Residual shear strength of soils in sliding zone during non-uniform deformation process of landslides

Landslides often exhibit step-like non-uniform deformation under external factors such as rainfall and reservoir water level fluctuation. The soils in sliding zone experience non-uniform shear cycles involving deceleration and acceleration, where their shear strength may act as a variable rather than a constant. Taking the Baijiabao landslide slip zone soil as an example, a series of multi-stage ring shear tests with stepwise acceleration and deceleration rates are conducted. The shear surfaces under varying rate are macroscopically analyzed using a 3D surface topography measurement system to investigate the changes of the residual shear strength under non-uniform shear conditions. The results demonstrate that the residual strength of the soils in sliding zone exhibits a decreasing trend, showing a relative reduction of 3.0% to 24.1% under stepwise acceleration shearing with rates ranging from 0.3 to 9.9 mm/min. Conversely, under stepwise decelerated shearing, the residual strength increases by 2.9% to 34.6%. The accelerated shearing progressively deepens scratch marks on the shear surface, while the decelerated shearing tends to be smooth and reorganize these scratches into more ordered configurations. Furthermore, the comprehensive internal friction angle (φ_e) of the soils in sliding zone displays a logarithmic decay relationship with the shear rate (v), which is quantitatively described by the equation φ_e=-1.56ln(v+0.15)+22.1. These findings systematically elucidate the evolution of residual strength of the soils in sliding zone under non-uniform shear deformation, providing critical insights for refining landslide stability assessments and enhancing early-warning systems.