| China's geological structure is very complex,most of the coal seams are composed of sandstone,limestone hard roof.The hard roof has the characteristics of high strength and strong integrity.After the working face is pushed through,it fails to caving in time,which easily leads to the formation of a large area of suspended roof in the goaf and provides space for the accumulation of gas in the goaf.In addition,when a large area of suspended roof in the goaf suddenly caving,a shockwave with great destructive force will be formed.At the same time,sparks may be generated due to the impact of gravel to cause gas explosion,threatening the safety of coal mine production.Aiming at the problem that the free surface of the hard roof in the goaf of coal seam is less,which leads to the unsatisfactory cracking effect,this paper puts forward the efficient static cracking technology of weakening the hard roof by filling the borehole with static crushing agent with circular grooves.The expansion pressure generated by the volume expansion of the static crushing agent filled in the borehole and the circular grooving in the limited space acts on the inner wall of the borehole and the circular grooving,so as to produce tensile failure in the parallel and vertical direction with the borehole,and then destroy the integrity of the hard roof,so as to achieve the purpose of timely collapse.(1)through the FLAC3D numerical simulation,studied the circular slot size cutting agent for crushing the static crack the influence law of effect of hard roof,found agent for crushing the static circular slot cutting width crack,and the effects of hard roof is lesser,and the length of the circular groove cutting agent for crushing the static crack effect is larger,the influence of the roof and the length of the circular groove cutting of30 mm can achieve good effect of crack.(2)A new type of circular grooving drill was independently designed to measure the expansion pressure development of static crushing agent in circular grooving structural steel pipe with or without circular grooving,and to measure the expansion pressure development curve of static crushing agent in circular grooving at different heights from the bottom of the hole.Compared with non-circular channel-cut steel pipe,the rate of static crushing agent generating expansion pressure in circular channel-cut steel pipe increases,and the maximum expansion pressure increases by 50%.With the increase of the distance between circular grooving and hole bottom,the maximum expansion pressure of static crushing agent in circular grooving is smaller,and the development time is more lag.(3)The static crushing experiment of the test block with or without circular grooving was studied.The fractal theory was used to quantify the complexity of the crack of the test block,and it was found that the circular grooving could better increase the crushing degree of the test block.At the same time,through the design of the bottom of the hole of different location of circular cutting research form for crushing the block to the bottom of the hole distance,the influence of relative to the circular groove cutting to the bottom of the hole distance is 100 mm block,to the bottom of the range of 0 mm to 50 mm hole at the bottom of the circular groove cutting can make the first can't spread to the upper block,cracking,inhibits the propagation of the crack block broken effect is poorer.(4)The change rule of plastic zone of roof under the action of static crushing agent is discussed under the condition that there is no circular grooving in borehole and different hole spacing factors.The results show that under the same hole spacing,the crack generated by ring grooving model can propagate farther and cause more damage to the roof.At the same time,the hole spacing has a great influence on the cracking effect of static crushing agent on the hard roof.With the increase of the hole spacing,the crushing effect becomes worse.There are 57 pictures,11 tables and 100 references in this paper. |
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