Study On Deformation-failure Of Overlying Strata Induced By Coal Mining And Its Permeability Assessment

The coal resources are very rich in Western China, coal reserves in which account for around 62.7% of the national coal reserves. Coal mines in northwestern China produced 29.6×108 tons of coal in 2013, accounting for about 80% of the total production. The energy structure of coal serving as principal energy in our country is difficult to change in a long period. To ensure energy security, coal production in China is transferring from east to west. However, Coal-rich regions are usually located in arid and semi-arid areas in northwestern China, where water resource is in great shortage, only taking up about 3.9% of the water resource nationwide,and annual evaporation is 6 times the amount of rainfall. In general, This district is very poor in ecological environment. Super large working face and full-seam mining technology are widely used during coal mining in western China. Due to shallow buried depth of coal seam, thin overburden and thick aeolian sand above surface, overburden deformation-failure usually only forms caved zone and fractured zone on the condition of high strength and holistic mining. Consequently, aquifer structure will be damaged severely, causing waterbody in bedrock aquifer and quaternary aquifer seepage into goaf, which will lead to leakage loss of groundwater resources, water inrush and sand inrush disasters, and bring large disturbance to water resources system and ground surface eco-environment in mining areas. Therefore, the study on deformation-failure and permeability of overlying strata during coal mining can be of great significance in the protection of underground water resources for coal mining in the western mining area of China.Based on Daliuta coal mine in Shendong mining area, applied theoretical knowledge such as coal mine engineering geology, rock(rock masses)mechanics, rock hydraulics and mining engineering, experimental study, theoretical analysis, physical simulation test and numerical simulation calculation are adopted to study on the deformation-failure of overlying strata induced by coal mining and its permeability assessment. Evolution laws of full stress-strain-permeability in various lithologic rocks are revealed and three-dimensional coupling relation and model between rock permeability and stress-strain are founded. Laws of overlying strata breakage and fracture evolution are studied and change laws of stress-displacement and its control mechanism for super large working face are revealed. Permeability coefficient field distribution and its control factors are also revealed. Based on distribution characteristics of porosity and permeability of rock mass in goaf caved and fractured zone, theory and method of overlying strata permeability risk induced by coal mining is founded, and horizontal partition and vertical zoning features of overlying strata permeability are revealed. Subsequently, longwall mining-induced overlying strata permeability in study area is evaluated. Those achievements provide theoretical basis for coal mine safety mining and water resource protection in the western mining area of China. Through the above research, The following results and understanding are achieved.(1) By means of permeability tests of rock under triaxial compression, permeability laws during the complete stress-strain path in coal overlying rock are obtained. The research result shows that the permeability of the rock with different lithology is a function of strain in the process of complete stress-strain. Primary pores and cracks can be easily compacted on the stage of microcrack closure and elastic deformation, leading to an obvious decrease in permeability. When the stress reaches the ultimate strength, rock failure occurs and cracks go throughout the samples; meanwhile, the permeability increases abruptly and peak permeating occurs in most cases in the strain-softening period. The permeability behavior varies for rocks samples of different lithology. Under the same confining pressure and pore pressure, the maximum permeability of the different types of rocks studied ranks as follows: mudstone<sandy mudstone<siltstone<fine sandstone<medium sandstone and gritstone. Mudstone and sandy mudstone has the comparatively lower permeability and, therefore, can be regarded as the most waterproof stratum. Three-dimensinal permeability and stress-strain relations and model for fractured rock mass under three-dimensional stress are founded by using porous elastic theory, these lay a theory foundation for permeability evaluation of coal roof and floor induced by coal mining.(2) Taking sedimentary rock masses of main mineable coal seam roof and floor in Daliuta coal mine as research objects, geological models with sedimentary lithology combination structure are established. Laws of overlying strata breakage and fracture evolution are studied through physical simulation experiment, then influence rules of sedimentary lithology combination on overburden failure, abutment pressure distribution and fracture evolution are analyzed. Calculation model of height of water flowing fractured zone is founded based on experimental results and large numbers of field measured data. The experiment results are showed as follows.(1)It is found that there are only two zones forming after overlying strata failure induced by high intensity mining in Shendong mining area,that is, the caved zone and fractured zone. Water conducting fractured zone is obviously higher and controlled by coal-series strata structure. Underground pressure don’t decrease with mining depth shallowing, instead, it generate intense strata behaviors.(2)There is difference between two physical models in sedimentary lithology combination structure of coal roof, deformation-failure, abutment pressure distribution and displacement change laws of roof rock mass are all different, which mainly manifested as follows: the first weighting interval of main roof, periodic weighting interval and height of caved and fractured zone decrease from sandstone combination to sandstone interbedded mudstone; lead abutment pressure and scope of abutment pressure zone also decrease, and peak position of abutment transfers far away from coal wall;subsidence curve coordination of different height roof strata gets worse and the roof separation value gets bigger.