EVALUATING THE INFLUENCE OF GRAIN SIZE ON SHEAR BAND THICKNESS UNDER PULLOUT CONDITIONS

The behavior of reinforced soil structures is largely governedby interaction mechanisms that develop betweenthe reinforcement inclusions,ll material. Because ofthe variety of inuential factors, the complex interactionmechanisms between soil,reinforcements are still farfrom clear. Dierent experimental methods, includingdirect shear,pullout tests, have been used to identifyand improve the understanding of soil-reinforcementinteraction. In the current research, pullout tests wereused to evaluate soil-geogrid interaction through relativesoil-reinforcement movements during pulling out of thereinforcement. In this regard, large-scale pullout tests(i.e., 100 x 60 x 60 cm) were carried out on samples reinforcedwith an HDPE geogrid, while digital images ofdeformed soil in close vicinity of the geogrid were capturedand image processing was applied using particleimage velocimetry (PIV) methods. Employing GeoPIVprogram, successive pairs of photographs were comparedto determine the incremental values of displacementsand strains at soil-reinforcement contact surfaces duringpullout test. Samples were subjected to normal pressuresof 25, 50,100 kPa. Two sandy,gravelysoils of dierent particle sizes were used in the studyfor the preparation of samples to assess the inuence ofgrain size on soil-geogrid interactions,the relativedisplacements. According to Unied Soil ClassicationSystem (USCS), sandy soils were classied as SP andSW,the gravely soil as GP. Results of the investigationshowed that soil particles in close vicinity oftransvers ribs are displaced in a circular manner, andthe thickness of the shear band around the geogrid increaseswith the increase in grain size. Punching shearfailure mechanism was found in SP soil, while generalshear failure mode was observed in SW,GP soilsduring pulling out of the reinforcement. Also, with theincrease in grain size, asymmetrical shapes of shear bandchange to symmetrical shapes.