| Mining under water body is a serious problem in China. The water prevention and control technology of intensive mining under thick soil layer and thin bedrock conditions have great influence on mining schemes in eastern mining areas. With the development of mining techniques for thick/ ultra-thick coal seams, water-preservation mining in western mining areas will encounter new technical challenges and require relevant theoretical breakthrough. The prediction of overlying strata failure caused by mining and the judgement and transformation of aquiclude are two key points of mining under water body. Systematic research was done in this paper based on the fact that overlying strata failure rules of full-seam mining for thick coal seams(including top coal caving mining and large cutting height mining) have no unified conclusions and the lack of research on clay aquiclude at the bottom of quaternary strata in eastern mining areas. By means of similar simulation, numerical simulation, field measurement, laboratory experiment, as well as statistics, grey theory, catastrophe theory, mechanics, mining science, geological subsidence science, this paper summarized the overlying strata failure rules of full-seam mining for thick coal seams, proposed the instability criterion of water-resisting property of clay aquiclude. The research results were applied to the mining practice of thin bedrock and shallow-buried coal seams under Weishan Lake in New East Four mining area of Yaoqiao coal mine. The main contents and conclusions are as follows.(1) The overlying strata failure rules and theories are improved.(1) Based on 153 series of measured data, The formulas for fully-mechanized caving mining and large cutting height mining(mining height is more than 3m) are proposed, and the selective methods of protective layer for high seam mining are proposed.(2) New methods to forecast the integrated mining thick and the heights of “two zones” of close distance coal seams are proposed. As a genereal rule, we can use to calculate the integrated mining thick, when the seams are extremely close(0≤h≤M) and the interval of mining is within half year, the formula (54)= ∑ 4) is more appropriate. Using the integrated mining thick (54), the heights of “two zones” of the ith coal seam can be forecasted, but the final heights of “two zones” of close distance coal seams mining are the highest elevation.(3) The overlying strata failure height is measured, which is caused by mining operation of lower close-distance coal seam group(#8 coal seam) under the goaf of #7 coal seam in Datun mining area. The caving zone(CZ for short) height of 8503 work face in Yaoqiao coal mine is 16.5 m. The water flowing fractured zone(WFFZ for short) height of 8172 work face in Xuzhuang coal mine is 84.74 m.(4) The overlying strata failure height and rules are no obvious differences between fully-mechanized caving mining and large cutting height mining for thick coal seams. With the increase of mining height, the resulting heights of “two zones” are not completely in conformity with the function type, and caving height is in linear relationship with mining height. Mining height and method are main factors controlling overlying strata failure height.(5) According to the redistribution rules of initial fissures and roof stress in overlying strata, “two zones” will be saddle-shaped. Inverted funnel shaped “two zones” proposed by some scholars based on similar simulation is incorrect because the rock strata simulated in experiments are assumed to be complete, while the rock mass in practical engineering often has weakness planes.(2) Mining deformation and instability mechanism of clay-rock structure under thin bedrock and thick soil layer conditions are studied.(1) The similar simulation test shows it is easy to form separated layers or fault space between sliding surface of subsidence basin and contact surface of clay. In this case, clay is likely to be instable, which is caused by shear and tensile forces. The significance order of affecting factors leading to separation layers and fault space △W is U> 4)> ε>h> K> fault> W. Because of the limitation of similar simulation test, the influence degree of mining height M and faults is ignored and weakened.(2) The thin bedrock roof is “all-soft overlying strata” roof type, where key strata do not exist. No matter it is sliding instability or deformation instability, bedrock has stress concentration to clay at the fracture point of clay-rock contact surface. Stress concentration can easily cause the instability of water-resisting property of clay. To avoid continuity damage in clay caused by shear fracture, the thickness of clay served as stable aquiclude needs to be more than 1 times the mining height M(the max thickness of △W).(3) The fold catastrophe model of clay-rock structure under static load is established. Catastrophe theory is as follows: clay catastrophe instability is decided by shear modulus and thickness between clay and rock strata working on it. In quasi-static state, when the shear modulus is larger and the thickness is smaller, the clay is more likely to be instable. When the clay thickness is close to or even larger than that of the rock strata working on it, and the shear modulus is smaller, the clay will not reach catastrophe instability, but gradually lose stability through creep deformation. The dynamic load that bedrock worked on clay during mining can increase the probability of the occurrence of clay catastrophe instability.