retention rate of BG1 is5198 When the photooxidation aging time is 1000 h thetensile strength retention rate of BG1 is 4832 When thephotooxidation aging time is 1100 h the tensile strengthretention rate of BG1 is less than 50 so the PP biaxialgeogrid losees efficacy From Figure 10(b) it can be seen thatthe tensile strength retention rate of BG2 is 5188 when thephotooxidation aging time is 1700 h and 4983 when it is1800 h less than 50 and thus the PP biaxial geogrid isinvalid

+e aging test curve changes greatly in the early stage ofaging and is relatively unstable +e prediction curve is notvery accurate in the early stage of aging but rather accurate indescribing the long-term performance of aging

42 Test of Load Bearing Capacity of Reinforced Sand Foun-dation considering theEffects ofAging +e tensile strength ofthe geogrid before and after aging is listed in Table 5

Load-settlement curves of the cushion are obtainedaccording to load at all levels as shown in Figure 10Compared with the nonreinforced sand cushion the set-tlement value of the reinforced sand cushion under the sameload decreases and the bearing capacity of the foundationincreases the settlement value of the reinforced sandcushion of BG2 geogrid was smaller than that of the rein-forced sand cushion of BG1 geogrid and the bearing ca-pacity of the cushion was increased the difference in theload-settlement curves of the reinforced sand cushion andthe nonreinforced sand cushion at the early stage was smallIt shows that the reinforcement effect was not obvious at theinitial stage of loading With the increase in load the spacingbetween the load-settlement curves of the pure sand BG1reinforced sand and BG2 reinforced sand cushions grad-ually increased+us the reinforcement effect is increasinglyobvious

+e P-S curve of geogrid-reinforced sand consideringthe aging effect is shown in Figure 11 Figures 11(a) and11(b) show that the P-S curves of aging geogrids coincidewith those of nonaging geogrids when the load P was lessthan 125 kPa With the increase in load the two P-S curvesbegin to separate and under the same load the settlementratio of the aging-reinforced sand cushion of the BG1geogrids to that of the non-aging-reinforced sand cushionwas less than that of the aging-reinforced sand cushion Ingeneral the bearing capacity of the aging-reinforcedcushion was slightly lower than that of the non-aging-

Table 4 +e results of gray model prediction

Aging time (h) 0 100 200 300 400 500 600

BG1 tensile strength retention rateTest value () 100 986 944 888 7980 737 697

Predicted value () 100 1001 931 866 805 748 695Relative error () mdash 152 138 248 088 149 029

BG2 tensile strength retention rateTest value () 100 978 964 908 865 853 801

Predicted value () 100 9870 9481 9108 8749 8404 8073Relative error () mdash 092 165 030 114 148 078

Advances in Civil Engineering 9

reinforced cushion but the effect is not obvious +ebearing capacity of reinforced soil decreases with the in-crease of aging time

+e effect of aging of geogrids in the early stage ofloading on the bearing capacity of reinforced soil is

negligible +e authors define an aging effect on the initialload When the load value is less than the initial load of agingeffect the aging effect is small on the settlement of foun-dation When the load value exceeds the initial load of agingeffect the aging effect on the settlement of foundation

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

Photooxidative aging time (h)

Test valuePredicted value

Tens

ile st

reng

th re

tent

ion

rate

()

40

50

60

70

80

90

100

(a)

0

200

400

600

800

1000

1200

1400

1600

1800

Photooxidative aging time (h)

Test valuePredicted value

Tens

ile st

reng

th re

tent

ion

()

404550556065707580859095

100

(b)

Figure 10 Prediction curves of tensile strength and photooxidation aging time (a) BG1 (b) BG2

Table 5 +e tensile strength of geogrid before and after aging

Geogrid Tensile strength of geogrid before aging(kNm)

Radiation intensity (Wm2)

Tensile strength of geogrid after aging of700 h

Aging time(h)

BG1 158 600 108 700BG2 255 600 1835 700

0 100 200 300 400 500p (kPa)

Pure sand Unaged BG1

Aging 500h BG1Aging 600h BG1

25

20

15

10

5

0

s (m

m)

(a)

p (kPa)

Pure sand Unaged BG2

Aging 500h BG2Aging 600h BG2

0 100 200 300 400 500 600

25

20

15

10

5

0

s (m

m)

(b)

Figure 11 Load-settlement (P-S) curve of geogrid reinforced sand considering aging effect (a) BG1 (b) BG2

10 Advances in Civil Engineering

gradually appears For BG1 the aging load has an initialvalue of 125 kPa Similar to BG1 the initial aging load ofBG2 bars is 175 kPa which is much larger than that of BG1bars (Figure 11(b)) +e aging effect of BG2 with highertensile strength on the initial load is greater than that of theBG1 geogrid with lower tensile strength It indicates that theaging effect of BG2 with higher tensile strength on thebearing capacity of the reinforced cushion foundation issmaller than that of the BG1 geogrid with lower tensilestrength

After the model test it was found that the defor-mation of biaxial geogrid was significant (see Figure 12)+e deformed geogrid was measured and the centralpoint of the biaxial geogrid shall be concave about17 mm

+e beneficial effect of reinforcement for increasing thebearing capacity is conveniently described by the bearingcapacity ratio (BCR) according to [20]

BCR qR

q0 (7)

In order to compare the strengths of geogrids underdifferent load levels the ultimate bearing capacity and ul-timate bearing capacity ratio are listed in Table 6 It can beseen that the bearing capacity of the geogrid-reinforcedcushion decreases with aging For BG1 the bearing capacityof the foundation decreases by 1333 and 20 respectivelyafter 500 h and 600 h of aging For BG2 aged 500 h and 600 hthe bearing capacity of the foundation decreases by 625and 1875 respectively

+e BCR of the geogrid-reinforced sand is listed inTable 6 It can be seen that the aging effect of the geogrid

leads to the decline of the reinforced foundation BCR whichreduces the bearing capacity of the geogrid-reinforcedfoundation For different types of geogrids the degree ofdecline in BCR owing to aging is different and the value ofdecline in BCR owing to aging is slightly smaller for thehigher-strength BG2 than for the lower-strength BG1

5 MechanismAnalysis of PhotooxygenAging inPP Biaxial Geogrid

+e energy of the ultraviolet light accelerates the agingprocess of the grid Photooxidation causes degradation andaging of the PP germanium geogrid decomposes the PPpolymer and destroys its internal structure [21]+e tensilestrength and fracture elongation of two kinds of PP biaxialgeogrids were reduced in different degrees after differentaging modes temperature aging time and aging condi-tions After thermal aging there is a decrease in cross-linking degree and in binding and friction betweenmolecular chains +e effects of photooxygen aging aregreater than the effects of thermal oxygen aging +esunlight ultraviolet light provides the energy required forchemical crosslinking of the PP biaxial geogrid +e tensilestrength and fracture elongation of the geogrid temporarilyincrease in a short period of time (100ndash200 h) with theextension of aging time

