Laboratory pullout test study on the influence zone of geosynthetics

Conventional pullout test devices are mainly designed to measure the shear strength indices of the geosynthetics–soil interface. This article introduces a new model container to study the influence zone of geosynthetics in pullout test. Several basic image processing methods were employed to analyze the grain motion of two gravel compositions in pullout tests. The trajectories of grains in the gravel differ at different heights; specifically, a more active motion was observed if the grain was closer to the geosynthetics. Moreover, grains were found to move not only horizontally but also vertically as well as to rotate on pulling out the geosynthetics. The range of a grain on the move is almost constant during the pullout and determines the influence zone of the geosynthetics. Image processing revealed that the influence zone was eight times the average grain size above the geosynthetics and 4–5 times the average grain size below the geosynthetics. The boundary condition thus significantly affects the influence zone.