Deformation and acoustic emission characteristics of coal with different water saturations under cyclic load

Coal pillars in coal mines are often subjected to the coupling effect of groundwater intrusion and cyclic loads. Consequently, the mechanical characteristics of the coal under different water saturations and cyclic loads are the crucial factors that affect the stability of coal pillars. In this study, the mechanical properties and acoustic emission evolution characteristics of coal samples with different levels of water saturations were investigated through cyclic loading experiments. The results show that the deformation modulus and peak stress of the coal samples show a consistently decrease trend with the increase of water saturation. The acoustic emission count step phenomenon was obvious in the early stage of each block of cyclic loading, which is an important period for the formation of new cracks and the multiplication of macroscopic cracks. Coal samples with low water saturation absorbed most of the energy to form elastic strain energy under the cyclic loading. In contrast, the energy absorbed by coal samples with high water saturation was mainly used to generate new plastic zones. The softening mechanism of water on coal samples was elucidated by revealing that the water saturation weakened the strength softening factor and reduced the cumulative acoustic emission count and cumulative dissipative energy of the coal samples. Coal absorbs the water into the pore space, which decreases the friction and bonding force between the coal matrix blocks and destroys the internal structure of the coal. A cyclic load can compact the cracks in the coal matrix blocks, thus weakening the softening effect of water on the coal. The results of this study can provide important theoretical and practical guidance for maintaining the stability of the coal pillars in the mine reservoirs under cyclic loading.