| In China, the exploitation of coal resources in Shendong area has made great contributions to the development of the national economy. Currently, the conditions of the coalfields in the region are mostly shallow and thin bedrock and thick unconsolidated layers. And the advantages of coalfields are small dip angle,large thickness of the coal seam and simple structures. In the region, it forms the characteristics of high strength exploitation such as for one-time to mining the overall height and the moving of the working face rapidly etc. However, under the condition of the mining environment and mining intensity, the fracture of coal and rock which is shallow buried and thin bedrock usually forms cracks which connect the working face and the layers of loose and loaded, even lead to longitudinal penetration on the surface of the ground. If the loose and loaded layers are sandy and rich in water, the working face will occur to huge accidents, liking the leaking of water and sand. It will restrict the high production and efficiency of the mine. If you can master the transfixion rules of the cracks which are under the conditions of shallow and thin bedrock, and put forward control countermeasures, it will be great significant for the types of mine which will be mining safe,highly effective and green. The paper based on the working face of the fourth group in Changhangou coal which is thin bedrock and shallow buried, it used the methods of the analog simulation and numerical simulation and mechanical calculation and in-situ test to analyze the rules of the developing process of the cracks in mechanics. And also obtain the following main conclusions and innovations:(1) Master the structural features and stability of the overburden rock in Chang Hangou four coal mine groupThe sedimentary environment of Chang Hangou four coal mine group is mainly middle and lower reaches of the river flood plain sedimentary environment. Within the study area, based on the ranges of sand and mud of the coal-forming environment, we divided intoⅠ~Ⅴgrade five sedimentary facie rock stability partition. TheⅠ~ Ⅴgrade coal district of Chang Hangou four coal mine group are distributed, and to gradeⅢ~Ⅴare the main partition. Looked at the results from the core, the distribution of the coal mine rock of Chang Hangou coal change little. There occurs fine-grained sandstone rock above the roof, above that is the medium-grained sandstone. And the medium-grained sandstones contain multiple thin layers of mudstone interlayer, local coarse-grained sandstone containing.(2) It reveals the evolution law of the face longitudinal through cracks in shallow bedrock.Using the way of similar simulation analysis, we get that the evolution when the face get through the longitudinal crack can be divided into four stages, such as:① Crack embryonic stage:After the basic roof first collapses backward, with the face advancing, it began to appear the longitudinal fissures in a certain range of the face, and with the gradual expansion of mining until the load layer through the face, accompanying the beginning of the first pressing cycle.②Crack opening stage: In the process of first cycle compression, the overlying bedrock longitudinally through the cracks of mined-out area to the direction of the axis of rotation gradually. When the overlying rock rotating, the opening degree of longitudinal fissures do not increase until the overlying rock goes back to the limit. When the mining height gets larger, the periodic weighting step will be smaller, the broken rock collapse expansion coefficient will be smaller, and the degree of opening of the longitudinal fissure will be more obvious.③Fissure rapidly closing stages: With the face advancing, especially when the mining height is large, the rock cuts off in the rear of the bracket when the first cycle pressure occurs. Bedrock overall cut off and reverse rotation, combined with rock collapse backward irregularities hulking structure and fracture surface, resulting in longitudinal through cracks quickly closed.④ Fissure compaction stage: As the second period began to pressure due to the rotary movement of overlying strata adjacent rock collapse has squeezed effect, resulting in longitudinal through cracks is further compacted.Using the way of similar simulation analysis, we work on shallow bedrock surface overburden longitudinal through cracks open, and the closing process research and further detailed analysis of the working face through longitudinal fissures opening and closing phenomenon. The basic structure of the top in the face advancing the state of the process of the formation and destruction and instability determines face longitudinal through crack propagation process of development. In the periodic weighting process, due to the rotation caused by the basic roof rock overlying rock fissures longitudinal dimension increasing, with the breaking of overlying strata overall cut off and reverse rotation occurs between the longitudinal through cracks began separating unit body re-engage, can be seen through the longitudinal fissure has been closed. It took place between the unit body and body when the next periodic weighting in breaking of overlying strata adjacent rotary compression has been in contact with a certain degree of re-embedding. Longitudinal through cracks is further compacted. Face value calculated longitudinal fissure through the entire process of opening and closing is similar to the simulation results basically the same.