Study on deformation and rock failure mechanism of floor rock mass with thin aquifuge near Ordovician limestone

In order to study the deformation and failure mechanism of thin aquifuge floor rock under the influence of Ordovician limestone,the influence of weathering zone was constructed based on the geological structure characteristics of the rock. The deformation and failure model of the lower floor rock mass was theoretically analyzed and verified. Based on the difference in vertical floor rock deformation,the depth of floor failure zone during ultra-thick coal seam mining was calculated and the disturbance deformation characteristics of the intact strata above the weathering zone under the elastic-plastic conditions and the tensile failure mechanism of the of the rock formation based on the type I stress intensity factor were analyzed. Finally,based on the mine strata structure and coal seam characteristics,a numerical model was established to simulate and analyze the deformation value and failure law of the floor rock mass under the conditions of near Ordovician limestone confined aquifer and thin aquifuge. The results show that the two ends of the rock stratum are compressed and the middle is tensioned,forming a similar thin plate structure,and the rock stratum is subject to upward vertical stress,which accelerates the destruction of the rock layer. The top weathering zone of Ordovician limestone is easily damaged due to weak lithology and is easily damaged by disturbance，and the damage area will develop upward; the failure of the rock stratum will continue to develop through the floor damage area and the weathering zone damage area to the middle. When mining extra-thick coal seams,the failure zone can penetrate the entire floor rock above the weathered rock. The transient displacement change of monitoring points shows that there are two obvious inflection points in the vertical direction of the displacement curve of monitoring points. The displacement curve of monitoring points is divided into 3 sections,which correspond to the rock strata above the weathered section,the weathering section and the Ordovician limestone section respectively. The Ordovician limestone weathering zone has obvious influence on the deformation of the floor rock,so corresponding reinforcement measures need to be taken. The whole displacement curve of the monitoring point is consistent with the transient curve,and the numerical results are consistent with the theoretical analysis.