6 Conclusion

In this study thermal oxygen test is carried out at 60degC 70degCand 80degC and ultraviolet photooxygen aging tests are carriedout at 70degC for two kinds of PP biaxial geogrids of different

(a) (b)

Figure 12 Geogrid-reinforced foundation damage

Table 6 BCR of geogrid-reinforced sand

Group Aging time Ultimate bearing capacity (kPa) BCR Percentage reduction in strength due to agingUnreinforced mdash 250 1 mdashBG1 0 375 15 mdashBG1 500 325 13 1333BG1 600 300 12 20BG2 0 400 16 mdashBG2 500 375 15 625BG2 600 325 13 1875

Advances in Civil Engineering 11

specifications +e influence of aging factors on the bearingcapacity of geogrid-reinforced foundations is considered

After investigating the effects of thermal oxygen andphotooxygen aging tests on tensile strength and fractureelongation of geogrids the following conclusions are drawn

(1) +e tensile strength of the geogrid decreases withthe increase in aging time and there is an increasein properties in aging period up to 100ndash200 hours+e comparison with the same thermal oxygenaging time shows that the tensile strength of thegeogrid decreases with the increase in thermaloxygen aging temperature With the increase inphotooxygen aging time the tensile strength of thegeogrid showed a significant trend of decrease +eeffect of ultraviolet light on the geogrid tensilestrength during aging is much greater than that oftemperature +e effect of photooxygen aging ontensile strength is greater than that of thermaloxygen aging for two different types of PP biaxialgeogrid Under the same temperature and agingtime the tensile strength of geogrid is reduced by nomore than 5 while the strength of geogrid isreduced by about 35

(2) +e retention rate of fracture elongation decreaseswith the increase in thermal oxygen aging tempera-ture and aging time+e elongation at break decreaseswith the photooxygen aging time and the influence ofultraviolet light on geogrid fracture elongation ismuch greater than that of temperature +e tensilestrength retention rate of different types of PP biaxialgrids showed different photooxygen aging charac-teristics+e aging resistance performance of BG2wassignificantly higher than that of BG1

(3) +e effect against tensile strength retention rate isbetter with the GM +e following conclusion camefrom prediction models with a blackboard tem-perature of 70degCplusmn 2degC irradiation of 8 h nonirra-diated condensation of 4 h relative humidity of70plusmn 2 and ultraviolet irradiance of 600Wm2the lower-intensity BG1 fails at approximately1100 h +e higher-strength BG2 fails at approxi-mately 1800 h

(4) Geogrid reinforcement significantly changes thebearing characteristics of a sand cushion by in-creasing the bearing capacity of the reinforcedcushion+e reinforcement effect of the geogrid withhigher tensile strength is more significant Agingchanges the interface characteristics between thegeogrid and soil In this test the bearing properties ofthe geogrid cushion after aging have certain changes+e aging behavior of the two geogrids reduces theload bearing capacity of the reinforced cushion by20 and 1875 respectively

Data Availability

No data were used to support this study

Conflicts of Interest

+e authors declare that they have no conflicts of interestrelated to this work

Acknowledgments

+e authors would like to acknowledge the National NaturalScience Foundation of China for the financial support forthis study (no 51578359)

References

[1] Y-L Dong J Han and X-H Bai ldquoNumerical analysis oftensile behavior of geogrids with rectangular and triangularaperturesrdquo Geotextiles and Geomembranes vol 29 no 2pp 83ndash91 2011

[2] A M R Ewais R K Rowe and J Scheirs ldquoDegradationbehaviour of HDPE geomembranes with high and low initialhigh-pressure oxidative induction timerdquo Geotextiles andGeomembranes vol 42 no 2 pp 111ndash126 2014

[3] Y G Hsuan and R M Koerner ldquoAntioxidant depletionlifetime in high density polyethylene geomembranesrdquo Journalof Geotechnical and Geoenvironmental Engineering vol 124no 6 pp 532ndash541 1998

[4] W-K Wong and Y G Hsuan ldquoInteraction of antioxidantswith carbon black in polyethylene using oxidative inductiontime methodsrdquo Geotextiles and Geomembranes vol 42 no 6pp 641ndash647 2014

[5] R K Rowe and H P Sangam ldquoDurability of HDPE geo-membranesrdquo Geotextiles and Geomembranes vol 20 no 2pp 77ndash95 2002

[6] H P Sangam and R K Rowe ldquoEffects of exposure conditionson the depletion of antioxidants from high-density polyeth-ylene (HDPE) geomembranesrdquo Canadian GeotechnicalJournal vol 39 no 6 pp 1221ndash1230 2002

[7] W Muller and I Jacob ldquoOxidative resistance of high densitypolyethylene geomembranesrdquo Polymer Degradation andStability vol 79 pp 161ndash172 2003

[8] S B Gulec T B Edil and C H Benson ldquoEffect of acidic minedrainage on the polymer properties of an HDPE geo-membranerdquo Geosynthetics International vol 11 no 2pp 60ndash72 2004

[9] R K Rowe and S Rimal ldquoDepletion of antioxidants from aHDPE geomembrane in a composite linerrdquo Journal of Geo-technical and Geoenvironmental Engineering vol 134 no 1pp 68ndash78 2008

[10] R K Rowe S Rimal and H P Sangam ldquoAgeing of HDPEgeomembrane exposed to air water and leachate at differenttemperaturesrdquo Geotextiles and Geomembranes vol 27pp 131ndash151 2009

[11] R K RoweM Z Islam and Y G Hsuan ldquoEffects of thicknesson the aging of HDPE geomembranesrdquo Journal of Geotech-nical and Geoenvironmental Engineering vol 136 no 2pp 299ndash309 2010

[12] F B Abdelaal and R K Rowe ldquoEffect of high temperatures onantioxidant depletion from different HDPE geomembranesrdquoGeotextiles and Geomembranes vol 42 no 4 pp 284ndash3012014

[13] X-H Bai X-Z Huang and W Zhang ldquoBearing capacity ofsquare footing supported by a geobelt-reinforced crushedstone cushion on soft soilrdquo Geotextiles and Geomembranesvol 38 pp 37ndash42 2013

12 Advances in Civil Engineering

a

u

⎡⎢⎢⎢⎣ ⎤⎥⎥⎥⎦ BTB1113872 1113873

minus 1B

minus 1B

TYn

B

b1 1

b2 1

middot middot middot 1

bnminus1 1

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦

Yn

y1

y2

middot middot middot

ynminus1

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦

m (n minus 1) 1113944nminus1

i1bi( 1113857

2minus 1113944

nminus 1

i1bi

⎛⎝ ⎞⎠

2

a (n minus 1)1113944

nminus1i1 biyi minus 1113944

nminus1i1 bi1113944

nminus1i1 yi1113876 1113877

m

u minus1113944

nminus1i1 bi1113944

nminus1i1 biyi + 1113944

nminus1i1 bi( 1113857

21113944

nminus1i1 yi1113876 1113877

m

(6)

