Failure Mechanism Of Hard Thick Coal Under Various Types Of Disturbance At The Front Of Mining Face

The fully mechanized top-caving coal mining is the main method to excavate the thick coal seam.The fractured efficiency of coal mass is very important during top-coal caving process.In generally,the top coal experiences two phases during the mining process,one is the excavation disturbance phase,and another is support disturbance phase.The soft or mid-hard coal mass can be fractured after these two phase,then transfers to loose block.However,only several cracks are generated in the hard coal mass after that two phase.Therefore,some measure often be taken to weaken and disturbance the coal mass so that the coal can be fragmented efficiently.The No.11&12 coal seam of Xinzhouyao mine in Datong coal district is the hard coal seam,which means the strength of coal sample is quite strong.According to the test result,the limit strength of coal sample reach to 44 MPa.It is very difficult to caving it during mining process.Only several large blocks fall behind the shield,and its size is greater than the size of drawing open window.Based on these difficulties,the thesis investigates several weaken coal mass method.Combining the geological background of No.8935 panel in Xinzhouyao mine,the theoretical,numerical and experimental methods are used to study the failure mechanism of coal mass under the disturbance of blasting,abutment and support shield.The work includes the following:(1)Several coal samples are obtained from the No.11&12 coal seam in Xinzhouyao mine,and a series compressive test are conducted to obtain the mechanics properties of coal seam.Based on the test results and some published research results,the mechanics properties of No.11&12 coal seam are decided: the uniaxial compressive strength is 29.29 MPa,the elasticity modulus is 26.9 GPa,the poison rate is 0.23,and the in-situ stress of 8935 panel is 10 MPa in general.(2)In the previous mining practice,the pre-split blasting method has a positive effect on hard coal mass fragment.However,there is few theory to explain the coal mass blasting mechanism.The design optimization mainly relies on evaluate the caving efficiency,which is not support by scientific theory.Based on this,a dynamic damage constitutive model is developed to describe the damage of coal mass during blasting process,the p-? state equation considering the blasting-induced pressure and porosity are adopted to the pressure and strain in coal mass,and the rate of accumulated plastic strain(??)to final plastic strain is defined to describe the crack initiation and propagation.Several modes are built to simulate the effect of delay time,varies borehole space,different in-situ stress and bedding planes on crack propagation.For evaluating the blasting-induced fragment,a new calculation method are introduced to describe the particle size distribution.The result shows that,a superposed stress waves are generated between the double holes in the no delay model,which can smashed the coal mass at bores connect line into pieces.This design can be applied into cumulative blasting and smooth blasting.The double holes delay blasting is helpful to spread the explosive energy in a wider extent,and to generate uniform distribution cracks.It is applicable for the blasting fragment project.Different space of holes models can generate various fragment size.The simulation method can be applied into optimize the design according to the needs of project.In the constant in-situ stress blasting model,the length of radial crack and the number of cracks decrease with an increasing of in-situ stress;in the different lateral pressure blasting model,the blast cracks trend to develop along the direction of maximum principal stress,while the length of crack trend to decrease along the direction of minimum principal stress.The blast stress wave is divided into two parts when it encounters bedding plane,one is reflection stress wave,and another is transmission wave.The reflection wave may induced tensile failure in coal mass if the distance between borehole and bedding plane enough short,and the transmission wave may induced new crack generation beyond the bedding if the tensile stress component induced by transmission wave exceeds its strength.The stress wave,which is parallel to bedding plane,has a positive effect on the crack growth at the tip of beddings.The schemes of triangle,quadrilateral and quadrilateral with center empty hole are all applied to simulate the coal mass blasting fragment,and the fragment size is counted using the Matlab software.The result show that the triangle and quadrilateral designs are beneficial to generate smaller fragment,and the percent of small fragment is about 95 %;while in the quadrilateral with center empty hole design,the empty hole can guide the crack growth and hinder crack development in other direction.These research results provide the optimization method for blasting design in coal mining.(3)Based on the previous research results about the abutment pressure distribution of top-coal caving panel,the elastic-plastic mechanics and fracture mechanics are used to analyze the crack initiation-development-propagation process.The equation are built for each phase,and the results indicate that the crack development is influenced by the maximum principal stress and minimum principal stress significantly.The number of new generated crack is related to the increasing or decreasing rate of principal stress.A UDEC simulation model are built to explore the effect of principal stress increasing or decreasing rate on the crack evolution in the coal mass.The principal stresses which are applied to the model boundaries refer to the stress variation at abutment pressure zone.The effects of depth,confined reduction rate,compressive strength on crack development of coal mass are studied,respectively.The simulation results show that the principal stress has an significant influence on the crack growth of coal mass at front of mining face.Then,a UDEC model is created to study the crack growth under excavation disturbance.The effects of in-situ stress,confined pressure reduction rate,strength of coal mass on shear and open crack generation are discussed,respectively.The results indicate that,during the axial stress increasing and lateral stress decreasing,a little shear cracks occur at elastic phase.The number of cracks increases with the increasing axial stress.After the axial stress reach to its peak,the shear cracks increase sharply with the increasing axial strain,and the open initiation cracks occur with the increasing lateral strain.When the increasing rate of lateral strain exceeds the axial strain,the shear crack transfers into open crack gradually.The number of shear crack decreases with the increasing of open crack.The in-situ stress also has an influence on the axial stress peak and its position.The larger in-situ stress,the greater peak axial stress,corresponding to the greater lateral stress.The proportion of open crack also increases with the increasing of in-situ stress.However,the stress concentration factor decrease with the increasing of in-situ stress.The failure stress and the number of cracks decreases with the increasing of confined pressure reduction rate.The confined pressure reduction rate has an positive correlation with the distance between coal mass and mining face.These research results provide a thoeretical suggestion for coal mass fracture in advanced abutment pressure zone.(4)Based on the previous research results,the failure types of top-coal at roof control zone is analyzed.The failure process is divided into two phase: initial support phase and lifting disturbance phase.A UDEC model is create based on the geological background of No.8935 panel.The fracture pattern of coal mass is explored during excavation process.The effect of lifting types on coal mass fracture is also studied.The results indicate that,the soft coal is in loose state after experiencing the mining disturbance,and it can cave by itself.The mid-strength coal is in embed shape above the support,and it can break into loose fragment under the disturbance of support.The coal mass can be broken uniformly using low support force and high frequency disturbance.The hard coal is in suspension shape after the support,and it can not be broken using support,it need be pre-split by blasting or hydraulic fracturing.These results give some suggestion about breaking top-coal mass at roof control zone.(5)Based on the previous theoretical and numerical results,a similarity simulation experiment is conduct to study coal mass failure process in top coal caving mining face.The fracture pattern is compared between hard coal and weakened coal during mining process.The result show that the blasting pre-split method can break hard coal into embed shape,and it can be transferred to loose fragment after excavation disturbance.Various support disturbance types and different support force are adopted to explore the optimal disturbance scheme.At last,the thesis summarizes the failure pattern of coal mass at in-situ zone,abutment pressure zone and roof control zone,respectively.A jointed method,which consider pre-split blast,excavation disturbance and support lifting disturbance,is proposed to weaken the coal mass.The method is introduced into weakening the hard coal in nearly horizontal thick coal seam and large inclination thick coal seam longwall mining.