| In the south of China,there exists a large karst area with coal seam mining,there are not only the Changxing limestone that exist in the roof of the coal seam,but the Maokou limestone exist in the floor of coal seam.The karst water existed in the floor poses a threat to the mine safety production,and the karst fissure development can destroy the ecological environment in karst region when coal seam mining.At present,the research on coal seam mining in karst area is focused on the influence of karst water to mine safety production.However,the surface ecological environment of karst area is fragile,and the coal seam mining has obvious influence on the shallow ecological environment of karst area.There are relatively few researches on the development of mining fracture and its influences on the surface deformation of shallow buried karst when there are karst caves exiting in the roof of coal seam.In view of this,from the perspective of ecological environmental protection in shallow karst area,this paper based on the national natural science foundation of china-Study on basic theory of coal mining for surface water environment protection in karst region of south China(51474104),by the research method of combining theoretical analysis,similar simulation experiment and numerical simulation,the coal mining working face stress features,regularity of strata movement,mininginduced fracture height and shallow buried karst cave surface subsidence mechanism have been studied and discussed,the main works and results are as follows:1)The breaking location and vertical fracture of strata have been analyzed by elastic foundation beam theory and plastic hinge,and the diversion fissure zone development has been studied by numerical simulation.The research results show that,the diversion fissure zone development should consider the structural differences of overburden,and in consideration of the influence of the key strata on strata movement,vertical fracture of key strata located upper part of the fracture zone can be analyzed by plastic hinge theory,thus,the water transmitting ability of the key strata can be determined,then the upper boundary of the diversion fissure zone can be judged.Based on the mining overburden fracture and the development rule of diversion fissure zone,combining the relative location of karst cave and mining working face,the generalization model of karst cave under mining influence was established.2)By the similar simulation experiment for strata movement when mining in karst region,the influence of karst cave on mining fracture development has analyzed.And the results show that,when there are karst caves in diversion fissure zone,the karst cave has a certain orientation and attraction to the development of mining fracture.For lack of adequate support,the subsidence of caves roof strata has an abnormal increase at the top of cave,which makes the mining fracture developed to the largest at the top of cave.In the horizontal direction,the karst cave has an obvious hindrance to the influence of mining.When the coal mining is only on the side of the cave,the mining side strata are strongly influenced by mining,and the influence is not obvious on the mining strata near integrated coal.The subsidence of mining side strata is significantly larger than the subsidence of the strata neat integrated coal,which result in incongruous deformation of the two sides of karst cave.3)The stress characteristics of working face and the development law of mining fracture when mining in karst area are simulated by discrete element numerical simulation software.And the simulated results show that,Compared with non-karst regions,the periodic caving span of working face in karst area is smaller than that of non-karst region,it is more frequent.When there is a karst cave with a certain distance from the upper boundary of diversion fissure zone,the height of mining fracture above the top of karst cave increase sharply and forms a "spike".And compared with the strong mechanical disturbances such as mining effects,the development of rock fracture is mainly controlled by the mechanics of rock mass,karst water has a relatively small influence on the development of mining fracture.With mining disturbance,karst water loss rate which caused by the development of mining fractures and karst fractures is far greater than that of karst water;when the karst cave is in shallow burial condition or water supply is low,the mining disturbance can cause the negative pressure inside the karst cave and accelerate the collapse of the karst cave.4)When there is combined action of the mining action and the karst water pressure,the karst fissure is more developed,and it is more likely that the mining fracture and the karst fissure will penetrate.Under the disturbance of mining,different karst water pressure leads to karst water different loss rate.That is when the water pressure value in karst cave is less than the security head value,the karst water leakage rate will at lower levels,once the water pressure value is more than security head value,The leakage rate of karst water will increase sharply,which not only affect the mine safety production,but also the shallow water ecological balance.And accordingly this,the mining method of “draining and anti-hydraulic pressure mining” and “local grouting and release water press mining” have been put forward.5)A numerical simulation is used to reconstruct the collapse of the shallow buried karst cave,the surface subsidence process of shallow buried karst cave can be divided into four stages: The latency stage of karst cave,the crack expansion stage of karst cave and forms the open karst cave,the development stage of the soil cave,karst ground collapse stage.With the mining disturbance,the forming conditions of the open rock karst caves,the ultimate developmental height of the soil caves and its influence factors,and the extreme development depth of the ground fractures have been analyzed theoretically.It is suggested that if the shallow buried karst caves collapse under the mining disturbance are not developed to the surface,the thickness of the covering layer should be greater than the sum of the ultimate developmental height of the soil caves and the ultimate development depth of the ground crack.6)According to the thickness of covering layer,the distribution of shallow buried karst cave and the layout of mining working face in PINGHU coal mine,shallow buried karst collapse prone area has been prejudged,which is consistent with the surface damage serious area in the actual production process.Based on the actual mining conditions and combining the research results of the paper,the working face under the shallow buried karst area of PINGHU coal mine have been optimized,the recovery rate of the working face is improved.Based on the previous mining experience,shortwall stripe-and-pillar mining with backfilling have designed to solve safe coal seam mining which is near the below the karst layer of PINGHU mine. |
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