Effect of Cyclic Loading and Unloading on the Deformation and Damage Properties of Sandstone from Beizao, a Coal Mine underneath the Bohai Sea in China

Mining under the sea is a challenging task in China. Affected by blasting, tunneling, and other engineering disturbance, surrounding rock is often in a state of cyclic loading and unloading stress. In this study, in order to investigate the effect of cyclic loading and unloading on the deformation and damage characteristics of sandstone underneath the Bohai Sea, the GCTS test machine is used to conduct cyclic loading and unloading tests on sandstone. The results show that under cyclic loading and unloading compression, the stress-strain curves of sandstone form a hysteresis loop. The axial residual deformation first decreases, then increases with the increase of cycle number and unloading stress level. Both the circumferential residual strain and volumetric residual strain decrease with the increase of cycle number and unloading stress level. The axial elastic deformation increases with the increase of the cyclic number and cyclic load. The volume deformation first increases, then decreases, and the circumferential strain gradually decreases. In the process of cyclic loading and unloading, the loading elastic modulus gradually increases. Affected by damage, the unloading stress difference of sandstone initially increases with the increase of cycles. Next, the effects of cycle number and unloading stress level on the damage parameters of sandstone are analyzed. Before brittle failure of the specimen, the absolute damage parameters of axial, circumferential, and volume show an increasing trend, and the increase rates of circumferential damage parameters and volume damage parameters suddenly increase, which is also the precursor of the sandstone specimen’s instability failure.