GEOGRID – SAND INTERFACE FRICTIONAL PROPERTIES IN DIRECT SHEAR MODE

1. Research Scholar,Govt: College of Engineering, Kannur, Kerala, India. 2. Prof. in Civil Engineering, Dept of Civil Engineering,Govt: College of Engineering, Kannur, Kerala, India. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History Received: 05 March 2020 Final Accepted: 07 April 2020 Published: May 2020


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The influence of passive resistance in a geogrid-soil interface under direct shear mode is a controversy. It is observed in a study by Lopez [1] that the contribution of passive shear resistance offered is not at all significant under direct shear mode where as in a study by Bergardo et al. [5] geogrid-geometerial properties impart much contribution to passive resistance in a direct mode shear test using a HDPE geogrid -soil interface.
The shear strength parameters, such as interface friction angle and adhesion, for unreinforced, reinforced with soft geogrid, and reinforced with stiff geogrid were studied by Seo, M [6]. In the case of unreinforced, as the maximum particle diameter increases, the internal friction angle also increases but the internal friction angle in the case of geogrid reinforced soil turned out to be lesser than that in the case of unreinforced soil. The tendency of decrease in the interface friction angle due to reinforcing with geogrid is similar to the results in his previous research. Theinfluence of soil particle size on soil-geosynthetic interaction is important, but its significance depends on several factors. With geogrids, it is the relative sizes of soil particles and geogrid apertures, and the thickness of the geogrid bearing members and soil-geogrid interface shear resistance. Tests on geogrids in which the bearing members had been cut, show a significant decrease in soil-geogrid interface shear resistance.
The correlation between certain physical properties of granular material such as the friction angle and the grain size distribution were studied by EsmaMostefa Kara [7]. Even though the contribution of transverse ribs to the soilgeogrids interaction under pull-out mode has been documented, the contribution of transverse ribs to the soilgeogrids interaction under the direct shear mode was not so clear. However, studies were conducted and found that transverse ribs of the geogrid provide approximately 10% of the interface shear resistance Chia-Nan Liu [8].It was observed by Chia-Nan Liu [8], that much attention was given to overall interface shear strength on soilgeogridstudies,but less emphasis on the credentials of different mechanisms causative to the interface shear resistance.
As the geogrid consists of longitudinal and transverse ribs with opening in the aperture area, the following mechanism is working during a shear test at the interface between soil and geogrid:(1) internal soil to soil resistance at the openings in the aperture area; (2) shear resistance between soil and surface of the geogrid ribs;(3) passive resistance offered by the transevers ribs. As schematic illustration of the mechanism is shown in Fig.1 by Wrigley [9], the soil reinforcement interaction is controlled by friction between the soil and the reinforcement, the friction between soil and soil, and the bearing resistance of the soil on the transverse member of the geogrid.
The Fig.1 shows the mechanism of interlock in gepgrid with soil samples.The soil particles having lesser size than the aperture is locked in the openonings of the geogrid and touching the ribs .Both longitudinal ribs (LR) and transversal ribs (TR) take role to contribute to the shear strength of geogrids. Fig. 2 is showing a typical view of geogrid with longitudinal and transverse ribs with opening of the aperture.The direction of shear force and the that of shear resistant direction along the ribs also marked.The total area of geogrid shall be calculated with the dimensions of it to calculate shear resistance by the geogrid soil interface.

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The authors [8,10] observed that at smaller displacements the shear resistant components are fully mobilized and the shear resistant by the way of bearing resistance is involved at larger displacements. It can be seen from the results of a direct shear test, Fig. 3 thatthe quantity of shear resistance component developed due to friction between soil to soil is greater than the resistance offered from soil to geogrid interfaces.
Jewell et al. [11] conducted the first theoretical study on soil geosynthetic interaction at direct shear mode.He suggested the basic equation to calculate the shear strength in a sand geogrid interface mobilized under direct shear mode as follows.  α ds =percentage open area of geogrid,, δ= interface friction angle between soil and geogrid,, ϕ'ds=  internal friction angle of sand from direct shear test Therefore, the total direct shear force can be given as: F s+g = F ss + F sg Where F s+g = Total direct shear resistance force F ss = soil to soil direct shear frictional force F sg = soil to geogrid direct shear frictional force In this study, a series of large scale direct shear tests was conducted with a same geogrid but with four samples of sand soil at different densities at different normal loads. The purpose of this paper is to investigate therole of interfacefrictional resistance offered by soil-soil, soil-geogrid and soil-transverse ribs interfaces to the total interface shear resistance by carrying out a series of shear tests with same geogrid and different soil samples .