Based on the experimental data of tensile strength andelongation at rupture of PP biaxial geogrids under photo-oxidation aging a GMwas established to predict the changesin tensile strength and elongation at rupture of PP biaxialgeogrids under long-term photooxidation aging +e pre-diction results are listed in Table 4

It can be seen from Table 3 that the GM (1 1) model isestablished for the tensile strength of samples BG1 and BG2under photooxidation aging for 0ndash600 h +e maximumrelative error of fitting the photooxidation aging test resultsof BG1 is only 248 the average error is only 149 and thetensile strength retention rate of BG1 and BG2 under 700 h isonly 662 +e GM is used to predict the tensile strength ofBG1 and BG2 +e calculated predicted value is 646 andthe relative error of data recovery is only 242 +emaximum relative error of fitting BG2 is only 165 theaverage error is only 030 and the relative error ofchecking data with 700 h is only 565 +is clearly showsthat the GM (1 1) model has better fitting effect

Prediction curves of tensile strength and photooxidationaging time are shown in Figure 10 It can be seen fromFigure 10(a) that the tensile strength retention rate of BG1 is5198 When the photooxidation aging time is 1000 h thetensile strength retention rate of BG1 is 4832 When thephotooxidation aging time is 1100 h the tensile strengthretention rate of BG1 is less than 50 so the PP biaxialgeogrid losees efficacy From Figure 10(b) it can be seen thatthe tensile strength retention rate of BG2 is 5188 when thephotooxidation aging time is 1700 h and 4983 when it is1800 h less than 50 and thus the PP biaxial geogrid isinvalid

+e aging test curve changes greatly in the early stage ofaging and is relatively unstable +e prediction curve is notvery accurate in the early stage of aging but rather accurate indescribing the long-term performance of aging

42 Test of Load Bearing Capacity of Reinforced Sand Foun-dation considering theEffects ofAging +e tensile strength ofthe geogrid before and after aging is listed in Table 5

Load-settlement curves of the cushion are obtainedaccording to load at all levels as shown in Figure 10Compared with the nonreinforced sand cushion the set-tlement value of the reinforced sand cushion under the sameload decreases and the bearing capacity of the foundationincreases the settlement value of the reinforced sandcushion of BG2 geogrid was smaller than that of the rein-forced sand cushion of BG1 geogrid and the bearing ca-pacity of the cushion was increased the difference in theload-settlement curves of the reinforced sand cushion andthe nonreinforced sand cushion at the early stage was smallIt shows that the reinforcement effect was not obvious at theinitial stage of loading With the increase in load the spacingbetween the load-settlement curves of the pure sand BG1reinforced sand and BG2 reinforced sand cushions grad-ually increased+us the reinforcement effect is increasinglyobvious

+e P-S curve of geogrid-reinforced sand consideringthe aging effect is shown in Figure 11 Figures 11(a) and11(b) show that the P-S curves of aging geogrids coincidewith those of nonaging geogrids when the load P was lessthan 125 kPa With the increase in load the two P-S curvesbegin to separate and under the same load the settlementratio of the aging-reinforced sand cushion of the BG1geogrids to that of the non-aging-reinforced sand cushionwas less than that of the aging-reinforced sand cushion Ingeneral the bearing capacity of the aging-reinforcedcushion was slightly lower than that of the non-aging-

Table 4 +e results of gray model prediction

Aging time (h) 0 100 200 300 400 500 600

BG1 tensile strength retention rateTest value () 100 986 944 888 7980 737 697

Predicted value () 100 1001 931 866 805 748 695Relative error () mdash 152 138 248 088 149 029

BG2 tensile strength retention rateTest value () 100 978 964 908 865 853 801

Predicted value () 100 9870 9481 9108 8749 8404 8073Relative error () mdash 092 165 030 114 148 078

Advances in Civil Engineering 9

reinforced cushion but the effect is not obvious +ebearing capacity of reinforced soil decreases with the in-crease of aging time

+e effect of aging of geogrids in the early stage ofloading on the bearing capacity of reinforced soil is

negligible +e authors define an aging effect on the initialload When the load value is less than the initial load of agingeffect the aging effect is small on the settlement of foun-dation When the load value exceeds the initial load of agingeffect the aging effect on the settlement of foundation

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

Photooxidative aging time (h)

Test valuePredicted value

Tens

ile st

reng

th re

tent

ion

rate

()

40

50

60

70

80

90

100

(a)

0

200

400

600

800

1000

1200

1400

1600

1800

Photooxidative aging time (h)

Test valuePredicted value

Tens

ile st

reng

th re

tent

ion

()

404550556065707580859095

100

(b)

Figure 10 Prediction curves of tensile strength and photooxidation aging time (a) BG1 (b) BG2

Table 5 +e tensile strength of geogrid before and after aging

Geogrid Tensile strength of geogrid before aging(kNm)

Radiation intensity (Wm2)

Tensile strength of geogrid after aging of700 h

Aging time(h)

BG1 158 600 108 700BG2 255 600 1835 700

0 100 200 300 400 500p (kPa)

Pure sand Unaged BG1

Aging 500h BG1Aging 600h BG1

25

20

15

10

5

0

s (m

m)

(a)

p (kPa)

Pure sand Unaged BG2

Aging 500h BG2Aging 600h BG2

0 100 200 300 400 500 600

25

20

15

10

5

0

s (m

m)

(b)

Figure 11 Load-settlement (P-S) curve of geogrid reinforced sand considering aging effect (a) BG1 (b) BG2

10 Advances in Civil Engineering

gradually appears For BG1 the aging load has an initialvalue of 125 kPa Similar to BG1 the initial aging load ofBG2 bars is 175 kPa which is much larger than that of BG1bars (Figure 11(b)) +e aging effect of BG2 with highertensile strength on the initial load is greater than that of theBG1 geogrid with lower tensile strength It indicates that theaging effect of BG2 with higher tensile strength on thebearing capacity of the reinforced cushion foundation issmaller than that of the BG1 geogrid with lower tensilestrength

After the model test it was found that the defor-mation of biaxial geogrid was significant (see Figure 12)+e deformed geogrid was measured and the centralpoint of the biaxial geogrid shall be concave about17 mm

+e beneficial effect of reinforcement for increasing thebearing capacity is conveniently described by the bearingcapacity ratio (BCR) according to [20]

BCR qR

q0 (7)

