## 1 Introduction  
To reduce carbon emissions, the world is undergoing a transformation of its energy structure to mitigate impacts on the ecological environment [1]. China is the largest producer and consumer of coal globally [2]. High-intensity, large-scale coal mining has resulted in the formation of numerous goaf areas. These substantial spatial resources present an excellent opportunity for adjusting the green and clean energy structure and for the comprehensive development of new energy sources [3]. Abandoned coal mine tunnels are viable sites for compressed air energy storage systems, whereas the goafs of such mines can be effectively utilized as key components in pumped storage power generation systems. Underground pumped storage hydropower stations (UPSH) represent an optimal choice for repurposing the underground space in mines [4, 5, 6, 7]. The underground water reservoir in coal mines is a critical component of UPSH, utilizing voids in the collapsed rock mass created by coal mining to connect coal pillars with artificial dam bodies, thereby forming the reservoir dam body [8; 9; 10; 11; 12]. This system also includes facilities for storing and utilizing mine water, as shown in Figure 1. Underground water reservoirs in coal mines serve dual purposes: they function as pumped-storage hydropower facilities while simultaneously providing water resources for domestic use by local communities and agricultural irrigation. During the operation of the underground reservoir, the coal pillar dam body remains immersed in mine water for extended periods. Therefore, it is essential to study the mechanical characteristics of coal subjected to long-term mine water immersion. Previous studies have demonstrated that the physical and mechanical properties of coal change after immersion in water. [PERSON] et al. [13] found that as moisture content increases, the compressive strength of coal decreases. [PERSON] et al. [14] reported that the peak strain and compressive strength of coal exhibit positive and negative linear relationships with increasing moisture content, respectively, while the elastic modulus shows a negative exponential relationship with moisture content. [PERSON] and [PERSON] et al. [15] indicated that as moisture content rises, the stress-strain curve of coal displays a multi-stage drop and manifests as a combination of shear failure and tensile-shear failure. [PERSON] [16] conducted a pressurized water injection simulation experiment and found that the water film formed on the surface of coal molecules can lubricate the coal body and increase its porosity. [PERSON] et al. [17] demonstrated that as immersion time increases, coal porosity gradually rises, while the mechanical parameters exhibit an exponential decay relationship with immersion time. Currently, research on the effects of water immersion on the physical and mechanical properties of coal primarily considers two factors: the pure water environment and moisture content. Coal pillars are affected not only by prolonged immersion in mine water but also by static and quasi-static loads on the coal pillar dam body due to disturbances from coal mining. This is equivalent to subjecting the coal pillars to varying loading rates. Higher loading rates enhance the ability of coal and rock to convert external energy into elastic strain energy, making coal more susceptible to non-steady state failure [18; 19; 20]. Generally, a higher loading rate correlates with greater strength exhibited by coal and rock [21; 22]. However, this trend does not persist indefinitely; mechanical strength does not continuously increase with rising loading rates and displays certain nonlinear characteristics. In the relationship where coal strength initially increases and then decreases with increasing  
Figure 1: Schematic diagram of an underground water reservoir in a coal mine [9]. loading rate, the loading rate at the turning point is known as the critical loading rate [23].