Evolution characteristics and mechanisms of shear bands in Yellow River Delta silt and their impact on submarine landslide stability

Submarine landslides triggered by shear band development pose a growing threat to marine engineering facilities in the Yellow River Delta (YRD). To clarify the evolution mechanisms of shear bands and their role in submarine landslide instability, a series of consolidated undrained (CU) triaxial tests with continuous full-process visualization of shear band development were conducted on representative YRD silt. Confining pressures of 50 kPa and 400 kPa were adopted to represent shallow and relatively deep submarine stress states, while three shear strain rates (0.04, 0.4, and 4 mm/min) were applied to reflect different deformation timescales from long-term loading to rapid disturbance. The results demonstrate that shear band morphology, thickness, and failure mode are jointly governed by stress state and loading timescale. Under low shear strain rates, deformation is dominated by wide and diffuse shear bands characterized by progressive particle rearrangement, corresponding to a gradual stable change mode. As the shear strain rate increases, shear bands become increasingly localized and thinner, accompanied by enhanced particle damage and excess pore water pressure accumulation, leading to a local gradual loss mode. At high shear strain rates, rapid pore water pressure buildup and intensive particle fragmentation induce an instantaneous brittle loss mode, in which a thin, fully connected shear band forms rapidly and governs instability. Dilatancy and the development of complex shear bands are notably suppressed under high confining pressures. These results offer experimental insights into the transition of YRD submarine landslides from slow deformation to sudden instability.