| Based on the generation and development of overlying strata frature of Longwall Mining Top Coal Caving with Split-evel Gate Roads (LTCCSGR), destressing results of protected strata, the establishment of Simultaneous Extraction of Coal and Gas system and optimization of mining technology, theoretical analysis, numerical modelling, physical modelling and field measurement were used to fulfill the research.First of all, the research on the stability and failure of the overlying strata of single working face of LTCCSGR and whole seam one-slice longwall mining (WSOSLM) was carried out, the following main conclusions were obtained:(1) Failure height of overlying strata is key to the gas drainage, therefore the failure height of the overlying strata of WSOSLM was determined first, the method of dividing the three zones is put forward based on the key strata theory. And the calculation was completed according to pratice, compared with field measurement, it is considered that the results of the new method is closer to the field measurement.(2) On the basis of determining the division of the three zones of WSOSLM, according to the characteristics of a single long working face, shown by lap joint The method of dividing three zones of overlapped favce of LTCCSGR is put forward for the first time.(3) For the study on the generation of fracture of overlying strata in along dip, the theory of beam breaking theory was used first, and the overlying strata of the first working face along dip was regarded as the two ends fixed support, the expression of the bending moment and the breaking criterion of the fixed supports are given. It is considered that the height of the overlying strata at the two ends of the working face is the same, and the expression of stress at any point in the working face is: σ=Mh’/J7=q/12(6Lx-6x2-L2)h’/J7And the criterion for generating cracks is presented by: 6x2-6Lx+L2+2(σ-X)h2/qtgφ≤0Since the initial working face is fixed at both ends along dip, it is considered that the height of f.racture at both ends of the initial working face is the same and the basically symmetncal.When mining succeeding internal-form working face of LTCCSGR without pillar, as no coal pillar between the two panels,the rock beam is equivalent to cantilever,and the internal stress is: σ=3ql2/h2(l/h+3ql4/2Eh4-1)The criterion for the occurrence of fractures in the roof is: 3ql2/h2(l/h+3dl4/2Eh4-1)≥(Ï„-C)ctgφOn this basis,the position of the f.ractures in the toof is given,i.e.the displacement S of rock stratum satisfVs: S=hε=3ql2.Eh(l/h+3ql4/2Eh4-1)In order to further show the influence of the mining on the stability of the overlying strata and the development of fractures,The mechanical model of elastic thin plate was used to build the model of the two sides of the working face so as to analyze the influence of internal stress and size on f.racture development,the following conclusions were obtainde:(1)Fracture on the two sides of the initial working face were found symmetrical, and the stress value was basically the same,maximum stress of two sides before the initial breakage of roof is: σ=-0.3qa2/h2The mining parameter of working face of the corresponding f.ractures is:After the roof breaks,the maximum stress along the dip of the working face is: σ=0.3378qa2/h2The mining parameter of working face of the corresponding f.ractures is:The stress of the main roof occurred before and after the breakage varies within a small range. It suggests that the development of the annular fractures on the two sides of the working face change only a little bit. When the succeding working face is mined, no coal pillar between two panels resulting in no new fractures on the top of the overlapped part, And along with the compaction of overlying strata, some fractures or cracks will be closed, two working faces reflect the characteristics of a single working surface which means fractured zone occurs within the two sides of the multiple overlapped working faces, the stress distribution is: σ=1.854qa2/h2The mining parameter of working face of the corresponding fracture is:After the first cave-in of the main roof, the stress distribution is:σ=1.962qa2/h2 The mining parameter of working face of the corresponding fractures is:Comparison between the two working faces, the fact was found that after miningthe succeeding working face, stress on the side of the solid coal is 6 times of the initial face, which is the same as the traditional coal mining method. And due to the cancellation of the coal pillar between panels, the stress on the side of the solid coal is larger, fracture develops more completely. (2) Further study on the development law of the lateral fracture in mining field wascarried out, during the initial working face was mining, the overlying strata cave in and are compacted, forming the "O" shape circle. During the mining of the succeeding working face, due to the cancellation of the coal pillar, caved zone of overlying strata and the initial working face form a whole, with the increase of the mining area, the whole gob is "O-L-O" shape.On the basis of the above research, further study on the pressure relief caused by the mining of protective seam between the protective mining with pillars and the pillarless mining is conducted, and the following conclusions were obtained:(1) In conventional protective mining with pillars retained, the protected area was directly affected by coal pillar size, if there is virgin in-situ stress zone in the middle of the coal pillar, it can be divided into four zones in the protected seams, the virgin in-situ stress zone, the stress increased zone, the partial destressed zone and the fully destressed zone.