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

The study of the deformation characteristics and damageevolutionlaw of the underground water-bearing rock mass under reciprocatingloads such as mine earthquake and mechanical vibration is a very crucialaspect of underground engineering. In this pursuit, the present studywas envisaged to assess the deformation characteristics and damageevolution law of sandstone with different water contents under variouscycles. Specifically, the uniaxial and cyclic loading and unloadingtests, X-ray diffraction (XRD), and scanning electron microscope (SEM)tests of the sandstone under dry, unsaturated, and saturated conditionswere carried out under laboratory conditions. Subsequently, the changelaws of elastic modulus, cyclic Poisson's ratio, and irreversiblestrain in the loading section of sandstone under different water contentconditions were analyzed. Based on the two-parameter Weibull distribution,the coupled damage evolution equations of sandstone under water contentand load were established. The results showed that with an increasein the water content in the sandstone, the loading elastic modulusof the corresponding cycles exhibited a gradual decrease. Microscopicanalysis revealed that kaolinite was present in the water-bearingsandstone in a lamellar structure, with flat edges and many superimposedlayers, and the proportion of kaolinite gradually increased with anincrease in the water content. The poor hydrophilicity and strongexpansibility of kaolinite are the key factors in reducing the elasticmodulus of sandstone. With the increase of the number of cycles, thecyclic Poisson's ratio of sandstone experienced three stages:an initial decrease, followed by a slow increase, and finally a rapidincrease. The decrease was mainly observed in the compaction stage;the slow increase existed in the elastic deformation stage; and therapid increase was seen in the plastic deformation stage. Furthermore,with the increase of water content, there was a gradual increase inthe cyclic Poisson's ratio. The concentration degree of thedistribution of the rock microelement strength (the parameter m) under the corresponding cycle of sandstone with differentwater content states exhibited an initial increase followed by a subsequentdecrease. With the increase in the water content, the parameter m under the same cycle gradually increased, and the changerule of parameter m corresponded to the development of internal fracturesin the sample. With an increase in the number of cycles, the internaldamage of the rock sample gradually accumulated, and the total damageincreases gradually but the growth rate decreases gradually.