## 5 Conclusions  
When a coal sample absorbs more water, the mechanical properties are greatly affected; for example, the coefficient of friction decreases, and crack development increases and leads to a decrease in the elastic modulus [33]. Using acoustic emission technology and digital image processing, we conducted uniaxial compression tests on coal rock samples immersed for varying durations. We uncovered the patterns of compressive strength degradation and fracture evolution of coal rock under immersion-induced weakening.  
1. The natural water absorption rate of coal rock reaches its peak after being immersed in water for 1 day. As the immersion duration increases, the absorption rate gradually stabilizes at around 20 days, indicating the gradual saturation of the coal rock with water absorption. The saturated natural water absorption rate is 1.97%. The longitudinal wave velocity of the coal rock tends to increase after water immersion.
2. Short-term water immersion results in an improvement in the plasticity of the coal rock with only a minor softening effect, resulting in a relatively complete damage morphology and a low degree of fragmentation. Conversely, prolonged water immersion results in the significant softening of the coal rock, which causes a shift in the damage morphology from shear fracture to longitudinal fracture. A negative exponential function was used to establish the relationship between the compressive strength and peak strain of the coal rock during continuous immersion, revealing a linear correlation between the elastic modulus and immersion duration.
3. The active periods of acoustic emission in coal rocks with different water immersion durations are comparable, with the most significant stress peaks occurring near the highest intensity. With increasing immersion durations, the cumulative number of acoustic events decreases, indicating the pronounced attenuation effect of water. Furthermore, the longer the immersion duration, the lower the extent of damage and the higher the level of integrity. The number of failure areas in the compressive fracture process of coal rock decreases after water immersion. The area of strain concentration forms a narrow strip in the direction of vertical lamination, and this is significantly influenced by the orientation of the primary joints within the coal.
4. The fitting curves of the compressive strength and elastic modulus under different immersion durations were obtained. The mechanical reduction relationship was applied to the failure patterns of coal pillars in underground reservoirs. The damage area of the coal pillar and the surrounding rock of the excavation area is expanded when increasing the duration of water immersion.  
The mechanical reduction relationship model of coal rocks with different immersion durations is based on a 30-day specimen, and future studies will increase the immersion duration to further validate the reliability of the mechanical reduction relationship model.  
**Author Contributions:** Investigation, H.Z.; methodology, P.W.; software, K.Z.; data curation, Y.G. and Z.Q.; writing--review and editing, P.W.; project administration, M.C. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This work was financially supported by the National Key R&D Program of China (No. 2023 YFC2907400, 2021 YFC2900500), the National Natural Science Foundation of China (No. 52074020), the specific research fund of the Innovation Platform for Academicians of Hainan Province (YSPTZX202504), and the Hainan Province Science and Technology Special Fund (ZDYF2024 GX)S307).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.  
**Conflicts of Interest:** The authors declare that they have no conflicts of interest.