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.

Jurassic Red-Bed sandstone is one of the main types of rock composing unstable slopes on both banks of the Three Gorges Reservoir (TGR) in China. It is vulnerable to tensile strength degradation when exposed to cyclic wet-dry conditions under reservoir operation. Therefore, the law behind this needs to be clarified in order to accurately evaluate the stability of a sandstone slope located in the drawdown area of the TGR. In this paper, Jurassic Red-Bed sandstone specimens from the Majiagou landslide in the TGR area were collected for experiments with cyclic wet-dry procedures. After each wet-dry cycle, efforts were made to implement splitting tests, measure clay mineral contents and ion contents in the soak solution, and observe changes in microstructures. The tensile strength is experimentally demonstrated to be a process of damage accumulation under the integrated actions of water pressure, calcite dissolution, and clay mineral hydration, among which hydration of illite contributes the most to the damage. The loss of the cement material and the cyclic expansion and shrinkage of clay minerals weaken the cementation between grains, leading to the tensile strength degradation.

This paper presents an experimental study of nanomaterials’ influence on improving the mechanical behaviour and microstructure of cemented soils. The strength characteristics were obtained through uniaxial compressive strength test. The influences of nanomaterials on the pore size distribution and micromorphology of cemented soil were investigated by nuclear magnetic resonance, scanning electron microscope, and X-ray diffraction. The results show that the uniaxial compressive strength of the cemented soil increases with the nano-SiO₂ content. When the content is 4%, the strength of the cemented soil increases by about 40%. Improvement with nano-Fe₃O₄ shows different trends. The strength of the cemented soil increases with the nano-Fe₃O₄ content, reaching a peak at 3% of the nano content, and then decreases with the increase in the content. The transverse relaxation time spectrum curve of the cemented soil is trimodal, and the main peak covers a dominant area. Adding nanomaterials improves the pore distribution, transforms large pores into small pores, and greatly reduces the pores of the cemented soil. The porosity of the cemented soil decreases exponentially with the increase of nano-SiO₂ content. On the contrary, with the increase of nano-Fe₃O₄ content, the porosity of the cemented soil specimen first decreases and then increases, the porosity reaches the minimum at 3% content. Nano-SiO₂ and nano-Fe₃O₄ can effectively fill the internal pores of the cemented soil and accelerate the hydration process. In addition, nano-SiO₂ participates in the hydration reaction of cement and has a good promoting effect on the mechanical properties of cemented soil.