Discrete-Element Numerical Investigation of the Dynamic Response Characteristics and Instability Mechanism of a Cross-Jointed Rock Mass Slope under Multistage Sustained Seismic Action

Frequent earthquakes, coupled with discontinuous geological conditions, complicate the earthquake dynamic response characteristics of slopes. To reveal the dynamic accumulative damage effect and instability mechanism of a cross-jointed rock mass slope under continuous earthquakes, two discrete-element models, a homogeneous slope and a cross-jointed slope, are established in this study via particle flow code (PFC2D). The results show that the cross-jointed slope has an obvious dynamic amplifying effect on the elevation and slope surface. Compared with the homogeneous slope, the cross-jointed slope has a more significant slope magnification effect. The seismic wave amplification on cross-jointed slopes is more significant under the influence of S waves than P waves. In addition, on the basis of the characteristics of crack propagation and particle bonding failure, the dynamic cumulative failure effect and evolution process of cross-jointed slopes under cumulative earthquakes are revealed, including the crack initiation stage (≤0.1g), crack accumulation stage (0.1–0.2g), crack propagation stage (0.2–0.4g), and crack penetration stage (0.4–0.6g). Through the analysis of particle bond rupture characteristics and displacement evolution, the mechanisms of dynamic failure and modes of instability for the cross-jointed slope are identified. Under a small earthquake (≤0.2g), the slope remains largely uncracked, and slope failure does not occur. Under a strong earthquake (≥0.4g), bond failure near the slope surface develops rapidly and intensively, and the sliding body gradually experiences instability failure. Moreover, joints control slope dynamic failure behavior. Shear failure and tensile failure mainly occur in cross-jointed slopes and homogeneous slopes, respectively.