(3) Compared with shallow clay, density of deep clay(depth is more than 70 m) is larger, while porosity and water content decrease. However, its mechanical parameters don’t change obviously. The clay state(liquidity index) decreases obviously with the increase of burial depth. When burial depth is beyond a certain range(the depth is different for different mining areas, for example, the depth of Haizi coal mine is 80 m and Xinglongzhuang is 70 m), the liquidity index is almost 0. In this case, the clay is in hardplastic or semi-solid state and has higher water resistivity.(4)Influenced by mining, horizontal migration occurs between strata in loose soil of the quaternary system. The direction of horizontal migration is random, and the amount increases with the decrease of the distance to the working seam; the aquifers exhibit compresses deformation, and the aquicludes exhibit swelling deformation due to mining in the loose soil. As a result of mining-induced fractures, aquifer was dewatered and pressure was released, leading to the increase of effective stress which is the principal reason of aquifer compression; the swelling of aquiclude in touch with water is mainly caused by the hydration expansion of smectite in clay. Omitting the small amount of horizontal strain in clay caused by overlying stratum, the stratum displacement and deformation parameters obtained by probability integral method can be used to estimating the displacement and deformation of clay.(5)The fluid-solid coupling numerical calculation model of stope roof was built, and the results are: the physicochemical property of rock-soil was changed because of the stope roof stress filed redistribution and rock soil layers movement and destruction, which then led to the change of the seepage filed; the water-resistivity of clay layers did not change abruptly but gradually due to its integrity failure, and it can be self-healing to a certain extent; under special conditions of mining and hydrogeology, the interaction between fracture propagation and water-resistivity in clay layers has a major influence on the final state of seepage field.(6)Quantitative criteria of clay impermeability were made(1) As a result of movement and deformation of strata, the state of clay layer has changed dramatically in terms of shearing and tensile deformation, or even destruction. Ultimate deformation experiments were designed for 10 different clay samples in 6 mines. Experiments show that: the change of clay layer water-resistivity with the increase of deformation includes maintaining stage, descent stage and losing stage, which is bounded by “abrupt node” and “losing node”. In “abrupt node”, the ratio of tensile length and thickness λL is 0.120, and the ratio of shearing length and thickness λJ is 0.105; in “losing node”, the ratio of tensile length and thickness λL is 0.156, and the ratio of shearing length and thickness λJ is 0.135.(2) Combined with soil test and the parameters of stratum movement and deformation, the quantitative criteria of clay impermeability were made by four indexes included 、(6、(7and. According to the criteria, clay impermeability is divided into 3 states:a. water-resisting property keeping state:b. water-resisting property losing state:c. no water-resisting property state:(7)Microscopic scanning technique can be applied in the study of the mechanism of clay permeability variation. Through the comparison analysis of the porosity change before and after deformation of clay samples, it shows that: the process of losing waterresisting property in clay is the increment of new fissures and the development of primary fissures; the fissures of different clay propagate in different ways after deformation: generally clay with high viscosity is likely to undergo native crack extension, while clay with low viscosity undergoes both native crack extension and new crack increment.(8)The calculating method that was used to compute the height of overburden fractures and the failure criteria of clay water-resistivity were applied to the engineering practice and verification in Xindong #4 mining district of Yao Qiao coal mine,and it showed that: the predicted height of overburden failure by the formula of caving "two zones" is within the upper limit which is replied by commission of Jiangsu province for #7, 8 coal seam in Xindong #4 mining district of Yao Qiao coal mine. For coal seam No.7, when the mining method is full-mechanized caving or full-thickness mining, the upper limit of coal pillar for quicksand is-157 m in elevation, and rock pillar is 28 m; when excavating merely bottom layer(mining height of 2.5 m), the upper limit is-135 m, and rock pillar is 15 m. For coal seam No.8, when it is near H43 or in the east of FW3 fault, the upper limit of full-seam mining is-173 m, and rock pillar is 15 m; for other areas, the upper limit is-140 m, and rock pillar is 42 m; if limited-thickness mining(mining height 2.5m) is adopted, the upper limit is-135 m, and rock pillar is 37 m. In the Xindong #4 mining district of Yao Qiao coal mine, after deformation of the critical layer for water-resistivity in loose layer due to mining, the four indexes AL, AJa,AJb and AW are lower than the limit of water-resisting property losing state. The clay layer maintains good water-resistivity and can guarantee safe mining under large-scale water body of Lake Weishan. |
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