In order to compare the strengths of geogrids underdifferent load levels the ultimate bearing capacity and ul-timate bearing capacity ratio are listed in Table 6 It can beseen that the bearing capacity of the geogrid-reinforcedcushion decreases with aging For BG1 the bearing capacityof the foundation decreases by 1333 and 20 respectivelyafter 500 h and 600 h of aging For BG2 aged 500 h and 600 hthe bearing capacity of the foundation decreases by 625and 1875 respectively

+e BCR of the geogrid-reinforced sand is listed inTable 6 It can be seen that the aging effect of the geogrid

leads to the decline of the reinforced foundation BCR whichreduces the bearing capacity of the geogrid-reinforcedfoundation For different types of geogrids the degree ofdecline in BCR owing to aging is different and the value ofdecline in BCR owing to aging is slightly smaller for thehigher-strength BG2 than for the lower-strength BG1

5 MechanismAnalysis of PhotooxygenAging inPP Biaxial Geogrid

+e energy of the ultraviolet light accelerates the agingprocess of the grid Photooxidation causes degradation andaging of the PP germanium geogrid decomposes the PPpolymer and destroys its internal structure [21]+e tensilestrength and fracture elongation of two kinds of PP biaxialgeogrids were reduced in different degrees after differentaging modes temperature aging time and aging condi-tions After thermal aging there is a decrease in cross-linking degree and in binding and friction betweenmolecular chains +e effects of photooxygen aging aregreater than the effects of thermal oxygen aging +esunlight ultraviolet light provides the energy required forchemical crosslinking of the PP biaxial geogrid +e tensilestrength and fracture elongation of the geogrid temporarilyincrease in a short period of time (100ndash200 h) with theextension of aging time

6 Conclusion

In this study thermal oxygen test is carried out at 60degC 70degCand 80degC and ultraviolet photooxygen aging tests are carriedout at 70degC for two kinds of PP biaxial geogrids of different

(a) (b)

Figure 12 Geogrid-reinforced foundation damage

Table 6 BCR of geogrid-reinforced sand

Group Aging time Ultimate bearing capacity (kPa) BCR Percentage reduction in strength due to agingUnreinforced mdash 250 1 mdashBG1 0 375 15 mdashBG1 500 325 13 1333BG1 600 300 12 20BG2 0 400 16 mdashBG2 500 375 15 625BG2 600 325 13 1875

Advances in Civil Engineering 11

specifications +e influence of aging factors on the bearingcapacity of geogrid-reinforced foundations is considered

After investigating the effects of thermal oxygen andphotooxygen aging tests on tensile strength and fractureelongation of geogrids the following conclusions are drawn

(1) +e tensile strength of the geogrid decreases withthe increase in aging time and there is an increasein properties in aging period up to 100ndash200 hours+e comparison with the same thermal oxygenaging time shows that the tensile strength of thegeogrid decreases with the increase in thermaloxygen aging temperature With the increase inphotooxygen aging time the tensile strength of thegeogrid showed a significant trend of decrease +eeffect of ultraviolet light on the geogrid tensilestrength during aging is much greater than that oftemperature +e effect of photooxygen aging ontensile strength is greater than that of thermaloxygen aging for two different types of PP biaxialgeogrid Under the same temperature and agingtime the tensile strength of geogrid is reduced by nomore than 5 while the strength of geogrid isreduced by about 35

(2) +e retention rate of fracture elongation decreaseswith the increase in thermal oxygen aging tempera-ture and aging time+e elongation at break decreaseswith the photooxygen aging time and the influence ofultraviolet light on geogrid fracture elongation ismuch greater than that of temperature +e tensilestrength retention rate of different types of PP biaxialgrids showed different photooxygen aging charac-teristics+e aging resistance performance of BG2wassignificantly higher than that of BG1

(3) +e effect against tensile strength retention rate isbetter with the GM +e following conclusion camefrom prediction models with a blackboard tem-perature of 70degCplusmn 2degC irradiation of 8 h nonirra-diated condensation of 4 h relative humidity of70plusmn 2 and ultraviolet irradiance of 600Wm2the lower-intensity BG1 fails at approximately1100 h +e higher-strength BG2 fails at approxi-mately 1800 h

(4) Geogrid reinforcement significantly changes thebearing characteristics of a sand cushion by in-creasing the bearing capacity of the reinforcedcushion+e reinforcement effect of the geogrid withhigher tensile strength is more significant Agingchanges the interface characteristics between thegeogrid and soil In this test the bearing properties ofthe geogrid cushion after aging have certain changes+e aging behavior of the two geogrids reduces theload bearing capacity of the reinforced cushion by20 and 1875 respectively

Data Availability

No data were used to support this study

Conflicts of Interest

+e authors declare that they have no conflicts of interestrelated to this work

Acknowledgments

+e authors would like to acknowledge the National NaturalScience Foundation of China for the financial support forthis study (no 51578359)

References

[1] Y-L Dong J Han and X-H Bai ldquoNumerical analysis oftensile behavior of geogrids with rectangular and triangularaperturesrdquo Geotextiles and Geomembranes vol 29 no 2pp 83ndash91 2011

[2] A M R Ewais R K Rowe and J Scheirs ldquoDegradationbehaviour of HDPE geomembranes with high and low initialhigh-pressure oxidative induction timerdquo Geotextiles andGeomembranes vol 42 no 2 pp 111ndash126 2014

[3] Y G Hsuan and R M Koerner ldquoAntioxidant depletionlifetime in high density polyethylene geomembranesrdquo Journalof Geotechnical and Geoenvironmental Engineering vol 124no 6 pp 532ndash541 1998

[4] W-K Wong and Y G Hsuan ldquoInteraction of antioxidantswith carbon black in polyethylene using oxidative inductiontime methodsrdquo Geotextiles and Geomembranes vol 42 no 6pp 641ndash647 2014

[5] R K Rowe and H P Sangam ldquoDurability of HDPE geo-membranesrdquo Geotextiles and Geomembranes vol 20 no 2pp 77ndash95 2002

[6] H P Sangam and R K Rowe ldquoEffects of exposure conditionson the depletion of antioxidants from high-density polyeth-ylene (HDPE) geomembranesrdquo Canadian GeotechnicalJournal vol 39 no 6 pp 1221ndash1230 2002

[7] W Muller and I Jacob ldquoOxidative resistance of high densitypolyethylene geomembranesrdquo Polymer Degradation andStability vol 79 pp 161ndash172 2003

[8] S B Gulec T B Edil and C H Benson ldquoEffect of acidic minedrainage on the polymer properties of an HDPE geo-membranerdquo Geosynthetics International vol 11 no 2pp 60ndash72 2004

[9] R K Rowe and S Rimal ldquoDepletion of antioxidants from aHDPE geomembrane in a composite linerrdquo Journal of Geo-technical and Geoenvironmental Engineering vol 134 no 1pp 68ndash78 2008

[10] R K Rowe S Rimal and H P Sangam ldquoAgeing of HDPEgeomembrane exposed to air water and leachate at differenttemperaturesrdquo Geotextiles and Geomembranes vol 27pp 131ndash151 2009

