Deformation characteristics and failure modes of sandstones under discontinuous multi-level cyclic loads

Rock often suffers from discontinuous fatigue damages in geotechnical engineering, so it is of great significance to study deformation characteristics and failure modes of sandstone under discontinuous multi-level cyclic loading and unloading. By conducting continuous (only dynamic loading) and discontinuous (combined static and dynamic loading) multi-level cyclic loading and unloading tests, this study explored deformation characteristics and failure modes of sandstone. The test results demonstrated that the sandstone mainly suffered from large deformation dominated by dilation in circumferential direction when it was damaged after compression. After multi-level loading and unloading, evolution characteristics of deformation parameters showed characteristics of stepwise growth. However, the introduction of static loading changed such characteristics. In the process of combined static and dynamic loading, due to short action time, dynamic loading mainly promoted nucleation and sudden changes of cracks, while static loading affected crack development process mainly in the early compaction stage and late crack propagation and coalescence stage. Moreover, static loading showed small effects in elastic stage. In the compaction stage, the compacted micro-cracks reopened and cracks were affected by inertial damages in crack propagation stage and interconnected more thoroughly, so that the sandstone was finally more fragmented.