| This paper takes Liujia Coal Mine as practical engineering background, comprehensively using the theoretical analysis, the computer numerical simulation, the indoor similar simulation,the field pressure observation and some other means, the overlying strata structure movement and the strata-pressure behavior of close distance coal seam mining have been further researched, getting the following conclusions:1. The mechanical model of roof breaking and instability characteristics(1)According to the overburden structure distribution of Liujia Coal Mine, the overburden structure under gob area is divided into two categories: no basic roof between coal seams, with basic roof between coal seams.When there is a basic structure between coal seams, the roof will have two forms of "Masonry beam" and "cantilever" after breaking.(2) According to the sheet mine pressure theory of working face, this paper had do some research about the roof breakage mechanics model, and had built many elastic thin plate in different supporting condition based on the roof strata’s different movement characteristics at different time. At the beginning of mining, the roof strata can be seen as a four edges clamped plate before the basic roof fractured. And when the basic roof had fractured, the basic roof can be thought as a trilateral clamped one side simply supported plate. For the working face during mining, before the first fracture of basic roof, we can see it as a trilateral clamped and simply supported plate, after fracture as two adjacent edges clamped and two adjacent edges simply supported plate.Through the energy principle and ANSYS numerical method, this paper have researched four plate models, and calculated the roof bending moments.(3)After building the basic roof fracture mechanics models, the conditions of basic roof for the first time and period breaking were calculated, so the conditions of the basic roof breaking with "Masonry beam" structure or "cantilever" structure had been received.2. The laws of pressure behaviors of mining face in close distance coal seam.By summarizing the pressure behaviors of three representative mining faces in close distance coal seam, and comparing the differences between them, the character behaviors of three representative mining face in close distance coal seam have been received:(1) The pressure behaviors of mining face in close distance coal seam is not obviously, and the dynamic load coefficient is low with mining pressure, it’s average is only 1.182. So, the increasing range is small of hydraulic support’s working resistance.(2) The weighting distance of mining face is uneven, which appears the periodic variation of long-short weighting distance. This circumstance is related to the periodic instability of basic roof. The periodic weighting distance of mining face is not long, the average is 19.6 meters.(3) The relation of first roof weighting in mining face and interlayer structure is closed. When the interlayer distance is short and which is only existed in immediate roof, the first weighting interval is short, and it’s no obviously different to the periodic weighting. When interlayer distance is long, the first weighting distance will be long.(4) If there is hard basic roof between coal seams, the working face will be under the double pressure of the upper layer and basic roof, the pressure increases than that with no hard basic roof between coal seams.3. The moving characteristics of overlying strata of close distance coal seam mining(1)The moving characteristics of overlying strata of the first mining coal seam is great different with the below coal seams. When mining the first coal seam, the moving regularity of overlying strata is similar to mining the single coal seam。With the advancing of working face, the overlying strata will go though these stages of immediate roof first falling, basic roof breaking, basic roof periodic breaking.(2) When mining the 2# coal seam which is very closed to the 1# coal seam( the nearest distance is only 2m), the interlayer will fall immediately because of there is no tough rock layer, so, the roof weighting of mining face mostly depends on the 1# coal seam’s basic roof structure. With face advancing, the broken lower basic roof occurs rotation, accompanied by slipping instability, resulting in face weighting, therefore, the upper basic roof breaking and the broken basic roof instability play a major role on 2 # seam face weighting.(3) Because there is a stability sandstone structure between the 2# coal seam and 3# coal seam which can form the basic rock layer, so, when mining the 3# coal seam, the weighting of working face is influenced by two factors: the periodic breaking of 3# coal seam’s basic roof, the instability of upper coal seam’s basic roof.4. The stress distribution of close- distance seams(1) The stress distribution of current coal seam and the lower coal seam has the following rules when the first coal seam is mining:①In the beginning of mining, the supporting pressure peak value in front of the working face gradually increases along with the advance of working face, vertical stress gradually reaches the maximum at 70 m advancing distance, and subsequently the supporting pressure in front of working face decreases and gradually stabilizes.②The supporting stress in caving rock compaction area of the gob center gradually increases along with the working face advancing; and when the working face advances to 110 m, the caving rock is compacted in the range of 60 m in gob center, the stress peak in compaction area exceeds the supporting pressure peak value in front of the working face.