[11] R K RoweM Z Islam and Y G Hsuan ldquoEffects of thicknesson the aging of HDPE geomembranesrdquo Journal of Geotech-nical and Geoenvironmental Engineering vol 136 no 2pp 299ndash309 2010

[12] F B Abdelaal and R K Rowe ldquoEffect of high temperatures onantioxidant depletion from different HDPE geomembranesrdquoGeotextiles and Geomembranes vol 42 no 4 pp 284ndash3012014

[13] X-H Bai X-Z Huang and W Zhang ldquoBearing capacity ofsquare footing supported by a geobelt-reinforced crushedstone cushion on soft soilrdquo Geotextiles and Geomembranesvol 38 pp 37ndash42 2013

12 Advances in Civil Engineering

reinforced cushion but the effect is not obvious +ebearing capacity of reinforced soil decreases with the in-crease of aging time

+e effect of aging of geogrids in the early stage ofloading on the bearing capacity of reinforced soil is

negligible +e authors define an aging effect on the initialload When the load value is less than the initial load of agingeffect the aging effect is small on the settlement of foun-dation When the load value exceeds the initial load of agingeffect the aging effect on the settlement of foundation

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

Photooxidative aging time (h)

Test valuePredicted value

Tens

ile st

reng

th re

tent

ion

rate

()

40

50

60

70

80

90

100

(a)

0

200

400

600

800

1000

1200

1400

1600

1800

Photooxidative aging time (h)

Test valuePredicted value

Tens

ile st

reng

th re

tent

ion

()

404550556065707580859095

100

(b)

Figure 10 Prediction curves of tensile strength and photooxidation aging time (a) BG1 (b) BG2

Table 5 +e tensile strength of geogrid before and after aging

Geogrid Tensile strength of geogrid before aging(kNm)

Radiation intensity (Wm2)

Tensile strength of geogrid after aging of700 h

Aging time(h)

BG1 158 600 108 700BG2 255 600 1835 700

0 100 200 300 400 500p (kPa)

Pure sand Unaged BG1

Aging 500h BG1Aging 600h BG1

25

20

15

10

5

0

s (m

m)

(a)

p (kPa)

Pure sand Unaged BG2

Aging 500h BG2Aging 600h BG2

0 100 200 300 400 500 600

25

20

15

10

5

0

s (m

m)

(b)

Figure 11 Load-settlement (P-S) curve of geogrid reinforced sand considering aging effect (a) BG1 (b) BG2

10 Advances in Civil Engineering

gradually appears For BG1 the aging load has an initialvalue of 125 kPa Similar to BG1 the initial aging load ofBG2 bars is 175 kPa which is much larger than that of BG1bars (Figure 11(b)) +e aging effect of BG2 with highertensile strength on the initial load is greater than that of theBG1 geogrid with lower tensile strength It indicates that theaging effect of BG2 with higher tensile strength on thebearing capacity of the reinforced cushion foundation issmaller than that of the BG1 geogrid with lower tensilestrength

After the model test it was found that the defor-mation of biaxial geogrid was significant (see Figure 12)+e deformed geogrid was measured and the centralpoint of the biaxial geogrid shall be concave about17 mm

+e beneficial effect of reinforcement for increasing thebearing capacity is conveniently described by the bearingcapacity ratio (BCR) according to [20]

BCR qR

q0 (7)

In order to compare the strengths of geogrids underdifferent load levels the ultimate bearing capacity and ul-timate bearing capacity ratio are listed in Table 6 It can beseen that the bearing capacity of the geogrid-reinforcedcushion decreases with aging For BG1 the bearing capacityof the foundation decreases by 1333 and 20 respectivelyafter 500 h and 600 h of aging For BG2 aged 500 h and 600 hthe bearing capacity of the foundation decreases by 625and 1875 respectively

+e BCR of the geogrid-reinforced sand is listed inTable 6 It can be seen that the aging effect of the geogrid

leads to the decline of the reinforced foundation BCR whichreduces the bearing capacity of the geogrid-reinforcedfoundation For different types of geogrids the degree ofdecline in BCR owing to aging is different and the value ofdecline in BCR owing to aging is slightly smaller for thehigher-strength BG2 than for the lower-strength BG1

5 MechanismAnalysis of PhotooxygenAging inPP Biaxial Geogrid

+e energy of the ultraviolet light accelerates the agingprocess of the grid Photooxidation causes degradation andaging of the PP germanium geogrid decomposes the PPpolymer and destroys its internal structure [21]+e tensilestrength and fracture elongation of two kinds of PP biaxialgeogrids were reduced in different degrees after differentaging modes temperature aging time and aging condi-tions After thermal aging there is a decrease in cross-linking degree and in binding and friction betweenmolecular chains +e effects of photooxygen aging aregreater than the effects of thermal oxygen aging +esunlight ultraviolet light provides the energy required forchemical crosslinking of the PP biaxial geogrid +e tensilestrength and fracture elongation of the geogrid temporarilyincrease in a short period of time (100ndash200 h) with theextension of aging time

6 Conclusion

In this study thermal oxygen test is carried out at 60degC 70degCand 80degC and ultraviolet photooxygen aging tests are carriedout at 70degC for two kinds of PP biaxial geogrids of different

(a) (b)

Figure 12 Geogrid-reinforced foundation damage

Table 6 BCR of geogrid-reinforced sand

Group Aging time Ultimate bearing capacity (kPa) BCR Percentage reduction in strength due to agingUnreinforced mdash 250 1 mdashBG1 0 375 15 mdashBG1 500 325 13 1333BG1 600 300 12 20BG2 0 400 16 mdashBG2 500 375 15 625BG2 600 325 13 1875

Advances in Civil Engineering 11

specifications +e influence of aging factors on the bearingcapacity of geogrid-reinforced foundations is considered

After investigating the effects of thermal oxygen andphotooxygen aging tests on tensile strength and fractureelongation of geogrids the following conclusions are drawn

(1) +e tensile strength of the geogrid decreases withthe increase in aging time and there is an increasein properties in aging period up to 100ndash200 hours+e comparison with the same thermal oxygenaging time shows that the tensile strength of thegeogrid decreases with the increase in thermaloxygen aging temperature With the increase inphotooxygen aging time the tensile strength of thegeogrid showed a significant trend of decrease +eeffect of ultraviolet light on the geogrid tensilestrength during aging is much greater than that oftemperature +e effect of photooxygen aging ontensile strength is greater than that of thermaloxygen aging for two different types of PP biaxialgeogrid Under the same temperature and agingtime the tensile strength of geogrid is reduced by nomore than 5 while the strength of geogrid isreduced by about 35

