Study On Permeability Evolution Mechanism Of Rock Fracture Process Based On Acoustic Emission Moment Tensor Theory

The fissure produced in the process of rock deformation and fracture is the fundamental cause of underground engineering water inrush disaster,and it is also one of the current research focuses in the field of rock engineering.The fracture expansion will change the connectivity of the fracture,and the opening and closing of the fracture will change the aperture of the fracture,resulting in the change of the permeability of the fracture rock.Therefore,an accurate understanding of the evolution law of fracture induced permeability in the process of deformation and fracture of rock is essential for evaluating the safety and stability of underground engineering,and is the basis for realizing the safety construction and operation of underground engineering.Although the permeability of fractured rock mass is an important factor affecting the safe construction and operation of underground projects,it is difficult to characterize the permeability of rock mass on site without hydrostatic test.Therefore,aiming at this problem,this thesis focuses on how to characterize the permeability of fractured rock mass without traditional hydraulic test.In this thesis,a series of triaxial hydraulic coupling acoustic emission tests were carried out on deep sandstone samples.The permeability evolution process is divided into five stages by using acoustic emission counting and the characteristics of stress strain curves.The effects of time-space distribution of acoustic emission events,magnitude,fracture mode and focal volume deformation on the permeability of the five stages are studied,and the effects of focal volume compression and volume expansion events on the maximum permeability in the whole loading process of sandstone samples under different effective confining pressures are analyzed.The conceptual model of permeability evolution is established,and the microscopic mechanism of permeability evolution is revealed.The results show that the increase of permeability is related to the spatial distribution and magnitude of source volume expansion events,and the decrease of permeability is related to the proportion of source compression events.It can be seen from the laboratory test results that the detailed information related to the evolution of rock permeability and fracture can be obtained under the test conditions.The evolution behavior of rock permeability depends not only on the spatial distribution of acoustic emission events(which can be expressed by fracture density and connectivity),but also on the magnitude of acoustic emission events(which can be expressed by fracture volume)and source volume deformation characteristics(which can be expressed by fracture opening and closing).In order to understand the influence of these parameters on permeability,it is necessary to obtain reliable information about the evolution of the spatial orientation,geometric characteristics,opening and closing characteristics of these fractures with load.According to the results of laboratory tests,the fracture is assumed to be a disc with a certain thickness in this thesis.The geometric size and spatial orientation of the fracture generated during the loading process of sandstone samples are determined by using the moment tensor inversion method.The initial fracture parameters are constrained by statistical laws and initial permeability.Based on the initial fracture and the inversion fracture parameters,the fracture network models at different loading times are established.It can be seen from the test results that the opening and closing of fractures have a great influence on the permeability.Therefore,an improved statistical permeability model is established by considering the opening and closing characteristics of fractures,and the permeability of the fracture network model in the process of sandstone loading is calculated.The validity of this method is verified by comparing with the laboratory results.The improved statistical permeability model only uses fracture geometry information(including fracture density,connectivity,length,aperture)and fracture opening and closing characteristics to estimate the permeability,realizing the research goal of characterizing the permeability of fractured rock mass without traditional hydraulic test.Based on the idea of fracture geometry parameters to calculate permeability,the verified improved statistical permeability model is used to simulate the permeability evolution during rock fracture by discrete element method.The core idea of the simulation program is to use the moment tensor algorithm to calculate the geometric parameters of the fracture during the fracture process,and at the same time,consider the effect of effective stress on the fracture aperture,and use the statistical permeability model to calculate the permeability of the representative volume unit during the fracture process.The established model is firstly verified by the mechanical and permeability data of triaxial hydraulic coupling test of sandstone,and the simulation results are in good agreement with the laboratory test results.Compared with the discrete element domain model,the calculation efficiency of this model has been greatly improved on the premise of ensuring the calculation accuracy.On this basis,the evolution of floor permeability caused by coal mining is numerically simulated,and the floor microseismic activity during coal mining is analyzed.In this thesis,the relationship between spatial location of AE events,focal mechanism and permeability evolution is analyzed through triaxial hydraulic coupling AE test of sandstone.On the basis of the experimental research results,an improved statistical permeability model considering the opening and closing of fractures is established,which realizes the research goal of characterizing the permeability of fractured rock mass through the geometric parameters of fractures and the characteristics of opening and closing of fractures without hydraulic test,and develops a discrete element numerical model based on fracture parameters to simulate the permeability.The results and conclusions obtained can provide reference for the calculation of fracture induced permeability evolution process caused by deep underground resource exploitation and roadway excavation,and for the monitoring and prevention of water inrush disasters in underground projects.In this dissertation,there are 75 figures,8 tables,and 252 references.