(3)Fissure rate changes after overlying strata breaks, the distribution of fissure rate is saddle-shaped horizontally, fissure rate in overlying strata located goaf boundary is much larger than goaf central. Fissure rate in vertical direction gradually decreases in a logarithmic function relationship. The higher mechanical strength of overlying strata is, the bigger fissure rate is, and the bigger rotating angel of roof broken rock is.(3) Taking sedimentary rock masses of main mineable coal seam roof and floor in Daliuta coal mine as research objects, numerical simulation calculation for the mining of super large working face has been carried out by using FLAC3 D software, change laws of stress-displacement for super large working face are revealed. Compared to traditional working face, the roof subsidence of super large working face increases obviously, and increase range gets larger with the increasing of mining height. when the length of working face is increased from 240 m to 360 m,subsidence values of main roof located at 33 m behind the working face and 26 m above mining coal, and increase by 19.5% in mining height of 5m, 29.8% in mining height of 6m, 42.9% in mining height of 7m. In addition, lead abutment pressure peak also increases obviously with the increasing of mining height, and increases by 14.2% in mining height of 5m, 18.5% in mining height of 6m, 27.1% in mining height of 7m.(4) The finite difference method is used to simulate the influence of mining factors and geological factors on the movement and deformation and stress of surrounding rock of super large working face, and the change mechanism of stress and displacement is revealed. The calculation results show that the mining height and working face length are larger, requirement for the stope support system is higher, the surrounding rock control is more difficult. It is manifested that the lead abutment pressure peak value increases sharply and destruction rate of coal wall nonlinearly increases, and the influence of abutment pressure zone transfers to the deep area of coal pillar. Overall coordination of overlying strata movement increases with the increasing of working face length and advance speed, which also can reduce movement and deformation of overlying strata and increase the range of even subsidence area. Faults can lead to initial stress field disturbance, forming the low stress and high stress concentration zones around fault zone, which can cause roof to sink violently during coal mining and high stress gradient of coal pillar to form between the coal wall and fault formation.(5) A finite difference model incorporated with coupled stress-strain and permeability in fractured rock mass is developed. The model is then applied to simulate mining influence features of working face, and surrounding rock permeability distribution laws of working face and its control mechanism are obtained. Areas with increasing permeability in mining-induced rock mass is larger than that rock failure zone, and the increase of permeability is greater but with a smaller influence range in horizontal direction compared to vertical direction. The horizontal permeabilityincreased zone is right above and below the central goaf and looks like an oval horizontally. The maximum height of horizontal permeability-increased zone appears in the goaf center. The vertical permeability-increased zone is not only right above and below the goaf, but also 30 m away from the goaf boundaries. The maximum height of vertical permeability-increased zone is generally inclined above of the goaf boundaries and the height is relatively lower above the central goaf, showing a saddle shape vertically.(6) Permeability of seam roof and floor rock shows regular variation with the deformation and failure of rock mass during coal mining. Strata permeability decreases in the situation of initial stress area and advanced stress compression area because of the lead abutment pressure appeared in front of the working face. Permeability increases in direct damaged area of mining stress, and maximum permeability appears generally in this area. The stress of coal and rock masses recover gradually and approach virgin state of stress in stress recovery area, leading to gradually decreased permeability. Permeability recovery of caved rock in goaf has relation to ground subsidence, mining height, bulking factor and coal bed burial depth, and relation between permeability recovery of goaf caved rock and ground subsidence value is derived.(7) Based on the relationship between porosity and bulking factors and the subsidence laws of overlying strata, calculation models about goaf caved zone rock porosity and permeability coefficient are founded, and distribution characteristics of porosity and permeability of rock masses in goaf caved and fractured zone are analyzed. Consequently, theory and method of permeability assessment of overlying strata induced by coal mining is established, and partition and zoning characteristics of goaf overlying strata permeability are revealed. And also, the permeability of goaf overlying rock mass induced by coal mining in the study area is assessed. It is shown that permeability coefficient of goaf caved rock exhibit an shovel shape transversely, it can reduce by two orders of magnitude from the coal wall and the two roadways to goaf interior. Permeability coefficient of rock masses in goaf caved and fractured zone decreases in a negative logarithmic function relationship from mining coal seam to its upward. Goaf caved rock can be divided into excellent zone, preferable zone and medium zone of porosity-permeating properties along goaf strike direction. Rock masses in goaf caved an fractured zone can be divided into four zones from the bottom to the top, which respectively are extremely strong permeability zone, strong permeability zone, moderate permeability zone and poor permeability zone.(8) According to experiment of water exploration and drain in Huojitu coalmine field, porosity empirical value of goaf caved rock is obtained with 0.158 approximately. The relationship between goaf porosity and roof lithology is founded in Shendong mining area. Based on assessment of porosity and permeability in goaf caved and fractured zone, calculation model of goaf water storage capacity is proposed, and water storage capacity in No.2-2 coal goafs for different storage elevation at Daliuta coalmine field is worked out. Those achievements can provide theoretical basis for ground water protection using coal mine underground reservoir in Shendong mining area.