(2) +e retention rate of fracture elongation decreaseswith the increase in thermal oxygen aging tempera-ture and aging time+e elongation at break decreaseswith the photooxygen aging time and the influence ofultraviolet light on geogrid fracture elongation ismuch greater than that of temperature +e tensilestrength retention rate of different types of PP biaxialgrids showed different photooxygen aging charac-teristics+e aging resistance performance of BG2wassignificantly higher than that of BG1

(3) +e effect against tensile strength retention rate isbetter with the GM +e following conclusion camefrom prediction models with a blackboard tem-perature of 70degCplusmn 2degC irradiation of 8 h nonirra-diated condensation of 4 h relative humidity of70plusmn 2 and ultraviolet irradiance of 600Wm2the lower-intensity BG1 fails at approximately1100 h +e higher-strength BG2 fails at approxi-mately 1800 h

(4) Geogrid reinforcement significantly changes thebearing characteristics of a sand cushion by in-creasing the bearing capacity of the reinforcedcushion+e reinforcement effect of the geogrid withhigher tensile strength is more significant Agingchanges the interface characteristics between thegeogrid and soil In this test the bearing properties ofthe geogrid cushion after aging have certain changes+e aging behavior of the two geogrids reduces theload bearing capacity of the reinforced cushion by20 and 1875 respectively

Data Availability

No data were used to support this study

Conflicts of Interest

+e authors declare that they have no conflicts of interestrelated to this work

Acknowledgments

+e authors would like to acknowledge the National NaturalScience Foundation of China for the financial support forthis study (no 51578359)

References

[1] Y-L Dong J Han and X-H Bai ldquoNumerical analysis oftensile behavior of geogrids with rectangular and triangularaperturesrdquo Geotextiles and Geomembranes vol 29 no 2pp 83ndash91 2011

[2] A M R Ewais R K Rowe and J Scheirs ldquoDegradationbehaviour of HDPE geomembranes with high and low initialhigh-pressure oxidative induction timerdquo Geotextiles andGeomembranes vol 42 no 2 pp 111ndash126 2014

[3] Y G Hsuan and R M Koerner ldquoAntioxidant depletionlifetime in high density polyethylene geomembranesrdquo Journalof Geotechnical and Geoenvironmental Engineering vol 124no 6 pp 532ndash541 1998

[4] W-K Wong and Y G Hsuan ldquoInteraction of antioxidantswith carbon black in polyethylene using oxidative inductiontime methodsrdquo Geotextiles and Geomembranes vol 42 no 6pp 641ndash647 2014

[5] R K Rowe and H P Sangam ldquoDurability of HDPE geo-membranesrdquo Geotextiles and Geomembranes vol 20 no 2pp 77ndash95 2002

[6] H P Sangam and R K Rowe ldquoEffects of exposure conditionson the depletion of antioxidants from high-density polyeth-ylene (HDPE) geomembranesrdquo Canadian GeotechnicalJournal vol 39 no 6 pp 1221ndash1230 2002

[7] W Muller and I Jacob ldquoOxidative resistance of high densitypolyethylene geomembranesrdquo Polymer Degradation andStability vol 79 pp 161ndash172 2003

[8] S B Gulec T B Edil and C H Benson ldquoEffect of acidic minedrainage on the polymer properties of an HDPE geo-membranerdquo Geosynthetics International vol 11 no 2pp 60ndash72 2004

[9] R K Rowe and S Rimal ldquoDepletion of antioxidants from aHDPE geomembrane in a composite linerrdquo Journal of Geo-technical and Geoenvironmental Engineering vol 134 no 1pp 68ndash78 2008

[10] R K Rowe S Rimal and H P Sangam ldquoAgeing of HDPEgeomembrane exposed to air water and leachate at differenttemperaturesrdquo Geotextiles and Geomembranes vol 27pp 131ndash151 2009

[11] R K RoweM Z Islam and Y G Hsuan ldquoEffects of thicknesson the aging of HDPE geomembranesrdquo Journal of Geotech-nical and Geoenvironmental Engineering vol 136 no 2pp 299ndash309 2010

[12] F B Abdelaal and R K Rowe ldquoEffect of high temperatures onantioxidant depletion from different HDPE geomembranesrdquoGeotextiles and Geomembranes vol 42 no 4 pp 284ndash3012014

[13] X-H Bai X-Z Huang and W Zhang ldquoBearing capacity ofsquare footing supported by a geobelt-reinforced crushedstone cushion on soft soilrdquo Geotextiles and Geomembranesvol 38 pp 37ndash42 2013

12 Advances in Civil Engineering

gradually appears For BG1 the aging load has an initialvalue of 125 kPa Similar to BG1 the initial aging load ofBG2 bars is 175 kPa which is much larger than that of BG1bars (Figure 11(b)) +e aging effect of BG2 with highertensile strength on the initial load is greater than that of theBG1 geogrid with lower tensile strength It indicates that theaging effect of BG2 with higher tensile strength on thebearing capacity of the reinforced cushion foundation issmaller than that of the BG1 geogrid with lower tensilestrength

After the model test it was found that the defor-mation of biaxial geogrid was significant (see Figure 12)+e deformed geogrid was measured and the centralpoint of the biaxial geogrid shall be concave about17 mm

+e beneficial effect of reinforcement for increasing thebearing capacity is conveniently described by the bearingcapacity ratio (BCR) according to [20]

BCR qR

q0 (7)

In order to compare the strengths of geogrids underdifferent load levels the ultimate bearing capacity and ul-timate bearing capacity ratio are listed in Table 6 It can beseen that the bearing capacity of the geogrid-reinforcedcushion decreases with aging For BG1 the bearing capacityof the foundation decreases by 1333 and 20 respectivelyafter 500 h and 600 h of aging For BG2 aged 500 h and 600 hthe bearing capacity of the foundation decreases by 625and 1875 respectively

+e BCR of the geogrid-reinforced sand is listed inTable 6 It can be seen that the aging effect of the geogrid

leads to the decline of the reinforced foundation BCR whichreduces the bearing capacity of the geogrid-reinforcedfoundation For different types of geogrids the degree ofdecline in BCR owing to aging is different and the value ofdecline in BCR owing to aging is slightly smaller for thehigher-strength BG2 than for the lower-strength BG1

5 MechanismAnalysis of PhotooxygenAging inPP Biaxial Geogrid

+e energy of the ultraviolet light accelerates the agingprocess of the grid Photooxidation causes degradation andaging of the PP germanium geogrid decomposes the PPpolymer and destroys its internal structure [21]+e tensilestrength and fracture elongation of two kinds of PP biaxialgeogrids were reduced in different degrees after differentaging modes temperature aging time and aging condi-tions After thermal aging there is a decrease in cross-linking degree and in binding and friction betweenmolecular chains +e effects of photooxygen aging aregreater than the effects of thermal oxygen aging +esunlight ultraviolet light provides the energy required forchemical crosslinking of the PP biaxial geogrid +e tensilestrength and fracture elongation of the geogrid temporarilyincrease in a short period of time (100ndash200 h) with theextension of aging time

