Experimental Study on the Mechanical Behavior of Coal Samples during Water Saturation

When mining-induced fractures reach overlying aquifers, water enters the mining area and the coal is under different natural water saturation conditions, which significantly affect the mechanical behavior of the coal. In this study, uniaxial compression tests were conducted on dry, partially saturated, quasi-saturated, and fully saturated coal samples. The mechanical parameters, acoustic emission (AE) activities, and failure patterns of differently saturated coal samples were analyzed. The effect of water content on the behavior of coal and suggestions to ensure safe underground coal mining were discussed. The results indicate that the water content in coal increases nonlinearly with intrusion time and can be regarded as a logarithmic function. With increasing water saturation, the mechanical strength of the coal decreases on the whole and the AE activities, crack development, and burst severity are weakened significantly. The failure pattern of the coal samples changes from a dynamic type to a quasi-static one and from a compressive-shear type to a tensile one. Water content has four main effects on the mechanical behavior of the coal samples. These are a liquid bridge force, a water softening effect, a wedge effect, and a lubrication effect. With increasing water saturation, the effect of water gradually increases and predominates the coal failure, leading to a continuous decline in the strength of the coal samples. When the coal around the mining space is subjected to water, the high degree of water saturation in the coal decreases the risks of coal bursts significantly; however, it causes a large deformation and instability of the roadways. To ensure safe mining, more measures should be taken to decrease the amount of inrushing water, reduce the stress, and reinforce the anchor bolting support.