Study on solid-fluid coupling similarity simulation test of water-sand inrush during mining of shallow buried thin bedrock roof

As a new type of mine geological disaster, water-sand inrush has the characteristics of concealment, sudden onset, difficulty in monitoring, and strong destruction. Therefore, exploring the characteristics of overlying rock mining degradation and the disaster mechanism of water-sand inrush has guiding significance for the prevention and control of loose layer water hazard. Firstly, the influence mechanism of different material proportioning on the strength and permeability characteristics of similar materials is studied through similar material proportioning and testing experiments, and a new solid-fluid coupling similar material suitable for simulating soft rock layers is developed. Then, by using the test system for water-sand inrush, similar simulation tests are conducted for mining under the condition of a single water-bearing structure capping thin and medium thick bedrock, and a double layered structure capping thin bedrock. The characteristics of mining crack and the disaster mechanism of water-sand inrush under the above strata structure conditions are analyzed. The test results show that the new solid-fluid coupling similar material has the characteristics of water resistance, anti disintegration, anti softening, low permeability, low strength, and low plasticity, and the permeability coefficient decreases exponentially with the increase of calcium carbonate content, while the uniaxial compressive strength exhibits a quadratic polynomial growth characteristic. The first weighting stage and the first periodic weighting stage of mining shallow and thin bedrock are easy to induce water-sand inrush. The mining cracks present an “OX” distribution on the plane, and as the thickness of bedrock and the clay layer at the bottom of the loose layer increases, the bedrock gradually transforms from tensile shear failure to tensile failure, and the morphology of the cracks also changes. At the same time, the opening and smoothness of cracks are reduced, which can reduce the inducing risk and harm degree of water-sand inrush. The ratio of crack opening to sand particle size , as well as the smoothness of crack, are the key factors determining the occurrence and severity of water-sand inrush.