Research On Mechanism And Control Of Stress Field Change Of Artificial Cracking Coal And Rock Mass

Large deformation of deep mine roadway,rock burst,coal and gas outburst,low permeability gas drainage,and strong ground pressure caused by the instability of hard roof are the key problems restricting the safe and efficient mining of coal mines.These key problems are closely related to the high stress circumstances.It has been proved that the effective means to solve these problems is to improve the stress environment and realize the stress transferring through the structural transformation of coal and rock mass.Artificial cracking is an effective method to transform coal and rock mass structure actively.However,the shape and parameters of artificial cracks produced by artificial cracking will affect high stress transfer and pressure relief and then the transformation of coal and rock mass structure and the improvement of stress field environment.Therefore,in this paper,the mechanical properties,stress field change mechanism and control of artificially cracked coal and rock mass are studied using physical experiment,numerical simulation,theoretical calculation and field application.The main achievements are as follows:(1)The mechanical characteristics and failure instability law of coal and rock mass with artificial fractures under different fracture parameters are revealed.It is concluded that with the increase of crack length and density or when the fracture angle is gradually approaching 30 °,the elastic modulus,strength and energy release of the test block are also gradually reduced.Y-type fracture is more advantageous to the macroscopic fracture of the specimen.The key fracture parameters which have significant influence on coal and rock mass failure are obtained.The damage equation of coal and rock mass with artificial fracture under tri-axial stress condition is established,and the damage variable is calculated.The influence law of fracture parameters on coal and rock mass damage is obtained.According to the damage degree of artificial fracture to coal and rock mass,the artificial fracture parameters are classified.The generation method of equivalent damage material is proposed and verified.(2)With the increase of crack length,the decrease of crack angle and the increase of fracture group number,the horizontal crack will have priority to crack;in the fracture group,the smaller the angle between the minimum principal stress and the fracture,the greater the contribution of stress transfer,and the area of cross fracture is obviously larger than that of single crack;with the increase of crack spacing,the high stress in the pressure relief zone changes from continuous to flat Under the action of mining stress,artificial fracture propagation and tip stress migration effect appear,resulting in the change and transfer of stress field distribution.The macroscopic stress field of coal seam is changed by changing the macroscopic stress field.(3)Considering the effect of weak structure on stress change,the concept of stress transfer based on weak structure of coal and rock cracking is proposed.The mechanical model of stress transfer of weak structure body is established,and four indexes for evaluating the stress concentration degree of weak structure body are determined.The factors influencing stress transfer under the condition of weak structure body are analyzed,and the parameters of stress transfer damage variable artificial fracture are determined And the quantitative relationship between them is also discussed.(4)Considering the effect of weak structure on stress change,the concept of stress transfer based on weak structure of coal and rock cracking is proposed.The mechanical model of stress transfer of weak structure body is established,and four indexes for evaluating the stress concentration of weak structure body are determined.The factors influencing stress transfer under the condition of weak structure body are analyzed,and the stress transfer,damage variable and artificial fracture are determined and the quantitative relationship between them is also discussed.(5)What influences the hydraulic fracturing extending and damage variables I analyzed.With the increase of volume and fracture length,the damage variables of weak structure increases linearly;With the increase of interval between segments,the damage variable increases first and then decreases;the dual vertical slotting fracturing whose interval between segments is 3 m and cutting angle is 30°?45° has maximum damage variables for weak structure.The sensitivity of hydraulic fracturing parameters affecting the damage of weak structure is analyzed,and it is considered that notch cutting,increasing fracturing segmentation and multi hole synchronous fracturing are effective to improve damage variable;the design of hydraulic fracturing process parameters based on damage variable and the calculation method of fracturing section range,borehole spacing,drilling angle,number of boreholes and the number of fracturing sections in each borehole are given.(6)This paper analyzes the mechanism of large dynamic pressure deformation of rail transportation roadway on the east wing of floor during the mining process of 313 working face in Qinan Coal Mine of Huaibei Mining Co.,Ltd.The control method of how hydraulic fracturing produces weak structure body pressure relief is put forward,and the parameters of artificial fracture are calculated,and the construction scheme of hydraulic fracturing is determined and implemented.Through the analysis of water pressure curve,hydraulic fracture morphology and roadway deformation,it is concluded that obvious cracks appear in the hole wall after fracturing,the deformation of roadway is significantly reduced,and the roadway is effectively controlled.There are 112 pictures,14 tables and 168 references.