| A new type of non-pillar longwall mining technology,named N00 mining method,is different from the traditional longwall mining(121 mining method)in the underground mining arrangement:? N00 method is free from the two roadways excavation in the traditional mining method in advance.The mining roadways are gradually formed in the process of mining,and the roadway is reserved located at the back of the working face after the coal extraction,and the reserved roadway can be used for the mining process of the next working face.? The N00 method is free from reserved protective coal pillar or artificial filling body between working faces that set up in the traditional mining design.Instead,the gob area is filled by the roof rock and forming one side-wall of the reserved roadway.The roof directional pre-split technology is employed to cut off the physical connection between the roadway and the gob roof rock layers,to promote the collapse of the roof in the range of the cutting seam.With the roof cutting and dilatancy of the collapsed rock in the gob area,the N00 mining method get rid of the phenomenon of stress concentration in the surrounding rock caused by the setting of coal pillar or filling body in the traditional mining method design.Comparing to the traditional longwall mining method,the N00 mining system has its particularities:The ventilation system of the N00 method has changed from U-shape and Y-shape ventilation system to Z-shape ventilation.In N00 mining method arrangement,the roof caving characteristics and ventilation mode are different from the traditional longwall mining arrangement,which leads to the changes in the seepage characteristics in the gob area and ventilation path at the working face.Therefore,the study on the law of permeability evolution in the gob area formed using N00 method is helpful not only for the understanding of the air flow migration characteristics,but also for providing the guidance to the prevention and control of spontaneous combustion and gas disaster in the gob area using N00 mining condition.This research work has its certain theoretical and practical value.In this paper,one N00 working face at Ningtiaota Coal Mine is taken as the field engineering case,the theoretical analysis,numerical simulation and field test methods are used to carry out the following research:(1)This paper expounds several kinds of current longwall mining technology in China,including traditional longwall coal pillar mining,mining with filling body along the gob area,the 110 non-pillar mining method,and the N00 non-pillar mining method.The comparative analysis is conducted from working face layout description,mining and ventilation modes introduction.In addition,the key technology of the no pillar mining N00 method are explained,which lays the foundation for the follow-up research.(2)Theoretical analysis and UDEC numerical simulation method are used to explain the "horizontal three zones" and "vertical three zones" of the gob area formed by under the condition of both the N00 method and the traditional longwall mining method.The characteristics of direct roof collapse and main roof fracturing behavior of the gob formed by N00 method are analyzed.Comparing to the traditional mining,the N00 method eliminates the influence of coal pillar or filling body that applied on the direct roof of gob.Thus,the "loose triangle area" caused by the reserved coal pillar or filling body is eliminated,which is beneficial to reduce the accumulation space of harmful gases such as methane and gas.It shows that the roadways on both sides of the working face all collapse and form the caving zone under the traditional longwall mining mode,while the fracture zone and bending subsidence zone are symmetrically distributed from the middle of the working face to the both sides.For N00 mining method,the stress transmission of the roof rock strata is cut off by the pre-roof cutting during the mining process at the end of working face,which makes the roof collapse more freely and forms a compacted stable area near the roadway.The distribution of the three transverse areas in the gob area is asymmetric.(3)According to the case analysis of the characteristic of the stress and deformation distribution of the surrounding rock near the retaining roadway,it is concluded that there is a free accumulation area of gangue within 40m behind the working face in the gob when using N00 method,and the porosity and permeability of porous media in the gob area are mainly affected by the gangue filling condition.The zone with the range of 40m to 160m behind the face is the over-load affected area,which is mainly affected by the broken expansion coefficient of caving gangue.There is a compacted stable area beyond 160m behind the face,and the gangue in this area is compacted,and the void and permeability of the gob area remain basically stable,which is determined by the residual dilatancy coefficient of the caving gangue.(4)Based on the characteristics of gangue caving and compaction in the gob area,the mathematical model of porosity and permeability of porous media in gob with N00 method is constructed,which lays a theoretical foundation for the further study of air flow migration law in gob.(5)The law of air flow movement at the working face using N00 method is analyzed using ANASYS-Fluent numerical simulation,and it is concluded that the main air leakage area of N00 working method is the corner between intake roadway and working face and the corner between machine tail and retained roadway.Among them,the air flow from the inlet corner into the gob area will enter the deep part and flow to the return air roadway along the gob,and most of the air leakage from the return corner will flow back into the return roadway within 50m from the back of the working face.Therefore,the air leakage at the inlet corner is more threatening.Field optimization measures are needed to be taken for the ventilation system.(6)To study the influence of different air volume injection on the air flow movement at N00 working face,the characteristics of air flow movement in working face with inlet air velocity various from 0.5m/s,0.8m/s,1m/s,1.2m/s to 1.5m/s are simulated,respectively.Results show that the larger the injection rate is,the more flow leakage into the gob.The higher the air leakage ratio leads to larger amount of air flow into the deep part of the gob,which makes it more difficult to prevent and control disasters such as spontaneous combustion and gas overflow in the gob area.(7)According to the characteristics of the air flow distribution at the N00 working face,it is proposed to set up wind-cloth curtains or pile separation wall at the inlet corner,and shotcrete to seal the gob-side retained roadway section.It can reduce the air leakage channel between the rear of the gob area and the return air roadway,so as to reduce the air flow from the deep part of the gob.By simulating the influence of different length of wind-cloth curtains and shotcrete surface treatment,it shows that the combination of 10m wind-cloth curtains with shotcrete is the best way to prevent the ventilation air leakage.Field observation shows that the wind reduction and pressure reduction,corner hanging wind-cloth curtains and roadway shotcrete surface treatment can effectively control the leakage air in N00 working face. |
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