6 Conclusion

In this study thermal oxygen test is carried out at 60degC 70degCand 80degC and ultraviolet photooxygen aging tests are carriedout at 70degC for two kinds of PP biaxial geogrids of different

(a) (b)

Figure 12 Geogrid-reinforced foundation damage

Table 6 BCR of geogrid-reinforced sand

Group Aging time Ultimate bearing capacity (kPa) BCR Percentage reduction in strength due to agingUnreinforced mdash 250 1 mdashBG1 0 375 15 mdashBG1 500 325 13 1333BG1 600 300 12 20BG2 0 400 16 mdashBG2 500 375 15 625BG2 600 325 13 1875

Advances in Civil Engineering 11

specifications +e influence of aging factors on the bearingcapacity of geogrid-reinforced foundations is considered

After investigating the effects of thermal oxygen andphotooxygen aging tests on tensile strength and fractureelongation of geogrids the following conclusions are drawn

(1) +e tensile strength of the geogrid decreases withthe increase in aging time and there is an increasein properties in aging period up to 100ndash200 hours+e comparison with the same thermal oxygenaging time shows that the tensile strength of thegeogrid decreases with the increase in thermaloxygen aging temperature With the increase inphotooxygen aging time the tensile strength of thegeogrid showed a significant trend of decrease +eeffect of ultraviolet light on the geogrid tensilestrength during aging is much greater than that oftemperature +e effect of photooxygen aging ontensile strength is greater than that of thermaloxygen aging for two different types of PP biaxialgeogrid Under the same temperature and agingtime the tensile strength of geogrid is reduced by nomore than 5 while the strength of geogrid isreduced by about 35

(2) +e retention rate of fracture elongation decreaseswith the increase in thermal oxygen aging tempera-ture and aging time+e elongation at break decreaseswith the photooxygen aging time and the influence ofultraviolet light on geogrid fracture elongation ismuch greater than that of temperature +e tensilestrength retention rate of different types of PP biaxialgrids showed different photooxygen aging charac-teristics+e aging resistance performance of BG2wassignificantly higher than that of BG1

(3) +e effect against tensile strength retention rate isbetter with the GM +e following conclusion camefrom prediction models with a blackboard tem-perature of 70degCplusmn 2degC irradiation of 8 h nonirra-diated condensation of 4 h relative humidity of70plusmn 2 and ultraviolet irradiance of 600Wm2the lower-intensity BG1 fails at approximately1100 h +e higher-strength BG2 fails at approxi-mately 1800 h

(4) Geogrid reinforcement significantly changes thebearing characteristics of a sand cushion by in-creasing the bearing capacity of the reinforcedcushion+e reinforcement effect of the geogrid withhigher tensile strength is more significant Agingchanges the interface characteristics between thegeogrid and soil In this test the bearing properties ofthe geogrid cushion after aging have certain changes+e aging behavior of the two geogrids reduces theload bearing capacity of the reinforced cushion by20 and 1875 respectively

Data Availability

No data were used to support this study

Conflicts of Interest

+e authors declare that they have no conflicts of interestrelated to this work

Acknowledgments

+e authors would like to acknowledge the National NaturalScience Foundation of China for the financial support forthis study (no 51578359)

References

[1] Y-L Dong J Han and X-H Bai ldquoNumerical analysis oftensile behavior of geogrids with rectangular and triangularaperturesrdquo Geotextiles and Geomembranes vol 29 no 2pp 83ndash91 2011

[2] A M R Ewais R K Rowe and J Scheirs ldquoDegradationbehaviour of HDPE geomembranes with high and low initialhigh-pressure oxidative induction timerdquo Geotextiles andGeomembranes vol 42 no 2 pp 111ndash126 2014

[3] Y G Hsuan and R M Koerner ldquoAntioxidant depletionlifetime in high density polyethylene geomembranesrdquo Journalof Geotechnical and Geoenvironmental Engineering vol 124no 6 pp 532ndash541 1998

[4] W-K Wong and Y G Hsuan ldquoInteraction of antioxidantswith carbon black in polyethylene using oxidative inductiontime methodsrdquo Geotextiles and Geomembranes vol 42 no 6pp 641ndash647 2014

[5] R K Rowe and H P Sangam ldquoDurability of HDPE geo-membranesrdquo Geotextiles and Geomembranes vol 20 no 2pp 77ndash95 2002

[6] H P Sangam and R K Rowe ldquoEffects of exposure conditionson the depletion of antioxidants from high-density polyeth-ylene (HDPE) geomembranesrdquo Canadian GeotechnicalJournal vol 39 no 6 pp 1221ndash1230 2002

[7] W Muller and I Jacob ldquoOxidative resistance of high densitypolyethylene geomembranesrdquo Polymer Degradation andStability vol 79 pp 161ndash172 2003

[8] S B Gulec T B Edil and C H Benson ldquoEffect of acidic minedrainage on the polymer properties of an HDPE geo-membranerdquo Geosynthetics International vol 11 no 2pp 60ndash72 2004

[9] R K Rowe and S Rimal ldquoDepletion of antioxidants from aHDPE geomembrane in a composite linerrdquo Journal of Geo-technical and Geoenvironmental Engineering vol 134 no 1pp 68ndash78 2008

[10] R K Rowe S Rimal and H P Sangam ldquoAgeing of HDPEgeomembrane exposed to air water and leachate at differenttemperaturesrdquo Geotextiles and Geomembranes vol 27pp 131ndash151 2009

[11] R K RoweM Z Islam and Y G Hsuan ldquoEffects of thicknesson the aging of HDPE geomembranesrdquo Journal of Geotech-nical and Geoenvironmental Engineering vol 136 no 2pp 299ndash309 2010

[12] F B Abdelaal and R K Rowe ldquoEffect of high temperatures onantioxidant depletion from different HDPE geomembranesrdquoGeotextiles and Geomembranes vol 42 no 4 pp 284ndash3012014

[13] X-H Bai X-Z Huang and W Zhang ldquoBearing capacity ofsquare footing supported by a geobelt-reinforced crushedstone cushion on soft soilrdquo Geotextiles and Geomembranesvol 38 pp 37ndash42 2013

12 Advances in Civil Engineering

specifications +e influence of aging factors on the bearingcapacity of geogrid-reinforced foundations is considered

After investigating the effects of thermal oxygen andphotooxygen aging tests on tensile strength and fractureelongation of geogrids the following conclusions are drawn