Test Apparatus:
The largescale direct shear test apparatus used for this study of shear strength on sand -geogridinterfaceis a shear box of dimensions 300 x 300 x 200 mm shown in Figure .4. The photograph of the equipment is shown in Figure .5. Theshearbox is made of MS plates having a thickness of 4mm.The whole set up is mounted on a MS testing bench with facilities to apply vertical and horizontal loads. Also, the horizontal force applied ismeasured through a 2 KN proving ring. Thehorizontal displacement is measured using a dial gauge of 0.01 mm sensitivity.  176

Materials Used:
The soil samples used for this study were a mix of 12 mm aggregate, 6mm aggregate and Sand. The three different samples were mixed in different proportions and each case the sieve analysis was carried out. Totalsuch four sand samples were prepared for the experiment study. The details of soil used are presented in Table 1 and Table 2. The properties of geogrid used are given in Table 3.The shear area is 0.09 sq.m and the total area of soil to soil contact is 0.061965sq.m and the soil to geogrid contact area is 0.028035 sq.m.   The soil samples were prepared for each mix ratios. The samples containing 12 mm aggregate, 6mm aggregate andSandwerewell mixed and the sieve analysis wascarried out to determine the gradation. For testing the soil samples alone, the lower half box is placed on the platform and the plate at its bottom base. Then the upper half box is placed over the bottom box and the two screws are inserted in the holes to arrest the movements of the two boxes. The soil sample is filled in the box lower and upper box in equal layers of 5 cm and each layer was compacted using a 1 Kg weight rod by tamping the surface. The weight of the soil is measured. The rigid bearing plate is horizontally placed on the surface of the soil and the sheartest was conducted at normal loads of 50,100 and 150 N. The horizontal load was applied by rotating the wheel which is exerting load on the upper half box at a speed of 10mm per minute. The vertical load is applied through the vertical rod touching the top plate placed over the top of upper box.The horizontal loading was stopped when the peak shear stress is obtained or till the shear strain reaches 5%.The horizontal load is read from the proving ring and the shear strain from the dial gauge at each 5 division on the PR .After completion of the test soil is fully taken out from the box and is weighed.
The shear test with geogrid is carried out the same way, but first the bottom half box is filled with soil in layers with proper compaction in layers of 5cm.Then the geogrid of size 290 x 290 mm is placed over the top of the soil without touching the sides of the box.Then the upper half box is placed oer the lower one and the screws are tightening to avoid movement of the boxes. The geogrid was very properly placed over the soil top surface and the upper portion is started filling as done before.Then the test is carried out as before and the weight measured to calculate the density of soil sample. All the soil samples are tested without the geogrid and with geogrid at four densities of each soil samples. The details are given Table.4 The interface shear stress τ p of the modified shear test can be defined as Where A p is the area of geogrid buried in the soil sample and T p is the horizontal force applied [10] The sand -Geogrid interface under direct shear mode may include following mechanisms. (1) Soil and ribs of geogridshear resistance (2)  Where σn is the normal stress, ρ is the per cent of open area of geogrid. ɸsg is the interface friction angle between sand and the geogrid and ɸss the soil angle of internal friction.
Soil Geogrid Interface Modeling: The large scale direct shear test was carried out with four nos sand sample, each at four different densities without the geogrid.The shear strength of the all 16 nos samples were obtained from the test at normalloads of 50, 100 and 150N. The normal stress against the shear stress was plotted and the angle of internal friction between soil to soil wasobtained. This data was used for determining the soil to soil shear strength in the contact area of aperture of the geogrid shear test with geogrid.
The interface frictional force between soil to soil Fss in soil-soil contact areas can be described in equation below. Fss = Ass . Ꞇss = Ass .σn.tanυss……………………………… (2) where: Ass = total shear area of the box Ꞇss=soil shear strength τ σn= normal stress applied on the soil sample υss = soil interface frictiona angle The tests were repeated with same nos of samples with geogrid in position. The force measured during the shear test without geogrid was the only the shear force experienced at interface of the soil to soil.When the test is done with geogrid, the shear force is experienced at soil to soil interface in the opening of the geogrid,soil to geogrid surface and the passive resistance offered by the transvers ribs of the geogrid. The normal stress against the shear stress was plotted and the combined angle of internal friction υs+gbetween soil to soil and soil to geogrid with transverse ribs 178 was obtainedIn this test, the total area A is 900sq.cm, area of aperture is 68.85% and that of geogrid surface is 31.15% of the total shear area.  7) On plotting the applied normal stresses against the interface peak shear stresses Ꞇsg of the soil gegridinterfaces, the interface friction angle Φsg will be obtained .Using this, the shear strength resistance offered by soil-geogrid surface can explained using Eqn(5). The υs+gυsg and υssobtained are given in Table.5 below and Table2.

