Experimental study on water-softening and seepage characteristics of weakly cemented sandy mudstone：Taking Shendong Coal Mining Area as an example

The weak cementation characteristics of bedrock in Shendong mining area are obvious, and the softening and seepage characteristics of rock mass after encountering water are common and basic problems in engineering fields such as softening treatment of roof water injection in underground coal mining, prevention and control of water inrush accidents, and construction of coal mine underground reservoirs. Aiming at the weakly cemented sandy mudstone in Shendong Buertai Mine, the paper systematically carried out rock water absorption, single triaxial compression, Brazilian splitting and seepage experiments, and the mechanical parameters and tensile-compression deformation of sandy mudstone as well as the seepage curves and their characteristics in natural and water-saturated conditions were obtained. The research results show that the water-rock solid softening effect and fluid mass transfer effect of weakly cemented sandy mudstone in Shendong mining area are significant, which must be fully considered in related underground engineering, as follows: (1) The plasticity of weakly cemented sandy mudstone after water injection Significantly enhanced, the softening effect is significant, and the compressive strength of rock decreases by 29%-57%, and the smaller the confining pressure, the more obvious the softening effect; the elastic modulus decreases by 25%-30% under different confining pressures. Under low confining pressures, the Poisson's ratio increases significantly, and under high confining pressure, Poisson's ratio does not change much. The post-peak dropping strain increases by 12%, and decreases with the increasing confining pressure. The uni-axial tensile strength decreases by 66%. (2) For the used sandy mudstone samples from Shendong area, the tensile strength is 1/10 of the compressive strength, and the ratio remains basically unchanged after the rock absorbs water. (3) The water seepage process of weakly cemented sandy mudstone easily leads to local collapse of fracture channels. The permeability of weakly cemented sandy mud-stone increases with the increasing gradient of injecting water pressure, but decreases with the increase of confining pressure. The permeability may decrease locally due to the collapse of fracture channels. The research results provide some theoretical basis for revealing the mechanical properties of water-rock coupling of weakly cemented sandy mudstone in Shendong mining area and for carrying out related engineering practice.