To study the interaction between sand particles and geogrids, pullout tests were performed on one type of geogrid in Pingtan medium sand. The sand motion around the ribs was captured by high-resolution digital camera and CCD camera. The images were analysed with the aid of digital photography measurement and micro-image analysis. Microscopic particle motion and displacement fields around the transverse ribs were then obtained to verify the macro behaviour of the sand–rib interaction. It was found that the thickness of shear band increases with the normal stress applied, and the shear zone is asymmetrical to the soil–rib interface. A near wedge shape shear zone was observed by analysing the shear strain around the ribs, which indicated that a punching shear failure mechanism may be applicable to the tested geogrid in medium-dense sand. Comparison of various models for bearing capacity of geogrids shows that the size and shape of bearing members should be considered in the calculation of bearing resistance of geogrids.
A two-dimensional Particle Flow Code (PFC2D) was applied to simulate sand liquefaction behavior induced by cyclic loading. The numerical sample was prepared with 4061 discs with its particle size distribution similar to Fujian Standard sand. Based on the theory of strain-controlled undrained cyclic triaxial test, a constant volume numerical test was carried out under cyclic loading with uniform strain amplitude. The macroscopic responses of the sample were obtained and the variation of average coordination number under cyclic loading was analyzed. The effects of cyclic strain amplitude and confining pressure on liquefaction resistance were further analyzed in numerical simulations. It was found that the numerical tests reproduced the general characteristics of liquefaction behavior of saturated sand under cyclic loading. The effects of cyclic strain amplitude and confining pressure on the liquefaction resistance of the numerical sample were comparable with reported experimental results.