Experimental Study on the Deterioration Mechanism of Sandstone under the Condition of Wet-Dry Cycles

The water content of rock in nature usually changes, because of the rainfall (Wet) and drought (Dry). This wet-dry cycle is one of the most important reasons leading to the deterioration of rock mechanical properties. In the paper, uniaxial compressive strength (UCS) tests of sandstone suffered different wet-dry cycles were conducted, and the ion content in the soaking solution was measured. In addition, the microstructure changes of the sandstone specimens were observed by scanning electron microscopy (SEM), and the pore changes were tested by nuclear magnetic resonance (NMR) technology. The UCS of sandstone decreases by 4.58% after 1 dry-wet cycle, and 18.35% after 20 dry-wet cycles. There are some ions in the soaking aqueous solution after each wet-dry cycle, such as Na+, Mg2+, K+, and Ca2+, etc, and the ion concentration decreases gradually with the increase of wet-dry cycles. The results of SEM and NMR show that the porosity of the sample increases from 10.22% to 12.30% after 20 wet-dry cycles. Based on recorded change of porosity, the damage evolution equation of sandstone is established. As the damage accumulates, the UCS of the specimen gradually decreases. The loss of some minerals and cyclic expansion-shrinkage of clay minerals during the wet-dry cycles are the main reasons for the damage of sandstone microstructure, which eventually results in the deterioration of mechanical properties.