| Based on the National Basic Research Program of China (973 Program) (2013 CB227903), development tendency and scientific production of coal in China were predicted according to the medium and long term strategy program of energy development. Before 2025, coal resource is still occupying the dominant position of energy production. Reasonable production is 3.3-5 billion tons. China must control her energy consumption at about 4 billion tons in 2020 and 4.5 billion tons in 2030, most of which are from the shallow coal seam in western China. "Three-dimention" is the developing direction of cities with intensive population. The development level of underground space is one important symbol to evaluate the city’s modernization.10 years in future is the golden times of subway construction and mining shallow seams. Structure formation and priority level of underground strata are proposed according to the property of rock. Research on the regulation of surface subsidence, temporal and spatial variation of strata movement, strata structure, movement velocity and stress change are beneficial to comprehending the source of ground pressure and key control area of excavation cavern so as to maintain the stability of excavation space and making ground pressure beneficial to us.By contrasting the stress, deflection and bending moment of different material’s bearing structure, structure characteristics and using its own nature of different structure are different. Disintegration route of underground strata is from shell to arch or board then to beam. In order to take full advantage of structure property of material itself, we need apply work or support to it to make the structure progress of material reverse from beam to arch or board then to shell. "Shellâ†'arch or boardâ†'beam" and their combination are predicted according to rock’s property of being good at resisting compression and failing at tension. These structures develop and range as face advances.Moving fields of subsidence and stress are proposed according to the characteristics of subsidence basin and stope stress. Moving fields of subsidence and stress are formed as face advances 4H/Ï€ gradually. According to the geological and mining conditions of shallow coal mine of liangshuijing coal mine, numerical simulation model, which has 500-meter-length,400-meter-width and 120-meter-height, is built to simulate strata movement progress from open-off cut to full subsidence. Formation and change of surface subsidence, velocity field and stress field can be reappeared. Three profiles of overburden strata are made to show the formation and change characteristics of displacement, velocity, and stress fields.Subsidence basin is gradually formed and subsidence curve is oval. Formation of subsidence basin can be divided into micro change stage, acceleration stage, insufficiency stage and sufficiency stage for analysis. Scope of influence to ahead of rib after subsidence field’s formation is H/Ï€ and 2H/Ï€ behind rib. The length of subsidence basin is L-4H/Ï€.Inflexion of subsidence curve starts from ten times of mining height (10M) away from coal rib. As face advances, inflexion transports to gob. Subsidence curve inflexion on the side of caving zone rapidly transfers to gob. When advancing H/Ï€, excursion distance is 8M. Then it gradually moves back. Excursion distance of inflexion is 3M in the gob. Excursion distance of inflexion in front and back of caving zone linearly transfers to goaf before roof failure. Then back inflexion hovers at 3 meters approximate equal to the thickness of the mining height (M) in caving zone. It is influenced by face advancing and fluctuates at 5M-12M in gob. Calculation formula of the advancing distance and position change of inflection point has been obtained; found that there is a Exponential function relationship between the distance and the position behind the goaf and a Power function relationship between the distance and the front position.According to roof and surface subsidence in combination with advancing distance and buried depth, strata movement can be divided into micro change (H/Ï€), mutation (H/π～3H/4), transition (3H/4～4H/Ï€) and stability (≥ 3H/4). In micro change stage, caving zone is small, deflection is small. Absciss layer develops. Displacement and stress is mainly in immediate roof. In mutation stage, "archâ†'beamâ†'oval curve" structure is formed in roof. In transition stage, displacement arch of different layers has different deflection. The closer the distance to coal seam, the higher the roof deflection is, the lower arch height is and the lower the rise span ratio is. It is called flat ach or parabolic arch. The higher layer, the lower deflection, and the smaller rise span ratio. It’s called circular arch. Strata structure from below to upper is flat archâ†'parabolic arch â†' circular arch. When circular arch develops to