Investigation on macro-meso cumulative damage mechanism of weak layer under pre-peak cyclic shear loading

To investigate the macro-meso cumulative damage mechanism of the weak layer considering the impacts of various factors, the pre-peak cyclic shear tests and PFC2D meso numerical simulations were conducted. The results indicate that: (1) Three stages, i.e., initial compression-shear nonlinear deformation (elastic zone), nonlinear deformation of the cumulative damage due to the stress climb (elastic-plastic zone) and plastic deformation with constant stress (plastic zone), are observed in the weak layer cyclic shear deformation evolution curve and strength evolution curve. (2) The peak (residual) strength and cumulative shear (normal) deformation of the weak layer decreases and increases respectively as the cyclic shear times increase under the same conditions of moisture content, normal stress, shear rate, shear amplitude or relative thickness; conducted with the same cyclic shear times, the peak (residual) strength decreases, increases, decreases, decreases and increases with the increase of the mentioned factors in turn, while the variations of cumulative shear (normal) deformation present the opposite trends. (3) The evolution curve of the meso cumulative damage crack quantity includes four stages, i.e., a slight-steep, slow, steep and slight increase in the initial, preliminary, middle and later stages, respectively. Meanwhile, the weak layer energy evolution curve includes three stages, i.e., a steep, slow and slight increase in the preliminary, middle and later stages, respectively; besides, the meso-damage particles are distributed on both sides of the shear plane in an approximately "banded S-shape". (4) The macro-meso cumulative damage failure modes of the weak layer can be summarized into three basic types, including the compacting-cracking failure (dilatancy effect), abrading-dislocating-gnawing failure (dilatancy-loosening-shrinkage effect) and penetrating-sliding failure (shrinkage effect).