A new loading–unloading experimental method for simulating the change of mining-induced stress

Based on a self-developed triaxial seepage device, a new loading and unloading experimental method is proposed in this paper to eliminate sample variations. The results show that the strength of sandstone sample and the axial strain at failure increased with the increasing initial hydrostatic stress, but decreased with the increasing loading–unloading rate. Following the alternating loading–unloading test, the stress–strain curves of specimens advanced in wave form, the waves' volatility decreased and then increased. It is found that near the ultimate strength, volatility is the biggest, and the stability of waves’ volatility increased along with the increasing initial hydrostatic stress. The similarity of stress–strain curves between the conventional loading–unloading tests and the alternating loading–unloading tests increased along with the increasing initial hydrostatic stress and the increasing initial velocity for the alternating loading–unloading method. Along with the increasing initial hydrostatic stress, the failure behaviour of the sandstone samples tested under loading–unloading methods changed from a tensile state to a tensile-shear coexisting state, and finally to a fully shear failure state. The degree of failure modes increased with the increasing loading–unloading rate.