Result and Discussion:-
The F s+g is the combined interface friction force obtained from the shear test with geogrid,F ss is the interface friction force of the opening area of the geogrid calculated using Equation (2) with the angle of internal friction obtained from the shear test carried out without geogrid and F sg is the interface friction force soil to grid contact area including passive resistance of ribs,,obtained using Equation (5) .Here the interface shear strength of sand-geogrid is taken from the graph drawn between the normal stresses against the F sg + F pr .i.e.F s+g -F ss .Using Φ sg obtained from the failure envelope , Equations (3) and (5), were used to predict Fsg.Here only the shear resistance of between the soil to geogrid surface is considered.The passive resistance is obtained by deducting Fsgfromcalculated from the F sg + F pr .The details are given in the tabled below for the 16 nos soil samples. It is reported Chia-Nan Liu [  It is seen that in most of the casesthe magnitude of predicted shear strength is smaller than the measured value by shear test. Using a geosynthetictexile for a shear test with sand, the shear strength at interface between soil and geotextile υ sg is used for predicting Fsg ,it is observed that peak shear strength of the interface between geotextile and sand developes at smaller displacements than the that of sand-geogridinterface.The test results and calculated data are given Table. 6 for soil samples S1 and S2.

Variation of Passive Resistance With Densities:
The variation of Passive resistance at different densities and normal loads were studied from the results of the test. The test conducted using four soil samples each at four different densities with a same geogrid at normal loads of 50N,100N and 150N.The contribution of passive resistance offered by ribs of the geogrid under the test conditions is approximately 17% on an average of the total combined shear resistance obtained from shear test with geogrid at different densities. The contribution of soil geogrid interface shear resistance is seen 21 % on an average under the test conditions. It can be seen from the Fig.5 to Fig.8, for all soil samples of S1,S2,S3 and S4,the passive resistance is seen reduced with increase of density of soil for the same normal stress. The passive resistance is reducing with increase of normal stress for soils with same densities in all the cases of soil samples tested. The frictional resistance at soil to soil interface increases with increases of normal load at same density and is decreasing with decrease of density at same normal load. The interface resistance offered by soil geogrid surface is seen increasing with increase of normal load for the same densities and is decreasing with increase of densities at same normal loads.Chia-Nan Liu [8] has reported that the passive resistance contribution is more significant at low normal stress levels. The authors [4] have observed in their studies that the passive resistance contribution was very diminutive for geogrids without the transverse ribs subjected to shear test.
When the geogrid is placed in position in the test box, it is surrounded by soil subjected to the shear resistance force, buckling like a small degree bending would occur to the ribs at the same direction of the applied shearload. So the applied load is taken care by the transverse ribs, the soil-geogrid contact surfaces and soil to soil contact area in the open area of the geogrid.When the applied load is increased, the frictional force shares outs non -uniformly with maximum load at the starting point of the load application and transmits progressively to the other free end. At this stage ,the relative displacement of soil and geogrid is taken place and the soil-geogrid static friction changes to dynamic condition ,rearrangement of soil particles happens leading to the interlocking of soil particles with geogrid at its surfaces and on the transverse ribs.When the normal overlaying load is comparatively diminutive, the soil particles can easily reorganise and adjust by movement and interlocking occurs , and finally ,the interface friction is reached to a greatest level .When the applied is load is increased further, only static friction between soil and geogrid is demonstrated dominantly.