(2) While mining protective seam using LTCCSGR, due to the cancellation of the coal pillar between overlapped panels, the characteristics of a single working surface is reflected by multiple adjacent panels, the corresponding position in the protected seams exist only fully destressed zone, and affected by the impact of repeated mining, the destressing effect was more completely.(3) Further combined with roadway support with pillars, borrowing mutation theory, equation of scientific pillar size was derived. When yielded region of coal pillar retained exceed 88% of the width of the coal pillar, mutation would occur. Then from the perspective of fully pressure relief of the protected seam, it is benefit for complete and continuous pressure relief of protected seam to determine the mutation and failure of the pillar. The reasonable size of pillar can be given as: a=|25mξ/22flnfR+kt/kt[1+f/ξ-1)ctgφ]|The corresponding time point for the failure of coal pillar is: t=η/ElnE+λ/KdHaL=25mξL/22flnfR+kt/kt[1+f(1/ξ-1)ctgφ]η/ElnE+λ/KdmAdvanced rate of working face must satisfy: v≥2222E/25mξη/lnfR+kt/kt[1+f(1/ξ-1)ctgφ]lnE+λ/KdmOn this basis, the corresponding mining sequence is as follows:face 1â†'face 4 â†'face 2â†'face 5â†'face 3.When the corresponding destressed zone is included in the protective panel 1, then the protective panel 1 only forms destressed zonel, after mining working face 4 and working face 2, pressure relief zone 2 formed. When mining working face 5 within the protective seam, pressure relief zone 1-2 forms in protected seam because coal pillar fails between panel 1 and panel 2. Similarly, after mining working face 3, pressure relief zone 1-2-3 forms, considering the pressure relief effect in the protective panel 4 and 5, the region of continuous pressure relief zone will form in protected seam.Compared with the traditional road way maintanence with chain pillar, with the mining of panel 1, the range of pressure relief of protected seam increases from L’ =L-2hctgδ to L’=2L+a-2hctg5 after mining of the panel 2.If using LTCCSGR, compared with the mining with coal pillars.First, the recovery rate can be improved; secondly, there is no coal pillar support challenges posed by instability; thirdly, the effect of delaying of cave-in on destressing result can be avoided.In order to verify the above research results, physical modelling and numerical modelling were studied. Conclusions are as follows:(1) During the mining of protective seam, with the increase of leghth of panel along dip, the range of failure of overburden increases both laterally and longitudinally.(2) It is benificial to achieve continuous pressure relief of protected seam by using LTCCSGR, it increases the pressure relief range along dip, and the scope of protection elevates.(3) Retaining coal pillar to maintain roadway results in increased stress zone within the protected seam above the pillars. Complete pressure relief area, partially destressed area and stress increased are form with the protected seam along dip.(4) When using LTCCSGR, it is equivalent to increase the mining area along dip, fractured zone elevate, level of protected seam is lowered within fractured zone, which is good for stress relief.(5) In numerical modelling, it is discoveried that annular cirle occurs on both sides of the panel longitudinally; Laterally, overlying strata undergoes"O" shape changes.(6) By adopting LTCCSGR with negative pillar, during the mining of succeeding face, the overlapped part is gradually compacted, and annular fracture circle at both ends of the panel occurs and the altitude is higher than single panel; laterally, during the mining of succeeding face, overlying strata undergoes"O-L-O" shape changes.(7) For comparison, numerical modelling of 20m coal pillar between panels were conducted and found that annular fracture circle were at both ends of the panel and the altitude is the same but lower than that of LTCCSGR. As a result of comparison, it is believed that mining while remaining coal pillar need to arrange panel separately, while in LTCCSGR there is no this demand but to wholy arrange.Finally, through summary and overview for characteristics of longitudinal and transverse fracture and collapse in LTCCSGR, and synthetically considers several panels that compose overlapped part of LTCCSGR without pillars, system of gas drainage by ground drill was established, U+L system+gas drainage system of upward drill and roadway gas drainage system of high level suction. Overall, drainage gas system in LTCCSGR without pillars is simpler than traditional longwall mining with coal pillar retained, which can significantly reduce roadway work quantity.Finally, integrating practical engineering,8# coal seam was mined,2# coal seam was protected. In order to improve the situation of large amount of roadway tunneling, small range of destressing zone, and large amount of gas emission, first adopting LTCCSGR with negative pillar to protect overlying 2# protected seam was proposed. It realizes a small amount of roadway tunnelling, continuous destressing of protected seam. Combined with influence of output and rate of advances on gas releases, reduceing the panel distance of the protective seam along dip, optimization of reducing the inclined length of the panel of protective seam along dip (250m in length in the original design,125m after optimizition) increasing the amount of daily advance (daily advance distance is 4.2 m) was further proposed. |
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