③The lower coal seam is less affected by mining than the upper coal seam, the supporting pressure in front of working face and the compaction area of goaf are both lower than the upper coal seam in the advancing process, and the influence degree decreases along with the increase of the distance from mining coal seam.(2) The lower coal seam stress distribution has the following rules:①Since the upper seam has been mined, the lower coal seam would be located at the upper coal gob area, and the distribution of supporting pressure would be significantly different. The working face would be gradually affected by stress reduction area, then the caving rock compaction area, then stress reduction area.② Along with the working face advancing, the overburden strata will be re-compacted in the coal seam working face gob, and gradually formed a new abutment pressure zone. The peak pressure is significantly less than the front of the working face and the compacted area of the gob.③The influence on 3# coal seam deceases, with the supporting pressure less than 2 # coal seam, close to the original rock stress level, and the increase of supporting pressure front of face was not obvious.④ The stress distribution of 3 # and 2 # coal seams is closer, since the stress of upper goaf compaction zone reduces, the work pressure peak in front of face is further reduced, but the stop mining coal line stress continues to increase.(3) The pressure behavior law of lower coal seam mining with different interlayer spacing:The pressure behavior law of lower coal seam mining with different interlayer spacing is similar, while the overburden movement characteristics have significant differences, when the interlayer spacing is small, there is only direct coal seam roof, with the interlayer spacing increases substantially, when there is hard interlayer roof, the face weighting is not only influenced by the upper layer, but also by the basic roof of the seam breaking effect.5. Indoor simulation experiment research of close- distance seams(1) The initial caving distance of immediate roof and initial fracture distance of basic roof of lower coal seam are less than the upper coal seam; In terms of the upper coal seam, when mining arrives at about 40 m, immediate roof caves for the first time, and when mining arrives at about 50 m, basic roof cracks for the first time; The immediate roof initial caving distance and basic roof initial fracture interval are about 30 m and 45 m respectively.(2) With the advance of working face, the supporting pressure peak value of coal wall supporting area in front of working face increases gradually, pressure relieving range and degree of the abscission zone behind goaf increases gradually; meantime, because of the overburden strata above goaf gradually touch the gangue and turn compacted, the compaction area stress reach the original rock stress level when the working face advance to 110 m.(3) In process of lower coal seam working face advances, supporting pressure peak value of the area in front of coal wall rises, but as a result of the lower coal seam located under the upper goaf, the upper roof compaction range is small, so the lower coal seam working face is in the stress reduction area on the whole, and stress peak value is less than the stress peak in the front of upper coal seam working face with the same mining distance.(4) Because there is a stability sandstone structure between the upper coal seam and lower coal seam, the weighting of working face is influenced by the periodic breaking of current coal seam’s basic roof, the instability of upper coal seam’s basic roof.6. The research of support’s reasonable working resistance and adaption in close-distance seams’ working face(1) Based on the structure model of three coal seams, and analysis of basic roof’s breaking characteristic, we built the model of support-surrounding rock in the three coal seams’ working face, and established the support’s working resistance formula in different models.(2) Based on the practical tested statistic data of pressure in WⅡN36-6 working face, according to the loading estimate method and measure method in working field, we established the support’s reasonable working resistance that satisfies the safe production.(3) Analyzed the load characteristics and adaption of support which is using in working face. By analyzing the acceptability and average of origin resistance we find that the support’s origin resistance in working face is lower, which can’t get to the specified origin resistance 75%, and the acceptability is less than 65%, which is far away to the requirement of support. So we have to take some effective methods to promote the support’s origin resistance and acceptability.(4) The distribution of support’s circulation terminal resistance is nearly revealed with the shape of the front is lower and the behind is higher, which means the lower working resistance just take small percentage while the higher working resistance especially that can get to the 90% specified resistance take great percentage. Which means the support’s resistance has not enough and to spare, but the percentage of the support’s resistance surpass the specified resistance is low, so the support can satisfy the production in working face.This paper takes Liujia Coal Mine as practical engineering background, comprehensively using several means, classifying the overlying strata structure, researching the overlying strata structure movement and the strata-pressure behavior in working face of close distance coal seam mining, and researching the basic roof’s breaking unstable stress model, the support’s working resistance and load characteristics, etc, and getting amount of useful conclusions. |
PDF Full Text Request