Residual Shear Strength Testing Protocol (Direct Simple Shear + Ring Shear)

Residual shear strength arises from the asymptotic shear resistance attained when soil undergoes extensive shear displacement—typically exceeding 10–30% strain—causing complete disruption of original particle structure and development of a preferred orientation (fabric anisotropy), especially in clay-rich materials such as smectite- or illite-dominated soils. This state eliminates dilatancy and interparticle frictional resistance beyond basal sliding, leaving only the intrinsic friction angle (φ_r) and negligible residual cohesion (c_r ≈ 0). Direct simple shear (DSS) testing applies horizontal shear displacement to a prismatic soil specimen confined between parallel platens, maintaining constant vertical load and enabling measurement of stress–strain–displacement behavior; however, DSS may suffer from non-uniform strain distribution and boundary effects at very large strains. Ring shear (RS) overcomes these limitations by using a toroidal (ring-shaped) specimen sheared along a closed circular path, ensuring uniform shear strain throughout the sample volume, continuous displacement capability (>1 m), and minimal end restraints—making it the preferred method for robust residual strength determination per ASTM D7608 and ISO/TS 21646. Practically, residual strength parameters are indispensable for back-analyzing historical landslides, calibrating runout models (e.g., DAN-W, RAMMS), designing remediation measures for reactivated slides, and assessing long-term stability of engineered slopes, tailings dams, and cut-and-fill embankments where cyclic or creep loading may induce progressive failure.