(3)Elaborating the reason of the collapse of sand and water from mechanical mechanismBy a shallow buried thin bedrock overburden rock periodic critical breaking the mechanical analysis, we found that the process of face advance, before the press after a cycle, the key rock before conducting a periodic weighting will cut occurs off phenomenon. The key role of rock tip angle as well as by the horizontal shear force and down, cut behind the structure of their formation can be simplified to "rock step beam" structure. Therefore, mechanical model, a detailed analysis of key features breaking rocks periodically, which explains the evolution of Face longitudinal through cracks from the mechanics.Face longitudinal through cracks appeared in the rock began periodically breaking key. Face in continuing to promote the process, the size of longitudinal through cracks as dynamic changes between fault block suffered broken horizontal force and displacement amount of rock gradually extended. When the crack propagation extreme limit, leading to pressure in the second period of the occurrence of critical rock cut off at the same time through the longitudinal fissure quickly closed after the through cracks is further compacted in the next crack of opening and expansion process.(4) Face made of sand collapse and collapse water control method①Arrange face advancing speedBasic roof weighting step cycle length directly affects the rotational angle of the rock breaking size. Periodic weighting step distance is longer, its rotation angle is smaller and face longitudinal fissure through the lower the degree of development. For periodic weighting step length of the face, preferably Swell degree bedrock fracture location or extent of the fracture surface irregularity is large, longitudinal cracks can think through, it may be appropriate to slow down the speed of face advance in this case. However, most in shallow bedrock Face periodic weighting step distance is relatively small, longitudinal through cracks open inevitable. Former should control certain face advance rate to pressure coming period, using grouting and other means "blocking" to live through the longitudinal fissure. After the press cycle, longitudinal through cracks quickly closed, it can handle. Meanwhile, before the advent of rock pressure cycle, should be appropriate to accelerate the speed of face advance, Face prevent longitudinal through cracks opened in the maximum advance is stopped, to minimize sand collapse and collapse of water accidents.②Increase the hydraulic supporting forceA lot of research and engineering practice shows that the overall cut off the bedrock in shallow thin bedrock mining coal and rock breaking basic form of movement. Cutting off the bedrock is inevitable, but the increase in hydraulic support supporting force can effectively delay the extent of expansion through the cracks. Therefore, engineering practice, shall ensure that hydraulic support working resistance and active area, try to slow down the degree of rotation bedrock cut off before the rock such that the longitudinal opening through cracks in a smaller size after a direct and rapid cut off, thereby enabling longitudinal through cracks quickly closed. Meanwhile, the need to suspend advance Face overhaul, grouting and other processes also try to arrange the crack closure phase. In addition, from the geometric relationship and on roof support beam of view, we should pay attention to the support beam supporting force, weakening the protective effect of the beam bracket.③Water-rich region limiter thick miningThe degree of filling of mined-out area directly determines the rotation deformation space rock breaking block body. The extent of the lower strata of rock bit filling and cutting height is directly related. For thin, medium thick seam depth and larger mines, mining height limit can be implemented to reduce the rotational deformation space broken bedrock fault block, thereby controlling the development of longitudinal fissures. But for high-intensity mining shallow cover and thin bedrock thick coal seam face, this approach is difficult to work unless the cost is enormous. From sand to prevent collapse and collapse water perspective, mining can be a full-height, fast forward and so on. The key is to take appropriate measures, as far as possible to promote rapid closure of the longitudinal fissure.(5)The formation of a working face water collapse and collapse sand control methods and engineering practiceWith seam mining, overlying strata fissures turned loose water loose layer, the upper layer of loose water will flow from top to bottom along the water flowing fractured. Reliable bracket support force, to ensure the safe passage of Face rock fracture, avoid large penetrating cracks; And accelerate the speed of face advance, but also make breaking rock face in the side as soon as possible and not broken rock at the lower end of the hinge section and pressed close, and further compacted in the overlying load, the rate of crack formation permeability, hydraulic conductivity decreased. Accordingly when mining coal mine 201 Han Chang Face, periodic work in different areas to pressure position to a position near collapse if there is pressure sand water accidents or sand collapse and collapse water signs the statistics. Statistics show that in order to arrange face advance rate, increasing hydraulic supporting force consisting mainly of sand collapse and collapse of a comprehensive water control methods, working surface during collapse sand hidden water does not appear to promote, achieve a safe mine, efficient mining. |
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