Damage Characteristics And Control Technology Of Surrounding Rock Cracks In Roadway Under Close Distance Coal Pillar

For down mining in closed distance seam,due to changes in coal mining methods,changes in development methods,and integration of mines,the positional relationship between the lower coal seam mining lane and remained coal pillars in the upper coal seam was intersected.Affected by the residual stress of remained coal pillars,large deformation and failure phenomena occur in the roadway of the lower coal seam,which restricts the safe and efficient mining of the mine.Based on No.3103 roadway under close distance coal pillar in Dengmaotong Mine,this paper is using a combination of field research,theoretical analysis,numerical simulation,mining pressure monitoring and other means to study the stress distribution and the distribution characteristics of the damage cracks on the floor of remained coal pillars.Based on the Trigon model,a numerical value of rock material is established to analyze the damage evolution characteristics of surrounding rock cracks in the roadway near the short distance coal pillars,and to propose the stability control technology for the surrounding rock of the roadway near the short distance coal pillars.The main research results are as follows:(1)Rock mass material numerical model: Trigon model.In uniaxial compression,the rock fractures are mainly shear fractures.When the stress is loaded to 20% to 40% of the peak strength,cracks begin to appear.When the stress is loaded to 70% to 80% of the peak strength,the number of fractures increase rapidly.The number of shear and tensile cracks is used to represent the degree of rock damage,and the damage mechanism of the rock is analyzed.The damage evolution process of Trigon model can be divided into four stages,initial damage stage,steady damage development stage,accelerated damage development stage and damage destruction stage.(2)The stress field and damage distribution of the remained coal pillars can be divided into four regions: high vertical stress region within 0~5m from the center of the remained coal pillar,and the rock layer is basically free of damage;the high-stress gradient region the distance within 5~15m from the center of the remained coal pillar,the shear fractures of the rock are severely damaged;the high-level stress region within 15~25m from the center of the remained coal pillar,the rock layers are mainly in tension and shear cracks;the original rock is more than 25 m away from the center of the remained coal pilla,crack closed area.There are the different damage deformation characteristics in NO.3103 intake roadway along the different scope of the remained coal pillar.(3)The deformation and failure of roadway surrounding rock is closely related to the evolution of surrounding rock fissures.There is consistency between the evolution process of surrounding rock fissures and the law of deformation and failure of surrounding rocks,and bolting support has a significant inhibitory effect on the expansion and penetration of surrounding rock fissures.Combining the characteristics of sub-zone damage in the lower coal seam,the principle of the control of the mining seam in the lower coal seam is proposed,including the “principle for coordinated support of key parts”,“principle for reinforcing the full length of fractured surrounding rock,” and “partition control principle” to solve the large deformation problem of mining roadway under coal pillars.(4)The specific support parameters of NO.3103 intake roadway surrounding rock control technology were applied to the site,and the supporting effect was evaluated through on-site monitoring data.The cumulative relative displacements of the two points of the measuring point #1,the measuring point #2,and the measuring point #3 are respectively 397 mm,336mm,and 241 mm.The cumulative relative displacements of the top and bottom of the roadway #1,#2,and #3 are respectively 354 mm,242mm,and 193 mm,the working load of the anchor bolts at measuring point #1,measuring point #2,and measuring point #3 are stable at about 131 kN,105 kN,and 64 kN,respectively,which is less than the yield load of the anchor bolts and the bearing status of the anchor rods is good.