# Water Infusion on the Stability of Coal Specim under Different Static Stress Conditions
## 1 Introduction  
In underground coal mines, the stability of coal pillars and the surrounding rock is a key factor controlling the stability of the cavity group system. Groundwater, as an important environment factor, significantly weakens the coal strength and stiffness, leading further to a series of mining disasters, such as gob collapse, pillar instability, roof fall and surface subsidence [1; 2; 3; 4; 5; 6; 7; 8; 9; 10]. When mines are abandoned, rock pillars are commonly in moisture conditions because of the rising water table. In addition, water injection is found to be effective for the mitigation and prevention of coal bump and rockburst [11; 12; 13; 14; 15; 16]. Therefore, it is worthwhile to study the mechanical behavior of coal rocks under watery conditions. Massive efforts have been made to explore the effect of water on the mechanical properties of coal [17; 18; 19; 20; 21; 22; 23; 24; 25; 26]. For example, [PERSON] and [PERSON] [17] conducted uniaxial compressive tests on Nemo coal specimens with different water contents. They found that with the increase of water content, the compressive strength and Young's modulus of the Nemo coal specimen decrease. [PERSON] et al. [22] carried out similar tests on Xishahe coal specimens. They discovered that the peak stress and modulus during elastic, strain softening and post-peak regimes decrease as water content increases while the failure strain increases. Based on the equivalent strain hypothesis and statistical damage theory, they derived a constitutive model for the coal specimens with different water contents [19]. [PERSON] et al. [20] obtained the damage evolution characteristics of coal specimens by means of acoustic emission technique. They found that the stress thresholds of coal are not affected by water, and they also stated that the presence of water promotes the shear failure in coal transformed from the split failure. [PERSON] et al. [26] determined the dynamic compressive strength of coal specimens under moisture conditions by using a split Hopkinson pressure bar. They suggested that both of water content and porosity strongly affect the dynamic strength of coal. In the majority of the above-mentioned studies, coal specimens were loaded under an increasing stress. However, the geo-stress acting on a pillar remains constant for a long time. The instability of a pillar in a stable stress field is essentially the result of the development and coalescence of inside micro-cracks, which is more likely to be creep deformation when the roof has a lower stiffness and to be stress relaxation when the pillar stands under a hard roof. By using some novel algorithms like the neural network and genetic algorithm, researchers are able to provide optimal estimation of the rheological parameters, which contributes to the prediction of the long-term rheological behavior of rock engineering [27]. As the degradation of the pillar is strongly sensitive to groundwater saturation, studying water's effect on rock and coal specimens is of great significance for a better understanding of the stability of pillars suffering from moisture or water immersion under geo-stress. However, this is impossible to determine by obtaining the whole instability process of pillars in underground mines. On the one hand, the instability of underground pillars is caused by combined factors including not only water and geo-stress but other complicated factors like defects, dynamic disturbance, weathering and so on, which makes it impracticable to quantitatively determine the water effect on pillar stability. On the other hand, the instability of pillars is unpredictable to some extent, which usually takes a long time, from months to years. Moreover, it is difficult to determine water distribution and its effects on the stability of rock engineering using the current monitoring techniques like microseismic monitoring.