Water inrush mechanism and engineering application of concealed collapse column in coal seam floor

Concealed collapse column in North China Coalfield is difficult to explore, easy to water inrush, hidden dangers, is a typical representative of coal mine safety hazards. Taking the hidden collapse column under the floor of 1501 working face in Shanyang Coal Mine of Chenghe Mining Area as the engineering geological background, according to the water-bearing ( water-conducting ) characteristics of the collapse column, the water inrush accident is divided into two modes : overall shear failure and tip splitting failure. Combined with higher rock mechanics and fracture mechanics, the hydraulic model of water inrush is established respectively. Through the critical equilibrium condition and fracture criterion, the analytical solution of the critical water pressure that the floor aquifuge can resist and the safety threshold of the thickness of the aquifuge are theoretically derived.The results show that the critical water inrush pressure consists of two parts, namely, the water pressure value of the floor itself and the water pressure value of the surrounding rock. The critical water inrush pressure is positively correlated with the thickness of key strata, and decreases first and then increases with the top width of collapse column. When the width of collapse column top is 11.2 m, the limit water pressure is mainly controlled by the thickness of effective aquiclude when it is less than this value, and the horizontal stress of surrounding rock has a great influence on the limit water pressure when it is greater than this value. The fissure extension length around collapse column is positively correlated with water pressure and negatively correlated with overburden pressure. The collapse column axis angle between 60° and 90° is more likely to induce flood disaster. The research results are applied to the risk assessment of the floor of 1501 working face. The theoretical analysis shows that the critical water inrush pressure of the floor under the influence of mining disturbance is 0.95 MPa, which is not enough to resist the pressure of the Ordovician limestone confined water pressure of 1.2 MPa, and the growth and development of secondary cracks around the collapse column after mining threaten the stability of the water isolation at the top of the collapse column. According to the suspected cavity inside the collapse column, the treatment method of ‘water plugging + double liquid grouting’ was scientifically constructed. After treatment, the water inflow of the working face was reduced from 65 m3/h to 0.3 m3/h, and the working face was successfully mined.