| With the excessive mining of coal resources in recent years, problems start to emerge such as depletion of resources and insufficient time of working face transfer. In order to improve this situation, many coal mines increased the upper limit of mining, decreased the height of safety coal(rock) pillar, and improved recovery rate in shallow areas. However, many disastrous accidents of water and sand burst happened, which caused huge economic loss and serious casualties. At present, the northwest region of China with geologic features of thick sand strata and thin bed rock is one of the main coal production bases, where water and sand burst accidents happen occasionally and threaten mine safety production constantly. Water and sand burst accidents belong to hidden disasters which often happen in the mining process under special geological conditions, and thus the mechanism requires comprehensive research of field observation, theoretical analysis, and laboratory test.This paper studies on water and sand burst accident and develops geologic model based on the geological condition of thin bed rock working face in Zhao Gu No.1 mine, proposes wedge-shaped structure of hydraulic pressure preservation, and studies the features of water preservation through laboratory tests; research has been done on the overburden failure features and the risk of mining under wedge-shaped structure of hydraulic pressure preservation based on the correlation theories of mine pressure and control, numerical analysis methods of finite and discrete element; the stability of weathered mudstone protective layer has been studied, while extension rules of fissure under high water pressure has been developed by using the testing apparatus of rock fissure extension under water flow, and the effect of fissure geometrical characteristics on water and sand burst was modeled by using testing apparatus of sand burst; water and sand burst mechanical mechanism caused by instability of sand prevention safety coal(rock) pillar under high hydraulic pressure through elastoplasticity, percolation and fracture mechanics is studied, and design method of sand prevention safety coal(rock) pillar under high hydraulic pressure and engineering criterion of water and sand burst prevention is proposed. The main research achievements of this paper are as follows:(1) Bedrock is divided into three types based on the thickness: thick bedrock, thin bedrock and ultrathin bedrock. The area where the thickness of bed rock is less than or equal to 50 m is categorized as thin bed rock, and the roof belongs to medium-hard type according to mining condition of Zhao Gu No.1 mine. There is a thick-bedded clay stratum in the middle of unconsolidated formation, so the shallow aquifer has no direct influence on mining; there is also a thick-bedded clay stratum under the bottom of the vast majority of Neogene stratum, and some parts are in gravel “skylight area” which has influence on mining; the water-abundance of weathered zone and roof sandstone aquifer is weak, which has limited influence on mining and can be prevented by the method of drilling drainage before and during mining operation.(2) Proposed wedge-shaped structure of hydraulic pressure preserve which consists of upper and lower clay pan, gravel “skylight area” and wedge-shaped weathered zone based on the geologic feature of thin bed rock area. The results of soil test, strength test of weathered zone rock and disintegration test indicate that the clay and the mudstone of weathered zone have strong water-resistivity and water retention property. High water pressure is generated in gravel aquifer under the influence of ultra-thick loose strata, and the aquifer becomes the “high pressure core” of wedge-shaped structure of hydraulic pressure preservation, which poses a threat to the working face with thin bed rock.(3) Based on the theoretical analysis of material mechanics and critical layer about the working face roof with ultra-thick loose strata and thin bed rock, it is concluded that the first and periodic weighting step of main roof is short, while only one critical layer exists in the roof and main roof tends to form a “short voussoir beam” structure. The “short voussoir beam” structure and “step beam” structure of roof will lead to deformation and slide instability easily, which can cause intense face weighting and deformation instability and failure of bottom claypan of unconsolidated formation.(4) According to the engineering characteristics of clay, thick clay strata have selfcarrying capacity which can decrease the transitive load from top soil layer to the critical layer. Load transmitting coefficient of loose strata is proposed when composite fracture occurs in the critical layer. The formula of load transmitting coefficient is derived between 0.24~0.30 for the area of thin bed rock based on the load transmitting theory of rock composite beam. Fissure aisle width formula of upper critical layer with composite fracture is also derived based on the roof deformation characteristics. Geometrical characteristics of fissure aisle can be divided into 9 types by analyzing the variation of revolution and breaking angle of main roof fractured rocks.