Studies on the Deformation and Macro–Micro-Damage Characteristics of Water-Bearing Sandstone under Cyclic Loading and Unloading Tests

[Image: see text] The study of the deformation characteristics and damage evolution law of the underground water-bearing rock mass under reciprocating loads such as mine earthquake and mechanical vibration is a very crucial aspect of underground engineering. In this pursuit, the present study was envisaged to assess the deformation characteristics and damage evolution law of sandstone with different water contents under various cycles. Specifically, the uniaxial and cyclic loading and unloading tests, X-ray diffraction (XRD), and scanning electron microscope (SEM) tests of the sandstone under dry, unsaturated, and saturated conditions were carried out under laboratory conditions. Subsequently, the change laws of elastic modulus, cyclic Poisson’s ratio, and irreversible strain in the loading section of sandstone under different water content conditions were analyzed. Based on the two-parameter Weibull distribution, the coupled damage evolution equations of sandstone under water content and load were established. The results showed that with an increase in the water content in the sandstone, the loading elastic modulus of the corresponding cycles exhibited a gradual decrease. Microscopic analysis revealed that kaolinite was present in the water-bearing sandstone in a lamellar structure, with flat edges and many superimposed layers, and the proportion of kaolinite gradually increased with an increase in the water content. The poor hydrophilicity and strong expansibility of kaolinite are the key factors in reducing the elastic modulus of sandstone. With the increase of the number of cycles, the cyclic Poisson’s ratio of sandstone experienced three stages: an initial decrease, followed by a slow increase, and finally a rapid increase. The decrease was mainly observed in the compaction stage; the slow increase existed in the elastic deformation stage; and the rapid increase was seen in the plastic deformation stage. Furthermore, with the increase of water content, there was a gradual increase in the cyclic Poisson’s ratio. The concentration degree of the distribution of the rock microelement strength (the parameter m) under the corresponding cycle of sandstone with different water content states exhibited an initial increase followed by a subsequent decrease. With the increase in the water content, the parameter m under the same cycle gradually increased, and the change rule of parameter m corresponded to the development of internal fractures in the sample. With an increase in the number of cycles, the internal damage of the rock sample gradually accumulated, and the total damage increases gradually but the growth rate decreases gradually.