Study On Roof Collapse And Expansion Mechanism And Its Mining Pressure Characteristics Based On Roof Cutting And Pressure Relief Technique In Ningtiaota Coal Mine

The mining of coal resources often breaks the balance of the initial stress in the stratum,and causes complex rock mechanics phenomena such as mining pressure relief and stress concentration,resulting in roof disaster of the working face.The research on roof disaster in the mining process of working face has always been a key issue in the field of coal mining.N00 longwall mining method with roof cutting and pressure relief technique is of great significance for solving the above scientific problem.However,the research associated with this issue is scarce.To solve this scientific problem,the research methods of combining physical simulation,numerical simulation,theoretical analysis and field test are adopted in this dissertation.The topic related to the mechanism of strengthening the degree of roof crack and expansion by N00 longwall mining method,the influence law of hulking coefficient on the evolution of basic roof stress and displacement,and the influence mechanism of expansion phenomenon of broken roof on the first and periodic weighting step and strength of the working face have been studied.The main results are as follows.(1)The shortcomings of the continuous materials to simulate the characteristics of discontinuous phenomenon such as roof collapse were expounded.A discontinuous material applied to physical simulation experiment was developed with loose stone,sand and barite powder as aggregate,and gypsum and rosin as cementing agents.Then,the knowledge of T-test in multivariate statistical analysis was introduced into the orthogonal experiment of rock mechanics.Moreover,multi-parameter regression equations with 90% confidence with the uniaxial compressive strength,elastic modulus,Poisson’s ratio,and volume-weight as the target functions were obtained and verified,greatly improving the material proportioning efficiency.(2)The phenomenon of roof caving and expansion in the mining process of working face under the condition of N00 longwall mining method were reproduced with physical simulation experiment.The bulking coefficient of broken rock at different positions of the working face was measured,indicating that the cutting side was the largest,the non-cutting side was the second,and the middle part was the smallest.The vertical displacement characteristics of the basic roof at different positions of the working face were analyzed.The results showed that the vertical displacement of the basic roof at the middle of the working face was the largest,the non-cutting side was the second,and the cutting side was the smallest.These results showed that the degree of collapse and expansion of roof has a controlling effect on the movement of the basic roof.The area with large bulking coefficient of broken rock has a small amount of basic roof subsidence,and vice versa.(3)Based on the Mohr Coulomb criterion,the UDEC uniaxial compression numerical experiments was carried out to check the mechanical parameters of rock and make the mechanical characteristics as similar as possible to the actual rock mass.The results of numerical experiments showed that the process of roof collapse and expansion can be divided into three stages,namely,the pre-fracture stage,the mining disturbance stage and the collapse stage.In the pre-fracture stage,the weak direct roof underwent obvious plastic failure under the action of advanced concentrated stress.In the mining disturbance stage,the rock and joints in direct roof had obvious tensile and shear damage.At the same time,the damage degree of measuring points on the roof-cutting side was greater than that in the middle of the working face.In the collapse stage,the broken rock moved downward rapidly.And collapse speed of the brocken rock at the roof-cutting side showed relatively large and good stability compared to the non-cutting side and the middle of the working face.(4)The strengthening mechanism of roof cutting on expansion of broken roof in N00 longwall mining method was reveared based on the results of physical simulation and UDEC numerical simulation,which was the comprehensive effect of structure effect and mechanical effect.Specifically,this phenomenon is mainly due to comprehensive action of coal and rock structure,stope structure,advanced pre-failure of roof mass,the relatively strong "tension compression transformation" caused by the mining disturbance,mechanical state deterioration caused by mining pressure relief,and downward movement caused by overlying rock pressure and gravity.(5)With the increase of bulking coefficient of broken rock,the width of the relief area of the basic roof in goaf and the width of the stress concentrated area in front of the working face were significantly reduced.At the same,the change range of the maximum principal stress at different positions of the basic roof rock beam was gentler,and value of that of the far-field roof was also smaller,indicating that increasing bulking coefficient of broken rock can optimize the stress state of the basic roof.In addition,with the increase of bulking coefficient,the subsidence of the basic roof at different positions was obviously reduced.At the same time,the subsidence of the measuring points in the rock layer above the working face also showed an obvious downward trend,indicating that increasing the bulking coefficient can slow down the movement of the basic roof and optimize the overlying rock structure under mining conditions.(6)A mathematical expression that can scientifically characterize the basic roof abutment pressure under the mining was obtained,with the advantages of clear physical meaning,and consistent with the actual.The structural mechanics models considering the supporting force of the crushed gangue before the first and periodic weighting were constructed,and the corresponding deflection,rotation angle,bending moment and shear force analytical solutions were obtained by using the Winkler Elastic Foundation Beam Theory.Using the control variate method,the influences of the support force of the crushed gangue on the subsidence,bending moment and strain energy of the basic roof were studied.The improving the support force of the crushed gangue,the increasing the basic top initial and periodic pressure step distance,reducing the pressure intensity,and reducing the number of times of pressure,thereby optimizing the mechanical state and improving the stability of mining space was revealed.(7)The research on the field monitoring data of hydraulic support working resistance in the working face showed that the first and periodic weighting step on the roof-cutting side was larger than that on non-roof cutting side,and the middle of the working face obtained the smallest value.At the same time,the pressure intensity at the roof-cutting side was less than that on the non-roof cutting side,and the middle of the working face obtained the largest value.The above results proved that the positive effect of rock crack and expansion on improving roof stability of the working face.Meanwhile,it also shows that the roof-cutting technique can optimize the basic roof stress environment and reduce the mine pressure of the working face.The research results are of positive significance for the prevention and control of roof disaster and surface subsidence in the process of coal resource mining.