(5) Based on the geological condition of thin bed rock area, mechanical model of numerical calculation about the large mining height working face of thin bed rock area is established and overburden failure rule is studied through finite element and discrete element numerical simulation. Overburden failure area continuously extends with mining and stops when sufficient mining is achieved. The plastic failure zone then becomes a “saddle” shape, and the movement angle of rock strata is about 75°. After mining, pressure in surrounding rock of goaf is released and stress concentration occurs in front of the coal wall. The plastic failure zone of overburden failure is divided into tensile fracture zone, tensile fissure zone, shear failure zone, and tension-shear zone. When mining height is 3.5 m, the height of caving zone is 12~15m, and the ratio of caving zone and mining height is 3.43~4.29; the largest height of fissure zone is 42~43 m, and the ratio of fissure zone and mining height is 12.00~12.29. The fissure zone can penetrate the protective layer of weathered zone and spread to wedge-shaped structure of hydraulic pressure preservation, which will finally cause the “high pressure core” leakage and will have direct influence on safety mining.(6) A testing apparatus of rock fissure extension under water flow is developed based on the similitude principle of simulation test and geological condition of thin bed rock area. 4 testing programs and 9 groups of tests are designed considering water injection pressure, crack width, subjacent crack width and rock physical properties. Test results indicate that crack extension of weathered mudstone has critical head pressure. Crack begins to propagate when head pressure reaches the critical value, and instability failure happens in the protective layer of weathered, which decreases the ability to prevent water and sand. Water pressure continues to decrease, as well as the speed of water pressure, and the variation of water inflow exhibits an “S” shape during the process. Crack extension has 3 stages: initial crack closing stage, extension stage and disintegration stage. The length of time and extension width are connected with water injection pressure, crack width and rock physical properties, which are the interaction of seepage field and stress field.(7) Under the condition of different parameters, the surface of weathered mudstone fissure appears different crack characteristics with current scour. Section of fissure experiences sheet erosion and liner erosion stages under current scour action. Based on silt flow theory, fissure extension mechanical model of weathered mudstone under water flow is established and instability condition of surface particles of rock mass under high water pressure is concluded. Fissure extension is caused by the instability of rock surface particles; the larger the hydraulic slope and the seepage force, the more easily particles become instable, and the higher degree the fissure propagates.(8) Influence of geometrical characteristics of mining fissure on water and sand burst is studied by using testing apparatus of sand burst with 3 testing programs and 24 groups of tests. The effects of geometrical characteristics of rock fissure and physical property of sand on critical hydraulic slope through empirical formula are analyzed. The process of sand burst experiences three stages: drip stage, instability of part sands and water and sand burst stage. Funnel-shaped cavity appears in the upper sand layer and spheroidshaped cavity appears in the lower sand layer after sand burst. Critical hydraulic slope and sand burst amount vary significantly with different geometrical characteristics of fissures. Critical hydraulic slope decreases with the increase of crack width and the decrease of sand layer compactness, which increases the possibility of water and sand burst.(9) The mechanical model of seepage-piping evolution of sand prevention safety coal(rock) pillar under high water pressure is proposed and the mechanical mechanism of water and sand inrush are studied based on the overburden failure law of thin bed rock area and the instability of the protective layer of weathered zone under high water pressure. Based on the pore-fissure elastic theory, weathered rock of the protective layer before and after mining and after the instability under high water pressure seepage is categorized into three types, and the water inrush and sand inrush in working face caused by the instability of sand prevention safety coal(rock) pillar undergoes four phases: seepage phase before mining, seepage solid-liquid coupling phase after mining, seepage phase in instability and piping phase. The solid-liquid coupling mechanical mechanism is built in each phase.(10) Improving the design method of safety coal(pillar) of mining under water bodies. Based on overburden failure law and the results of laboratory tests, considering the disadvantage effect of high water pressure on sand prevention property of protective layer of weathered mudstone with mining-induced fractures, method of designing sand prevention safety coal(rock) pillar which is suitable for high water pressure condition is proposed. By analyzing the detection results of roof hole in thin bedrock area and in water and sand inrush working faces, the engineering criteria of preventing water and sand inrush are proposed. In order to minimize the influence of leakage from high pressure core aquifer under wedge water-preservation pressure structure on sand prevention safety coal(rock) pillar, the pressure-decreasing water pumping measures in face roof are formulated. The research results were applied to other working faces with thin bedrock and safety mining and remarkable economic benefits were achieved. |
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