Three-Dimensional Mining-induced Stress And Rock(Coal) Dynamic Damage At The Working Face-end

The mining stress generated by the development of coal resources is the key factor affecting the stability of surrounding rock in mining and roadway.In the past,the study on the stability of surrounding rock in working face and roadway is mainly based on two-dimensional,and the problem is usually simplified into plane stress or plane strain,with the continuous improvement of the technical level of coal mining,more and more experts and scholars realize that the three-dimensional mining stress in coal mining process plays an important role in the damage and damage of coal and rock,especially the working face-end.The working face-end is the link between the working face and the roadway.It not only has the characteristics of periodic fractures of the roof,leading to large area weighting,on the working face,but also has the characteristics of serious deformation of the roadway surrounding rock,large advance support pressure and serious floor heave of the goaf.Characteristics above make the study about surrounding three-dimensional mining stress as well as coal and rock dynamic damage crucial rock at the end of the working face.Therefore,this dissertation will study the dynamic evolution of mining stress and the dynamic damage characteristics of coal-rock mass by theoretical analysis,field measurement,laboratory experiments and numerical simulation.It mainly carries out research from the following five aspects:(1)Roof Fractures Models and Stress Characteristics at Working Face-end.According to the stress,deformation and failure characteristics of the roof at the working face,the plate fracture models of roof at Face-end were established,and the deflection equations before the frist or periodic fractures were obtained.The arc length theorem and Hooke’s law were used to solve the roof rock formation.And the maxmum tensile stress is used as the criterion of roof failure.Based on maximum tensile stress criterion,the threshold of the initial and periodic breaking length of the end roof is deternined;the stress state of surrounding rock at Face-end is analyzed and divided into compression-compression zone,tension-tension zone and tension-compression zone.(2)Three-Dimensional Mining Stress and Coal Rock Damage Characteristics studied by Field Measurement and Simulation at the Face-end.Dynamic evolution characteristics of three-dimensional mining stress are obtained through real-time three-dimensional mining stress monitoring and 3DEC numerical simulation in coal entity and pillar in front of 3313 working face of Tang,an Coal Mine.The research shows that with the advancing of working face three-dimensional mining stress increases firstly then decreases similarly in different area.By analyzing the pressure data of sensors installed in goaf and backfill wall,it is concluded that the pressure of goaf and filling wall increase dramatically,which further proves the periodic breaking law of Face-end roof.It can be also concll,ded that the surrounding rock has regional breaking characteristic by analyzing the GPR signal of the surrounding rock at Face-end.(3)Partition Failure Characteristics of Rock Material Loading and Unloading Based on Mohr-Coulomb CriterionBased on the Mohr-Coulomb Criterion,this article analyzed the rock materials loading and unloading processes under the states of compression-compression,tension-tension and tension-compression,and found out the corresponding critical failure equation.On this basis,the relationship between time-weighted average loading and unloading rate of rock materials and the destruction level of rock materials were further obtained,and according to the different time-weighted average loading and unloading rates,this article also distinguished whether the rock materials is destroyed.This research adopted the FLAC3D conventional three-axis numerical simulation experiments of 7 different loading paths,the result found out when the material is damaged,the coincidence degree of the principle stress peak and the theoretical calculation result is as high as 98%.At the same time,this article also conducted simulation experiments under the conditions of 7 kinds of axial and radial loading rates ratio and 4 types of loading rates.The test results indicated that,the material is able to be destroyed when the ratio of the axial and radial loading rates reach within a certain range.The loading rate is positively correlated with the maximum principal stress peak which is happened at the state of material failure,and negatively correlated with the ratio of the maximum to the minimum principal stress.(4)Physical and Mechanical Parameters and Acoustic Emission Response Characteristics of Coal and Rock.Raw coal is selected at the end of 3313 working face of Tang’an Coal Mine and made into standard sample.The basic mechanical parameters of raw coal specimens are obtained for uniaxial compressive strength,tensile strength,Poisson’s ratio,elastic modulus and dynamic elastic modulus through ultrasonic testing,uniaxial compression test and Brazilian splitting test.During uniaxial compression process,the acoustic emission response characteristics of raw coal specimens were obtained in order to provide accordance for verifying the three-dimensional reconstruction model.The mineral composition of samples is acquired by X-ray diffraction experiments,which provide a basis for threshold segmentation in the process of three-dimensional reconstruction.(5)Dynamic Damage Characteristics of Coal and Rock Based On Three-Dimensional Reconstruction of CT Images.CT images of raw coal sample are obtained by industrial CT scanning.The spatial network of internal cracks and mineral composition is extracted and reconstructed by three-dimensional reconstruction technology.The spatial network is imported into FLAC3D for modeling,then the original CT image and reconstructed numerical simulation model is fractal dimension calculated respectively in order to verify reconstructed model.The parameters of the numerical simulation model are revised by comparing the stress-strain curve of CT specimens and reconstructed model in uniaxial compression process.The dynamic damage variable equation of uniaxial compressed sample is established by analyzing the proportion of plastic elements in uniaxial compression process.Similarly,the dynamic damage variable equation of triaxial compressed sample is also established by analyzing the proportion of plastic elements in triaxial compression process.