| Large mining height and full thickness mining technology is one of the important development directions of thick coal seam mining technology which is widely used in coal seams with a thickness of about 6.0m in coal seams such as Jincheng,Shenfu and Dongsheng coalfields in China.However,due to the increase of mining height and mining intensity,the gas emission intensity of the working face is increased and the gushing law is unbalanced.The concentration of gas associated with coal as clean energy is extremely easy to exceed the limit in the stope and return flow.It restricts the safe and efficient mining of coal.At present,the large mining height workface adopts multi-lane ventilation system such as "two intake one return" and "three intake two return" to reduce the gas concentration in the stope and goaf,but this system has the safety hazard of goaf ventilation.The safety hazard of district ventilation is seriously inconsistent with the"intake and return air of the mining face shall not pass through the goaf or the roof falling zone" as stipulated in the "Safety Regulations for Coal Mines",and the mine gas is discharged into the atmosphere with the exhaust air,which is polluted,polluting the air.wasting coalbed methane resources.Therefore,research and solve the problem of gas overrun in large mining height workface,change the existing multi-lane ventilation system pattern,realize mine gas drainage and utilization,not only can eliminate safety hazards,improve mine safety conditions and economic benefits,but also reduce coal pillars.And these measures are of great significance for reducing greenhouse gas emissions and protecting the environment.In this paper,theoretical analysis,laboratory test,numerical calculation,field measurement and engineering application are combined with the gas drainage-seepage model to study the characteristics of unloading gas migration in large mining height workface,and based on the theory of unloading rock mass mechanics,the relationship between the gas channel and the mining height of the roof of the large mining hight workface and its unloading damage evolution are studied in detail.The main conclusions and innovative achievements are as follows:(1)The on-site stress measurement shows that the maximum ?H of Changping Mine is 7.81 MPa.the lateral pressure coefficient is less than 1,and the vertical stress is the main,and the roof of the 3#coal seam is medium-grained sandstone with an average uniaxial compressive strength of 75.7 MPa.The drilling peep found that when the advanced working face is 5.4m and the top surface of the coal seam is 5.1m,there is a fracture gap of about 10mm between the rock masses.The direct roof is broken at 5.4m in front of the workface:there is a fault-breaking gap at the goaf of 18m from the top surface of the coal seam and 11m behind the working face.The wrong distance is more than 25mm.the fall zone is at least 18m,and the fracture zone height is between 30-55m.(2)Based on the mining dynamics test,the evolution of the roof fissures in the goaf is analyzed.The results show that the higher the mining height,the greater the supporting pressure of the roof coal and rock mass in the goaf,the more the fractures develop;the key layer theory and the fracture zone with the empirical formula of calculation,the theoretical upper limit height of the fracture zone is 39?51 m.It can be combined with the"masonry beam" structure and the location of the key layer to accurately arrange the high-efficiency drainage holes in the gas transitional flow channel zone developed in the fracture zone.(3)According to the mining dynamics process of the roof rock mass,the influence of mining height on the stress unloading of the fracture rock mass and the damage evolution of the gas channel in the goaf is analyzed by using the theory of unloading rock mass mechanics,and the relationship between damage factor and unloading amount are derived.The quantity and the discrete element software 3DEC is used to simulate the roof unloading and gas channel evolution in goaf under different mining heights.Based on the coal-rock seepage-mechanical test and effective stress concept,the relationship between damage factor and permeability is established.The unloading damage range of the gas channel under different mining heights,and pointed out:?As the mining height increases,the unloading capacity of the roof in the goaf increases,the damage factor increases.The damage of roof fracture rock mass and gas passage is aggravated by unloading,and the number of fractures is increased.When the unloading stress or unloading amount reaches a certain value,the damage propagation and penneability of the gas channel will be unstable and abrupt.Compared with the common mining height,the unloading stress and the unloading rate increase of the large mining height workface are slower,and the unloading stress and unloading capacity of the fractureed rock mass in the goaf are nonlinearly increased with the increase of the mining height.As the mining height increases,the increase of unloading stress and unloading amount of roof in goaf leads to the increase of deep horizontal displacement,and the large mining height is more conducive to the roof fracture and the lateral penetration of the gas channel.?As the mining height increases,the permeability of the fractureed rock mass in the goaf is increased after unloading damage.As the distance from the top surface of the coal seam decreases,the permeability of the rock mass increases gradually.When the fracture rock mass is reduced to a certain value from the top of the coal seam,the permeability suddenly increases,and the higher the mining height,the higher the height of the roof fracture expansion and permeability mutation point in the goaf,and the higher the upper limit of the height of the gas channel development.The permeability breakthrough point of the roof unloading damage in the goaf can be used as the basis for determining the development of the fracture zone of the large mining height workface and the height of the gas channel.(4)Based on the current situation of large mining height workface ventilation system and gas drainage,a gas control model for goaf in large mining height workface is established.The gas distribution characteristics of large mining height workfaces under different ventilation systems are analyzed by using the drainage-seepage model of gas diffusion and migration in goaf.The Fluent numerical software was used to calculate the gas migration characteristics and treatment effects of large mining height workface under different ventilation systems,and the following conclusions were drawn.?With the increase of mining height,the height of flow field in goaf increases and bulking factor changes.Gas emission from the stope under different ventilation systems and extraction modes will produce different degrees of diffusion and movement.Changing the boundary conditions of ventilation system by air drainage,directional borehole extraction or through borehole extraction can systematically change the gas diffusion and movement in high mining height workface,and affect the gas distribution in the corner of workface and goaf.The guiding function system of the extraction drilling hole changes the flow field of the workface and the goaf,and the high concentration gas moves to the inlet of the extraction hole with the extraction and makes the low gas area in the lower part of the goaf near the stope.But the strong guiding effect of penetrating borehole drainage on upper corner airflow greatly weakens the intensity of airflow flowing through upper corner and gas drainage roadway,and greatly reduces the gas concentration.The medium-high directional drilling and penetrating drilling combined with the U-shaped ventilation system formed a large range of low-concentration gas area along the lower part of the vertical goaf of the coal seam,and the treatment effect was the best.(5)According to the characteristics of multi-lane ventilation system in large mining height stope,the disadvantages of multi-lane ventilation are analyzed.The application of high-flow directional drilling and penetration drilling combined with high-flow extraction technology in goaf is proposed.To solve the key technical problems of the U-type ventilation system,the field engineering application optimization has determined the arrangement parameters of the middle and high level directional drilling and the penetrating drilling holes in the coal pillar.The results show that:?The drainage flow rate of large diameter boreholes with 153mm is 2-3 times that of 96mm in medium-high fracture zone,and the gas drainage concentration of boreholes in the development area of medium-high gas passage is 2.4 times that of low-middle boreholes.Large mining height is beneficial to the unloading damage evolution of the gas channel in the roof of the goaf and the formation of the dominant gas channel.?The gas drainage effect of 250mm large flow penetrating borehole with 5 m interval is the best.The gas concentration in the upper corner is maintained at 0.55%?0.6%after combined with directional borehole drainage in middle and high fracture zone,which avoids gas exceeding the limit.The safe and efficient mining of large mining height stope under the U-shaped ventilation system of Changping Mine has been successfully realized. |
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