EXPERIMENTAL STUDY ON PULLOUT BEHAVIOR OF EMBEDDED GEOGRID IN UNIFORM SANDY SOIL UNDER STATIC, CYCLIC, AND POST-CYCLIC LOADS

Geosynthetics are mainly used to stabilize and reinforce different types of earth structures such as slopes, retaining walls, bridge abutments, and foundations. In these cases, the interaction between soil and geosynthetic plays a significant role. In order to investigate the factors affecting the static, cyclic, and post-cyclic pullout behavior of a type of geogrid produced in Iran under the brand name of GPGRID80/30 embedded in uniform sand, an experimental study was carried out using a large-scale pullout apparatus. In order to study the monotonic and post-cyclic pullout behavior of geogrid in different conditions, a series of monotonic pullout tests and multistage pullout tests were performed. Given the effect of vertical effective stress on the pullout resistance, the maximum apparent friction coefficient of the surface of the geogrid and soil and deformation along the geogrid was investigated using monotonic tests. In the multistage pullout test, the influence of vertical effective stress, cyclic load amplitude, frequency, and number of tensile load cycle on the post-cyclic pullout resistance was studied. The results indicated that with an increase in the vertical effective stress, the pullout resistance of the geogrid and the maximum apparent coefficient of friction would increase and decrease, respectively. A comparison of the results of the multistage pullout tests and constant rate pullout tests with the vertical stress of 60 kPa showed that the cyclic loading had no significant effect on the post-cyclic pullout strength compared to the static pullout strength of the embedded geogrid in the sandy soil; however, with vertical effective stresses of 20 and 40kPa, a reduction in the maximum post-cyclic pullout strength was more evident than the pullout strength. Increasing the effective vertical stress and cyclic load amplitude in the second stage of the multi-stage test would enhance the cumulative displacements along the geogrid sample. A comparison between the loading-unloading tensile stiffness at the end of the second stage and tensile stiffness at the beginning of the second stage suggested that the cyclic loading would increase the tensile stiffness and finally, at the third stage of the experimental multistage test, the tensile stiffness would decrease as the displacement increased until it reached the corresponding value in the constant-rate displacement test.