Shaking table investigation into seismic failure mechanisms of bedrock-underlain stratified slopes: influence of soil layer configuration

Soil layer configuration governs the seismic failure characteristics and dynamic responses of layered slopes. However, the seismic damage evolution and mechanical mechanisms of clay-over-sand and sand-over-clay slopes remain poorly understood. This study integrates shaking table tests and cyclic triaxial tests to investigate how soil layer distribution controls slope failure mode, acceleration amplification, and dynamic stress evolution. The results indicate that: (1) Differences in soil mechanical properties across layers give rise to distinct seismic damage patterns. The upper-clay/lower-sand slope exhibits progressive plastic sliding with significant accumulated deformation, whereas the upper-sand/lower-clay slope undergoes brittle failure characterized by surface tensile cracking and shallow instantaneous sliding. (2) The acceleration amplification factor increases nonlinearly with peak ground acceleration (PGA), peaking no later than the critical state and decreasing sharply once the slope fails, thereby serving as a precursor to seismically-induced slope instability. (3) Soil layer configuration governs the spatial evolution of dynamic stress. In the upper-clay/lower-sand slope, the soft clay layer in the upper-middle section exhibits large stress fluctuations and frequent redistribution, forming the primary damage zone. In the upper-sand/lower-clay slope, the lower clay layer serves as a stable basal boundary, intensifying dynamic stress responses in the middle-lower section; slope stability is governed by basal restraint. (4) Quantitative stress–volumetric strain constitutive models were developed from triaxial tests at varying confining pressures, elucidating the transition from dilative behavior at low pressures to contractive behavior at high pressures and revealing how confining pressure governs soil deformation and failure. This study advances the understanding of seismic instability mechanisms in layered soil slopes and informs seismic reinforcement strategies, hazard prediction, and slope protection engineering.