| In order to realize the safe and efficient retracement equipment of fully mechanized mining face,most coal mines adopt the pre-excavation retracement roadway to realize rapid retracement.Because the pre-excavation retracement roadway is affected by many factors such as advanced mining stress and lateral stress in the end mining period of the working face,it is easy to induce strong mine pressure phenomena such as roof step subsidence,support crushing,coal wall spalling in the working face,and large deformation of surrounding rock in the retracement roadway,which affects the safe and efficient retracement of the working face.Based on the engineering geological conditions of 31107 working face in Huoluowan Coal Mine,this paper adopts the comprehensive methods of field measurement,numerical simulation,theoretical analysis and field industrial test to carry out relevant research on the roof instability mechanism and roof cutting and pressure relief technology of the retracement roadway under the remaining coal pillar.The hydraulic fracturing roof cutting and pressure relief control technology is proposed and applied to the engineering site,and remarkable control effect is achieved.The main research results are as follows :(1)At the end of the mining of the underlying working face,due to the existence of the overlying remaining coal pillars,the stress concentration of the surrounding rock below the coal pillars is caused.At the same time,the mine pressure is more intense due to the superposition of multiple types of stress during the final mining of the working face.The law of mine pressure in the end mining period of the working face under similar engineering geological conditions is compared and analyzed.It is concluded that the support pressure is about 42 MPa and the roof subsidence is 102.4cm when there are residual coal pillars above the retracement roadway,which is generally greater than the support pressure under the goaf 35 MPa and the roof subsidence 69.5 cm.It is analyzed that the severe mine pressure under the remaining coal pillars is due to the high degree of stress superposition in the final mining period due to the stress concentration under the remaining coal pillars.It is revealed that the dynamic load caused by the failure and instability of the hard high and low roof above the working face is the main reason for the strong rock pressure behavior in the retracement roadway.(2)According to the engineering geological background of 31107 working face,the FLAC3 D numerical calculation model is established,the stress concentration range under the residual coal pillar of 2-2 coal seam is determined,and the stress superposition law of the retracement roadway at the end of mining is calculated and analyzed.By establishing the mechanical model of the " linkage instability structure " of the upper and lower hard roofs of the remaining coal pillars,the roof load of the retracement roadway when the high and low roofs of the remaining coal pillars are linked to instability is calculated.When the hanging roof length of the high roof is greater than 60 m,the working resistance of the support is greater than 26411.2k N;when the length of the high roof suspension is less than 20 m,the working resistance of the support is less than 13355.2 k N.(3)By analyzing the distribution characteristics of the supporting stress of the remaining coal pillar roof,it is concluded that the stress environment in the peak area of the supporting stress is approximately consistent with the direction of the maximum principal stress.Based on the close correlation between the direction of hydraulic fracture propagation and the direction of maximum principal stress,a directional hydraulic fracturing technology for high-level roof is proposed.The directional drilling is used to construct a horizontal directional hole in the peak area of the abutment stress of the remaining coal pillar roof.The hydraulic fracture can achieve vertical expansion and achieve the purpose of cutting off the roof above the remaining coal pillar.(4)The mechanical model of cantilever beam structure of low hard roof is established,and the influence of hard roof with different length and thickness on the final mining period of the retracement roadway is analyzed.At the same time,the influence of low hard roof on the deformation of surrounding rock of equal pressure coal pillar and retracement roadway at different fracture positions is simulated and analyzed.The results show that when the fracture position of the low roof of the retracement roadway is close to the side of the goaf,the support load in the retracement roadway is the smallest,and the characteristics of the mine pressure are not obvious.It is proposed that the strong mine pressure behavior of the retracement roadway can be effectively controlled by artificially changing the fracture position and cantilever length of the low hard roof,so as to determine the hydraulic fracturing scheme of the low roof of the retracement roadway.(5)The combined control technology parameters of ’ high directional hole regional hydraulic fracturing + low straight hole local hydraulic fracturing ’ roof cutting in the final mining period of the working face under the remaining coal pillar are determined,and the test is carried out in 31107 working face of Huoluowan Mine.Through the monitoring and analysis of the mine pressure in the final mining period of the working face,the total subsidence of the roof during the retracement of the working face is 193 mm,and the support pressure is 36 MPa,indicating that the hydraulic roof cutting and pressure relief technology can effectively control the high stress concentration and strong dynamic load in the final mining period,so that the roof stress above the retracement roadway can be effectively controlled,and the goal of stable surrounding rock of the retracement roadway and safe and efficient retracement of the working face can be achieved.The research results can provide reference for the safe retracement of the working face under similar engineering geological conditions.The thesis has pictures 57,tables 7,and references87. |
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