Numerical investigation on the fatigue failure characteristics of water-bearing sandstone under cyclic loading

The strength of sandstone decreases significantly with higher water content attributing to softening effects. This scenario can pose a severe threat to the stability of reservoirs of pumped storage power stations developed from abandoned mines, especially when subjected to the cyclic loading condition caused by the repeated drainage and storage of water (fatigue damage). Based on this, it is essential to focus on the fatigue failure characteristics. In this study, the mineral composition of the used sandstone of Ruineng coal mine in Shanxi Province, China, was first tested to elucidate the rock softening mechanism after absorbing water. Next, a numerical model for replicating the mechanical behavior of water-bearing sandstone was established using two-dimensional particle flow code (PFC2D) with a novel contact model. Then, 16 uniaxial cyclic loading simulations with distinct loading parameters related to reservoir conditions (loading frequency, amplitude level, and maximum stress level) and different water contents were conducted. The numerical results show that all these three loading parameters affect the failure characteristics of sandstone, including irreversible strain, damage evolution, strain behavior, and fatigue life. The influence degree of these three parameters on failure behavior increases in the order of maximum stress level, loading frequency, and amplitude level. However, for the samples with different water contents, their failure characteristics are similar under the same loading conditions. Furthermore, the failure mode is almost unaffected by the loading parameters, while the water content plays a significant role and causing the transformation from the tensile splitting with low water content to the shear failure with higher water content.