Impact of Wetting–Drying Cycles on Mechanical and Microstructure Properties of Rocks: State-of-the-Art Analysis

This paper synthesizes recent advancements on the effects of wetting–drying (WD) cycles on mechanical and physicochemical properties of sandstone, granite, coal, and mudstone. WD cycling initiates complex physicochemical degradation processes contributing to rock weathering and long-term geological transformations. The analysis focuses on key parameters, including unconfined compressive strength (UCS), tensile strength, elastic modulus, porosity, water absorption, microstructure, chemical composition, and wave velocity. The results indicated that prolonged soaking periods of greater than 48 h and drying duration with temperatures less than 60 °C have significantly accelerated rock deterioration. Sandstone exhibited the highest resistance, while coal was the most susceptible. On average, sandstone showed 35% increase in porosity, 17% reduction in wave velocity, and 48% reduction in UCS, comparable to the combined effects observed in other rock types. Tensile strength and elastic modulus in sandstone declined by 41% and 48%, respectively, whereas other rocks experienced an average of 50% reduction in tensile strength. Microstructure analysis confirmed pore degradation and mineral disintegration. Notably, the role of scale effects remains underexplored in experimental studies. However, machine learning (ML) models highlight sample size as a critical factor influencing the strength variability. Additionally, pore fluid chemistry exhibited a nonlinear impact on degradation. A predictive ML model identified wetting duration, sample size, number of cycles, and mineralogy as the dominant variables accounting for 38% strength reduction. This analysis highlights the need for scaled-up modeling to replicate field conditions better and to improve the predictive accuracy of the degradation process under natural environments.