Investigation On The Responses Of Overburden Stress And Water Pressure To Mining Under The Ordovician Nappe

This paper focused on the responses of overburden stress and pore water pressure in confined aquifer to mining under the Ordovician limestone nappe aquifer. Based on the engineering geological prototype of coal Seam 32 mining in the first eastern mining area of the Qianyingzi Coalmine, a mining hydrogeological and engineering geological model was established. According to the fracture intensity criterion of overlying strata and critical conditions of crack arresting under the action of compression-shear stress, a formula for calculating the height of water conducting fractured zone during mining under the Ordovician limestone nappe aquifer was proposed. Analogue simulation test device which can be applied to coalmining under the Ordovician limestone nappe aquifer was developed. Analogue simulation experiments of mining under the Ordovician limestone nappe aquifer were carried out. Lastly, numerical simulation was used to analyze the characteristics of plastic zone development, rock stratum displacement, and the variations of overburdern stress and pore water pressure in confined aquifer in mining area. The main research results are as follows:(1) A mining hydrogeological and engineering geological model for studying the responses to coalmining under the Ordovician limestone nappe aquifer was established. The engineering geological conditions, hydrogeological conditions and mining conditions were taken into consideration in the model. Engineering geological conditions included physico-mechanical parameters of engineering geological types, characteristics of structural plane, in-situ stress, and etc. Hydrogeological conditions included lithology, water yield property and connectivity of aquifer and aquifuge, hydrogeologic boundary conditions, and etc. Mining conditions included mining methods and roof management methods.(2) A formula for calculating the height of water conducting fractured zone during mining under the Ordovician limestone nappe aquifer was proposed. The formula was on the basis of the fracture intensity criterion of overlying strata and critical conditions of crack arresting under the action of compression-shear stress, and from the perspective of the evolution of the crack propagation of the damaged rock mass, taking tectonic stress, the influence of fault, the mechanical effect of the seepage field on the stress field and the rock softening with water into consideration. The formula was applied to calculate the height of water conducting fractured zone above panel E3213 of the first eastern mining area of the Qianyingzi Coalmine. Comparative analysis of the results show that the formula improved the calculation precision.(3) An analogue simulation test device was developed for studying the responses of overburden stress and water pressure in confined aquifer to coalmining under the Ordovician limestone nappe aquifer. The test device was composed of five parts: model box, water supply system, stress monitoring system, water pressure monitoring system and image acquisition system, which meet the requirements of mining step by step and controlled the swelling deformation of material box effectively.(4) Based on the developed test device, analogue simulation experiment of mining under the Ordovician limestone nappe aquifer was carried out to investigate the responses of overburden stress and pore water pressure in confined aquifer. The results show that the variations of stress and pore water pressure in overburden were affected by mining effect and geological srtucture, which were closely related to the position in the overlying strata, and the horizontal distance to the DF200 fault. Pore water pressure increased with the increase of mining distance. The pore water pressure monitoring points were located in the Ordovician limestone aquifer as well as the front of the goaf and no water inrush occurred within the scope of the experimental design of mining.(5) Numerical simulation for fluid-solid coupling interaction in mining aera revealed the fault activation due to shear failure, the variation of overburden stress and pore water pressure in confined aquifer. The results show that a shear failure with a length of about 90 m along the DF200 fault surface was caused by mining effect. In the nearby of the fault shear failure area, pore water pressure decreased and a water pressure reducing area was formed in the confined aquifer. At this time, plastic zone around the fault did not connect with the water conducting fractured zone and no water inrush occured. With mining advancing, plastic zone keeped developing and the scope of water pressure reducing area extended. The value of pore water pressure further decreased at the moment of water inrush with larger decreasing amplitude than before. After mining process, the vertical stresses, horizontal stresses and their corresponding variations from working panel to the foot wall of DF200 fault were obviously larger than the upper wall of DF200 fault, which indicated that DF200 fault had iris action on stress transmit.