Critical State Interface Shear Strength Between Soil and Geogrids at Varying Degrees of Saturation

This study investigates the critical state interface shear strength between soil and geogrid reinforcement under varying moisture conditions, using large-scale direct shear and pull-out tests. The experimental program examines soil specimens at different water contents to evaluate how moisture influences soil behaviour and its interaction with geogrids. Emphasis is placed on the critical state shear strength at the interface, with analyses conducted using the effective stress framework. The results show that, for soils with low to moderate plasticity, if suction effects are appropriately accounted for through effective stress calculations, the critical state interface shear strength at each moisture level can be uniquely correlated with the corresponding critical state strength of the soil. This correlation is governed by the ratio of the geogrid's apparent opening size to the soil's average particle diameter. Based on these findings, a simple predictive model is proposed to estimate the critical-state soil–geogrid interface strength from the soil’s critical-state strength. The model is validated with pull-out test results from both this study and the literature. The proposed framework provides a practical and efficient tool for predicting soil–geogrid interface strength across a range of moisture conditions, hence reducing the need for extensive interface testing. With further development and validation with broader datasets encompassing a wider range of test conditions, the model may ultimately eliminate the need for interface testing in certain applications.