Study On Mining Pressure Behavior Of Zone And Fracture Characteristics Of Roof In The Condition Of Residual Coal Mining Face Passing Through Roof Falling Zone

Reasonable and efficient mining of residual coal resources is of great social and economic significance. It is an important way to achieve the sustainable development of coal energy and solve the problem of low recovery rate of coal resources as well as extend the service life of coal mine.In this paper, Shenghua Coal Mining Face 1301# is selected as the object of study, in order to provide a scientific basis for surrounding rock control of residual coal mining face, the type of roof falling zone is classified, mining pressure behavior of the zone and fracture characteristics of roof in the condition of residual coal mining face passing through roof falling zone is also studied based on on-the-spot investigation and data analysis, combined with the method of theoretical calculation, physical analogy simulation experiment and on-thespot measurement. The specific research contents and conclusions are listed as follows:1.The caving type caused by previous mining is, based on the preliminary field investigation, summarized and divided into following six types: along the bottom holding the top coal seam falling caving zone, along the bottom holding the top direct roof falling caving zone, along the bottom holding the top all fall caving zone,along the middle holding the bottom coal seam falling caving zone,along the middle holding the bottom direct falling caving zone, along the middle holding the bottom all falling caving zone.2.Physical analogy simulation experiment shows:① The basic roof along the edge of the falling zone is easily fractured, which can furtherly cause a advanced fracture of large thickness and long span when residual coal mining face reach a certain distance away from the roof falling zone. ② The working resistance of residual coal mining face present irregular characteristics. One of the most influencing factors is the advanced fracture. The ranges of pressure distance is 9.2 m ~ 26.3 m, the pressure intensity is 8508 KN ~ 16200 KN, the ranges of roof falling zone height is 24.3 m ~ 32.8 m according to the simulation computation. Second cycles pressure strength caused by the advanced fracture is 16200 KN, which is 3.21 times the one in initial mining. So the supporting measures should be well done before actual mining. ③ The working resistance of residual coal mining face increases with the decrease of distance between the support and the falling zone. Peak resistance appears where the intermediate coal pillar completely rupture or the area of working face breakthrough the roof falling zone. Its value increases with the raise of the span of roof falling zone. In the condition of residual coal mining face passing through roof falling zone the working resistance will decrease because the top coal can’t make a complete compaction with the support beam.3.Mechanics model of roof fracture shows that that roof falling zone can weaken the transfer of mining pressure along the cantilever beam and the basic roof is easily broken before it across roof falling zone with very long rock produced meanwhile. Then the support will be subjected to more loads, which is a dangerous state. Based on the theoretical analysis above, the computing method about working resistance of support are obtained in the condition of residual coal mining face passing through roof falling zone and the requirement for fracture but no instability of basic roof so as to establish a theoretical basis for the safe passing of residual coal mining face through roof falling zone.4.On-the-plot measurement showed: ① Roof of residual coal mining zone may rupture in advance, then the broken roof would sink along the front of working faces, the pressure front pillar of the support afford would increased, and which may finally lead a serious unbalance loading and distribution of hydraulic support. On-the-plot measured data shows that the stress of the back pillars is only about 13.8% of the front pillars. ② Because of the pressure forced by the roof of residual coal mining zone is irregular, the standpoint that force loading conditions of back support pillars should be used as the criteria of discrimination for the characteristics of roof pressure is proposed according to the fact that the pressure back pillar afford is zero, which is greatly different with that of front support pillar. ③ Along the working surface, the hydraulic support is under complex conditions of surrounding rock. It is big in the middle and small at both ends. ④ Working resistance of support is obviously increased when the residual coal mining face draws near the roof falling zone(3~5m), there is a positive correlation between resistance and width of the roof falling zone. And if the support is just working at the roof falling zone, its working resistance will decreases greatly.