When load is applied on the upper half box of the shear box, the ribs will be exerting force on the soil particles in the direction of force in contact with it.As the force is increasing gradually, the geogrid ribs will be yielding to touch the particles in front of it and it buckles, the soil particles behind ribs will be moved towards the ribs touching the ribs andexerting load in it.So ribs are exertingpassive load on the soil particles in front of it and soil particles behind the ribs are in contact with the ribs. The soil particles are mobilised at the low vertical loads and interlocking of particles occur around the ribs also. The soil particles will be in contact with the ribs at both front and back side of it in the direction of the applied force such that the movement of ribs are arrested further,under the constant vertical load.As the soil particles are in a state of interlocked between themselves and with the geogridsurface and when the vertical load is increased further gradually, the particles on the soil-soil interface and soil-geogrid interface are subjected further rearrangements and interlocks ,the friction load bearing by these interfaces are increased ,but the ribs are in a state of loaded condition by the passive forces by the soil particles in contact with ribs from both sides under the constant vertical load .So additional force is taken by soil -soil and soil -geogrid interfaces and hence the load taken by the ribs are reduced, at a constant normal force and density of soil.
When the normal load is increased at the constant same density, the particles will have more contact area with the themselves and geogrid.Therearrangementtendency of soil particles will be reduced because of the high vertical load 181 as compared to a low vertical load.The active and passive loadings on the ribs will be reduced because of it as compared to a low vertical load.This tendency will be further reduced as the vertical loads are increased further. So the passive resistance force taken care by ribs will be reduced asthe vertical loads are increased where as friction resistance will be more at interfaces between soil to soil and soil geogrid areas. Similarly, when the density of the soil is increased, the more contact area between particles and the geogrid will be available and the frictional resistance will also be increased. Variation of Passive resistance with particle size: From the Table.6 and Table 7, itcan be seen that soil samples S1D2, S2D3,S3D3 and S4D2 have almost the same densities. Table.1 gives the details of particles size distribution of all the four soils taken from the sieve analysis report of the samples.
The variation of D 10 ,D 30 ,D 50 , and D 60 with % passive resistance contribution of soil samples S1D2, S2D3, S3D3 and S4D2 fromTable.6 and Table 7 is analysedat normal loads of 50N,100N and 150N at same density soils. Fig.9 to Fig.11 shows the variation of passive resistance with respect the particles sizes of D 10 ,D 30 ,D 50 , andD 60 at same density of four soil samples tested at constant normal stress. The figures show that at 50N normal load, when the D 30 ,D 50 , andD 60 are increasing the passive resistance is alsoincreasing, but increase of passive resistance with increase of D 50 is less compared to D 30 and D 60 .Similar the case when the normal load is 100N and 150N.Also, the passive resistance is increasing without much increase of D 10 particles. In all soil samples, the presence of D 50 and D 60 soil particles have big range of sizes than D 10 and D 30 varying from size 6mm to 2.1mm for D 50 and 4mm to 7.9mm for D 60 .The presence ofhigher size particles in the soil samples imparts the increase of the passive resistance. The thickness of transverse rib is only 1mm and the particles having size greater than 1mm would be more effective to interlock the ribs with soil particles than particles having size less than 1mm. D50 particles have less sizes than