RESISTANCE TO PERMEABILITY DAMAGE AND PILLAR SAFETY OF COAL MINING UNDER GIANT THICK LOOSE SEAM—TAKE KOUZI￣DONG MINE AS AN EXAMPLE

With the shift of coal seam mining from deep to shallow,the water-bearing bodies of huge thick loose layers covering the coal seams have become a safety hazard that cannot be ignored in Huainan mine area. So the safety evaluation of reserved coal pillars is particularly important. This paper takes the Kouzidong mine as an example,presents the comprehensive and systematic collection and collation of geological and hydrogeological data revealed by long-term observation of mine water dynamics and underground mining works in Kouzidong mine in recent years. It uses the numerical simulation and theoretical analysis methods and obtained the following findings. (1)Based on the borehole column diagram of the classic block section in the mine area,a FLAC^3D based mechanical model is established to simulate the workface retrieval by step. The displacement cloud diagram,maximum principal stress diagram and plastic damage diagram are observed and analyzed to recognize the mining overburden damage law. The height of the hydraulic fracture zone is about 56.6 m. The hydraulic fracture zone cannot be developed to the height of the aquifer. (2)According to the hydrogeological data of Kouzidong mine,a hydrogeological model based on GMS(Groundwater Modeling System) is established. The bending deformation of the overburden rock caused by mining lead to the changes in its vertical permeability coefficient and hydraulic gradient. The stress effect of mining on the model in FLAC^3D is used as the basis. The stress magnitude is transformed into the vertical permeability coefficient of the bending zone as a variable reflected in the GMS model. The accuracy of the model is checked and the changes in the seepage field before and after mining is observed. It is found that the changes in the seepage field are very small,which also ensures the safety of leaving the coal pillars. (3)The control factors of water gushing sand is analyzed. The mine pumping test data is used. The iterative calculation program is developed using computer. The two critical hydraulic gradients(J_cr) are calculated as follows: J_cr for the fourth aquifer=1.66 and J_cr for the red layer=1.62. The critical head height of each point takes the value range of 10 to 25 m and is far less than the actual water level. These values are used as a basis for the evaluation of resistance to permeability damage. At present,when mining coal seams under water-bearing seams,the factors for setting up waterproof coal pillars mainly include mining height,seam inclination,roof lithology and its mechanical properties. The hydraulic properties of water-bearing seams and their changes after the change of seepage field are not taken into consideration. The above-mentioned properties of bedrock weathering layers are also not taken into consideration. This paper provides a scientific method and basis for evaluating the safety of coal pillars.