Shear behaviors of freeze–thaw interface in moraine soil: Experimental and theoretical approaches

In high-altitude cold regions, repeated freeze–thaw cycles induce significant hydrothermal migration within moraine soil slopes, leading to the formation of distinct frozen zones. The interface between frozen and thawed zones is a critical factor triggering slope instability. This study investigates the mechanical behavior of the moraine soil freeze–thaw interface by direct shear tests under varying initial water and fines contents. The shear characteristics and strength degradation patterns are analyzed. Furthermore, a shear strength degradation model for the interface is established based on the Mohr–Coulomb criterion. The results indicate that: (1) Shear stress-strain curves primarily exhibit strain-hardening behavior at low water contents; however, at high water contents, increasing the fines content promotes a transition from strain softening to strain hardening. (2) The shear strength of the interface decreases with increasing water and fines contents. Higher water content significantly reduces both the internal friction angle (by 6.12%–18.50%) and cohesion (by 31.83%–38.88%), whereas increasing fines content enhances cohesion. (3) The degradation mechanisms are attributed to the coupled effects of reduced ice cementation, enhanced lubrication, and alterations in particle interlocking and fragmentation. (4) Key parameters for shear strength are identified, and the developed degradation model for moraine soil is proposed, and its applicability is demonstrated. This study provides a theoretical foundation for understanding the failure mechanisms of moraine soil slopes subjected to freeze–thaw cycles. • Shear deformation patterns under varying water and fines contents were investigated. • The interface shear strength decreased as the water and fines contents increased. • The shear failure mechanisms at the moraine soil freeze-thaw interface were revealed. • A strength degradation model considering water and fines contents was proposed.