Experimental Investigation of Granular Soil and Clay Interfaces with Direct Shear Tests

This study experimentally investigates the shear strength behavior of interfaces formed between granular soils and clay under drained conditions, with particular emphasis on peak-to-residual strength evolution. Large and small-scale direct shear tests were performed on clay, granular soils (sand and gravel), and their interfaces, and shearing was continued to large displacements to reliably capture residual behavior. Unlike most previous studies that focus on soil mixtures, this study explicitly quantifies interface-specific shear strength parameters and highlights their distinct mechanical response. The results show that while interface cohesion remains comparable to that of clay, the interface friction angle is consistently higher. Specifically, under residual conditions, the friction angle of the clay (12.9°) increased to 16.4° for the sand–clay interface and to 19.8° for the gravel–clay interface. These findings demonstrate that adopting clay residual parameters for granular soil–clay interfaces may be overly conservative and that interface-specific residual friction angles should be considered in stability analyses of slopes and earth structures.