Study On Stability And Controlment Of Hard Roof Stope In Qing-dong Mine

Given the huge demand in the coal market right now,geological conditions of coal mine production tend to be more complicated.Due to the severe and sudden pressure of the stope caused by high strength,the hard roof has severely restricted the safe and efficient production of the working face.Based on the 828 hard roof working face of extra-thick coal seam in Qing-dong coal mine,this paper studies the special ground pressure behavior and failure mechanism of hard roof working face.The theoretical analysis,numerical simulation,similar simulation test,laboratory coal rock test and field monitoring are used to analyze the mechanical properties of coal and rock mass in stope,the ground pressure behavior in hard roof working face,the fracture interval,breaking structure and instability criterion of hard roof,overlying strata structure and its influencing factors,ground pressure behavior and its influencing factors,reasonable width of section coal pillar,the support load of ground pressure,the feasibility and effect comparison of pre-mining top slice,the adaptability of pre-splitting blasting,the basic parameters of stope working face,the matching and adaptability of stope equipment and the monitoring of field application.The results of the study are as follows:(1)According to the fundamental conditions of the coal minc basing on the on-site research,the author considers that the key factors to affect the productions are the hard roof in the 828 working face through analyzing the geological conditions and the mining conditions of-the mining stope.Testing the mechanical properties of the coal and rock mass in the mining stope,the results show that the average compressive strength is 113.24MPa,and the elasticity modulus is 88.83GPa,and the poission ratio is 0.293.The point load test results show that the compressive strength of the hard roof in the axial test is 54.6MPa,the tensile strength is 2.75MPa.The author thinks the hard roof can storage too much energy which is from overburden strata,and the energy can cause intense pressure for the working face when the roof instability is happened.(2)A physical simulation test is used to test the mining of the 828 working face,and the research shows that the large roof weighting step,the high ground pressure strength,the sudden of the roof-weighting,the wide range of roof pressure damage are the features of this working face.The maximum mining pressure reaches about 34MPa,the influence scope being 60m,and the stress concentration factor is 2.4 when the working face is in the normal mining.And at the same time,the first weighting interval reaches 60m,and the periodic weighting length is on 30m,and the height of the fractured zone is 90m.In the effect of the hard roof,the overlying strata in the stope shows a sudden and large-scale decline.(3)The first breaking interval and the periodic breaking interval of main roof are calculated,and the mechanical structural model based on the broken structure is established.According to the amount of roof subsidence established a criterion of instability.And according to the sliding rock of roof whether or not touch the waste rock,the rock instability mode criterion has been established.The beam theory of elastic foundation is used to calculate the breaking interval of main roof.The first weighting interval of the stope is 52m,and the periodic weighting interval is 25m.According to the articulated structure of the breaking rock block of main roof,the main roof breaks into the voussoir beam structure for the first weighting,and the cantilever beam structure is formed during the periodic weighting.When the first weighting happens,it needs to reach a subsidence value of 1.65m and the rock block rotates by 29°,and the allowable maximum turning angle of the goaf area is 14°.That is the first breaking is sliding of main roof key blocks.According to the maximum compressive strength of rock and its own load strain,when the periodic weighting happens,cantilever beam structure will start to rotate and rotate at 0.3 degrees.That is when the periodic weighting happens,the roof will rotate and collapse,and the maximum subsidence displacement will be 0.57m.(4)When the first weighting is coming,according to the theory of voussoir beam,the support load is about 29447kN.In the meanwhile,the load formula of periodic weighting is F2=Q?+[q·(3L2-2L)-6h·a·?max·?max]/6?max,and the support load is about 25997kN at the field.Though the comparison,the author suggest that the coal mine must take effective actions to release the stress to slow down the intensity of roof weighting because of the exorbitant support load.(5)Udec software was used to analyze the influence of the overburden destruction structure and form of the stope in the hard roof working face,roof lithology,protective layer and mining height effect.It is believed that when the stope is pushed to 100m,the overburden rock is destroyed.The first weighting interval is 60m,and the periodic weighting interval is 40m,the fall zone is 35m,and the fractured zone is 95m.The use of pre-mining top slice and reduction of the strength of the hard roof can effectively contain the strong mine pressure behavior of the stope;when the mining height is less than 9m,the mine strata intensity of the stope increases approximately linearly.When the mining height is greater than 9m,the intensity of the stope pressure shows a stepwise increase.The existence of direct roof releases the accumulated load of overburden in stope ahead of time and effectively relieves the intense strength of ore pressure.(6)Using Flac3d numerical simulation software,the influencing factors of stope pressure behavior in stope are analyzed by factors such as stope propulsion,mining height,presence or absence of direct roof,presence or absence of pre-plumbing stratification,hard roof strength and hard roof thickness.The studies found that affected by mining,the supporting pressure range is about 50m;the maximum stress in the stope is 34MPa;the maximum displacement of the roof in the gob area is 8.3m;the coal body around the stop is loaded into the plastic zone with a width of 10-20m.The increase in mining height will further increase the stress concentration area and strength of the stope,resulting in an increase in the scope of displacement and plastic zone damage.Direct roofing has a buffer effect on the reduction of mining pressure intensity in stope.The thickness of hard roof has a critical value for stope pressure.When the critical thickness is less than the critical value,the mine pressure increases with the increase of the thickness of the roof.When the critical thickness is greater than the critical value,the intensity of mine pressure is weakened.It is believed that the thickness of the hard roof is greatly disturbed at this time.Normal mining pressure in the stope.The intensity of mine pressure exhibited by the ordinary strength lithologic roof working face is obviously lower than that of the hard roof.The pre-acquired protective layer(7#coal seam)pre-disturbed the original rock stress of the stope,and the 8#coal seam face production was in an infinitely static state.It is considered that the pre-acquisition of No.7 coal seam and the reduction of the strength or thickness of the No.8 coal seam roof can provide superior working conditions for the No.8 coal seam.(7)The author establishes the stress distribution map of coal pillars in the lateral section of the stope,and according to the theoretical calculation,the optimal width of the coal pillars is about 8-18m when the protection pillar use the way of-narrow pillar.According to the numerical simulation to analyze the behavior law of-mine pressure of the mining stope under the conditions of different width's coal pillars when excavating the roadway and extracting the working face,the results of the numerical simulation gives a comparison of the roadway stability and the width of the reserved pillars,and the author suggests that it's reasonable to set 15m of the coal pillar comprehensive above factors considering all of the above factors.(8)The hard roof is only 5.6m intact after the failure of the pre-mining top slice to the hard roof,and the simulation experiments were carried out to verify the results.The pressure step distance was only 18.8m and 10.6m and determined that the low strength of the stope.A similar simulation experiment was designed to verify the above theoretical calculation and comparative analysis of the mine pressure in the stope of 8#coal seam with or without protective layer mining.The study showed that the pre-mining top slice can be significant in reduce the mine pressure intensity of mining seam in No.8 coal seam without top layer.The pre-splitting blasting method was designed for the No.8 coal seam without top layering to reduce the strength and heights of the hard roof,and it is determined that the hard roof of pre-split blasting has higher applicability.(9)In order to ensure the efficient production of the working face,the best basic parameters of the working face are designed according to the actual situation of the site,and the coal machine equipment supporting and power supply equipment plans that can effectively adapt to the hard roof stope are designed according to the annual output.The on-site drilling and peeping through the roof,the monitoring of the surrounding rock deformation of the roadway,the monitoring of the support resistance determined that the stope and the roadway are in excellent operating conditions,and it is considered that the strata behaviors is well controlled.