## 5 Conclusions  
This investigation involved conducting unidirectional compression tests on sandstone specimens subjected to different immersion durations, namely 0, 7, 14, 30, 60, and 90 days. The research investigates the impact of sustained water-rock interaction processes on the deterioration of mechanical properties. The principal findings of the research are outlined below:  
(1) The XRD results indicate that the samples were primarily composed of a quartz framework, clay minerals, and filler materials, with the most significant expansion of clay minerals occurring between 30 and 60 days. SEM analysis reveals the evolution of the sandstone's pore structure through microscopic structural changes, including microcrack formation and changes in clay mineral morphology. As soaking time increased, microcracks initially formed at the cementation points between the quartz framework and clay minerals. These microcracks then expanded and propagated within the clay minerals, weakening their cohesion. After 60 days of soaking, the microcracks within the clay minerals progressively connected, leading to weak interconnections between the clay minerals.  
(2) As soaking time increased, both the strength under compression and the elasticity of the sandstone decreased, with the most significant reduction occurring between 30 and 60 days, followed by a minimal decline from 60 to 90 days. These results were consistent with the XRD and SEM findings. The increase in Poisson's ratio suggests a shift toward more plastic behavior of the rock under prolonged soaking.  
(3) AE analysis was employed to investigate the impact, energy rate, and dominant frequency characteristics of sandstone samples. The results show that tensile cracks were approximately three times more prevalent than shear cracks in dry sandstone. After 60 days of soaking, shear cracks accounted for over 80% of the total, confirming the transition of failure modes from tensile to shear cracks under prolonged soaking conditions.  
(4) From both microscopic observations and macroscopic mechanical test results, it can be concluded that under prolonged immersion, the rebinding effect reduces the critical stress required for crack initiation at crack tips. When expansion stress reaches a sufficient magnitude, it displaces quartz particles, promoting crack propagation and the formation of new intergranular cracks, thereby lowering the fracture energy of the rock. Furthermore, the ongoing expansion and compression of clay minerals facilitate water penetration, leading to interconnected crack development and an increased likelihood of shear slippage.  
Methodology and project administration, H.L.; writing--review and editing, W.D., J.L., and J.Z.; field research, T.Y.; manuscript writing, S.Z. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Natural Science Foundation of China (Grant number 52174070), the National Key Research and Development Program of China (Grant number 2022 YFC2903902), and the Key Science and Technology Project of Ministry of Emergency Management of the People's Republic of China (Grant number 2024 EMST080802).  
Not applicable.  
The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding author.