ground surface, it breaks into approximately elliptic curve.The abutment pressure fluctuates during the process of face advancing, the pressure coefficient fluctuate between 1.5～3.4, the peak position in front of the coal rib is between 0-35m. Pressure in middle part of working face is higher than which in the ends. Comparing with the ends, the peak pressure is not always consistence with the position. The influence range of abutment pressure is mainly located in the front of the coal 40m. The first weighting happens while the advancing distance is H/3-H/2, pressure reached the maximum while the distance is Ï€H/2, The middle part of working face should be focused while L ∈ (H,W). In the ends of working face, local maximum pressure appears while advancing distance is Ï€H/A, so the ends should be focused while Ï€H/A and L=W.Mobile velocity field is composed of velocity beam and velocity arch, generally beam on the bottom and arch on the top. The length of beam is between 30m-45m and the velocity distribution is arched. The central of beam body is higher than that of front and back of the area. The velocity arch of different rise span ratio acts on different position of velocity beam. Beam is far greater than the rate of the strata above the arch, the effect degree of mining field pressure is different, there are transfer block strata separation rate, the rate of arch beam the effect is more obvious. Formation rate from declining, the effect of boundary ellipse distribution, can be regarded as the inverse trapezium, the length of hemline is between H/3-2H/3 and the length of up boundary is between 2H/π～4H/Ï€, the boundary advance with the advancing of working face. The influence of the mined out area is a coal rear H/2 mining field, gob side 3H/4 of the rate of decay to zero.Combining with the mobile rate beam of immediate roof, whose length is in the range of 23-38m. The periodic fluctuation of the rate of immediate roof, with the advancing of working face, has a range of fluctuation as 10-25m and the average is 17.5m. The average periodic weighting length of working face in the field production is 16m, which is consistent with the simulation results.The distribution range and the change rule of mobile principal stress field and velocity field are relatively similar. The effect boundary in the range of 35m(H/Ï€) in front of coal and 80m (2H/Ï€) at the side of the mined-out area. The distribution pattern of the boundary is elliptical in two-dimensional and ellipsoid in three-dimensional. Meanwhile, the boundary is constantly advancing with the advancing of working face. The opening of arch of velocity field is upward while the opening of arch of principal stress field is downward. The mobile stress field is the combination of these two fields.Aiming at the strength and thickness characteristics of rock strata, with the combination of bore histogram and mining conditions of ten shallow seam working faces, the overlying strata of shallow seam could be divided into four kinds of strata as Weak and Laminated Strata (WL), Weak and Massive Strata (WM), Strong and Laminated Strata(SL) and Strong and Massive Strata(SM). Subsequently, combining these four kinds of strata with mining height (M), buried depth (H), length of working face (W) and then conducting principal component analysis to the support pressure in stope using these 7 impact factors; found that there is a severe collinearity between the buried depth H and the factors as (WL), (WM), (SL), (SM). Two factor model could be obtained. The mining height, buried depth and working face length as well as mining height, working face length, (WL), (WM), (SL), (SM). Model 1 obtained mining height M, length of working face L and the four kinds of strata, totally 6 impact factor. Model 2 consist of three impact factor as mining height, length of working face and buried depth. Calculation formula 1 and 2 could be obtained by conducting multiple regression analysis to stope support pressure, respectively. The formula show that the stope pressure has a positive correlation with mining height, working face length and buried depth, the weight of mining height is larger than working face length and the weight of working face length is larger than the buried depth. Analysis showed that these two formulas are pretty effective in shallow seam field (H<400m), Calculation accuracy of formula 1 is higher than formula 2. However, formula 1 can only be effectively used in shallow seam, formula 2 could be widely used in different buried depth for the calculation of support pressure. Calculation software of hydraulic support resistance was obtained using VB programming to the calculation formula of stope pressure. The software draw the correspondence relationship of support pressure and buried depth, working face length and mining height. |
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