(1) +e tensile strength of the geogrid decreases withthe increase in aging time and there is an increasein properties in aging period up to 100ndash200 hours+e comparison with the same thermal oxygenaging time shows that the tensile strength of thegeogrid decreases with the increase in thermaloxygen aging temperature With the increase inphotooxygen aging time the tensile strength of thegeogrid showed a significant trend of decrease +eeffect of ultraviolet light on the geogrid tensilestrength during aging is much greater than that oftemperature +e effect of photooxygen aging ontensile strength is greater than that of thermaloxygen aging for two different types of PP biaxialgeogrid Under the same temperature and agingtime the tensile strength of geogrid is reduced by nomore than 5 while the strength of geogrid isreduced by about 35

(2) +e retention rate of fracture elongation decreaseswith the increase in thermal oxygen aging tempera-ture and aging time+e elongation at break decreaseswith the photooxygen aging time and the influence ofultraviolet light on geogrid fracture elongation ismuch greater than that of temperature +e tensilestrength retention rate of different types of PP biaxialgrids showed different photooxygen aging charac-teristics+e aging resistance performance of BG2wassignificantly higher than that of BG1

(3) +e effect against tensile strength retention rate isbetter with the GM +e following conclusion camefrom prediction models with a blackboard tem-perature of 70degCplusmn 2degC irradiation of 8 h nonirra-diated condensation of 4 h relative humidity of70plusmn 2 and ultraviolet irradiance of 600Wm2the lower-intensity BG1 fails at approximately1100 h +e higher-strength BG2 fails at approxi-mately 1800 h

(4) Geogrid reinforcement significantly changes thebearing characteristics of a sand cushion by in-creasing the bearing capacity of the reinforcedcushion+e reinforcement effect of the geogrid withhigher tensile strength is more significant Agingchanges the interface characteristics between thegeogrid and soil In this test the bearing properties ofthe geogrid cushion after aging have certain changes+e aging behavior of the two geogrids reduces theload bearing capacity of the reinforced cushion by20 and 1875 respectively

Data Availability

No data were used to support this study

Conflicts of Interest

+e authors declare that they have no conflicts of interestrelated to this work

Acknowledgments

+e authors would like to acknowledge the National NaturalScience Foundation of China for the financial support forthis study (no 51578359)

References

[1] Y-L Dong J Han and X-H Bai ldquoNumerical analysis oftensile behavior of geogrids with rectangular and triangularaperturesrdquo Geotextiles and Geomembranes vol 29 no 2pp 83ndash91 2011

[2] A M R Ewais R K Rowe and J Scheirs ldquoDegradationbehaviour of HDPE geomembranes with high and low initialhigh-pressure oxidative induction timerdquo Geotextiles andGeomembranes vol 42 no 2 pp 111ndash126 2014

[3] Y G Hsuan and R M Koerner ldquoAntioxidant depletionlifetime in high density polyethylene geomembranesrdquo Journalof Geotechnical and Geoenvironmental Engineering vol 124no 6 pp 532ndash541 1998

[4] W-K Wong and Y G Hsuan ldquoInteraction of antioxidantswith carbon black in polyethylene using oxidative inductiontime methodsrdquo Geotextiles and Geomembranes vol 42 no 6pp 641ndash647 2014

[5] R K Rowe and H P Sangam ldquoDurability of HDPE geo-membranesrdquo Geotextiles and Geomembranes vol 20 no 2pp 77ndash95 2002

[6] H P Sangam and R K Rowe ldquoEffects of exposure conditionson the depletion of antioxidants from high-density polyeth-ylene (HDPE) geomembranesrdquo Canadian GeotechnicalJournal vol 39 no 6 pp 1221ndash1230 2002

[7] W Muller and I Jacob ldquoOxidative resistance of high densitypolyethylene geomembranesrdquo Polymer Degradation andStability vol 79 pp 161ndash172 2003

[8] S B Gulec T B Edil and C H Benson ldquoEffect of acidic minedrainage on the polymer properties of an HDPE geo-membranerdquo Geosynthetics International vol 11 no 2pp 60ndash72 2004

[9] R K Rowe and S Rimal ldquoDepletion of antioxidants from aHDPE geomembrane in a composite linerrdquo Journal of Geo-technical and Geoenvironmental Engineering vol 134 no 1pp 68ndash78 2008

[10] R K Rowe S Rimal and H P Sangam ldquoAgeing of HDPEgeomembrane exposed to air water and leachate at differenttemperaturesrdquo Geotextiles and Geomembranes vol 27pp 131ndash151 2009

[11] R K RoweM Z Islam and Y G Hsuan ldquoEffects of thicknesson the aging of HDPE geomembranesrdquo Journal of Geotech-nical and Geoenvironmental Engineering vol 136 no 2pp 299ndash309 2010

[12] F B Abdelaal and R K Rowe ldquoEffect of high temperatures onantioxidant depletion from different HDPE geomembranesrdquoGeotextiles and Geomembranes vol 42 no 4 pp 284ndash3012014

[13] X-H Bai X-Z Huang and W Zhang ldquoBearing capacity ofsquare footing supported by a geobelt-reinforced crushedstone cushion on soft soilrdquo Geotextiles and Geomembranesvol 38 pp 37ndash42 2013

12 Advances in Civil Engineering

[14] S Jahandari J Li M Saberian and M ShahsavarigougharildquoExperimental study of the effects of geogrids on elasticitymodulus brittleness strength and stress-strain behavior oflime stabilized kaolinitic clayrdquo GeoResJ vol 13 pp 49ndash582017

[15] U Rajesh S Sajja and V K Chakravarthi ldquoStudies on en-gineering performance of geogrid reinforced soft subgraderdquoTransportation Research Procedia vol 17 pp 164ndash173 2016

[16] M Singh A Trivedi and S K Shukla ldquoFuzzy-basedmodel forpredicting strength of geogrid-reinforced subgrade soil withoptimal depth of geogrid reinforcementrdquo Journal of Trans-portation Infrastructure Geotechnology vol 33 p 1 2020

[17] Chinese Ministry of Housing and Urban-Rural DevelopmentldquoKey points for shallow plate load testingrdquo Code for Design ofBuilding Foundation GB50007-2012 p 87 Chinese Ministryof Housing and Urban-Rural Development Beijing China2012

[18] ASTM G154-16 Standard Practice for Operating FluorescentUltraviolet (UV) Lamp Apparatus for Exposure of NonmetallicMaterials American Society for Testing Materials WestConshohocken PA USA 2016

[19] J L Deng ldquoControl problems of grey systemsrdquo Systems ampControl Letters vol 1 no 5 pp 288ndash294 1982

[20] J Binquet and K L Lee ldquoBearing capacity analysis onreinforced earth slabsrdquo Journal of Geotechnical EngineeringDivision vol 101 pp 1257ndash1276 1975

[21] K Grabmayer G M Wallner S Beiszligmann et al ldquoCharac-terization of the aging behavior of polyethylene by photo-luminescence spectroscopyrdquo Polymer Degradation andStability vol 107 pp 28ndash36 2014

Advances in Civil Engineering 13