| The damage and failure of engineering rock mass is caused by the full development of cracks under the combined action of surrounding rock stress and frequent external stress disturbance.The development and propagation of cracks is essentially a process of damage accumulation and progressive failure of rock.Therefore,focusing on the theme of rock damage induced by crack development under multistage stress disturbance,this thesis adopted the methods of laboratory test,numerical simulation,theoretical analysis and engineering practice to analyze the damage and deterioration laws of rock mechanical properties and crack development characteristics under the influence of various multistage stress disturbance factors,which revealed the macro and micro damage mechanism of each multistage stress disturbance factor and the crack evolution law.Based on the crack development under multistage stress disturbance,a multi-parameter damage evaluation index was established to characterize the damage,and a comprehensive rock damage evaluation method was constructed theoretically.Based on the crack development law under multistage stress disturbance,the multi-parameter damage evaluation index was introduced to construct a comprehensive rock damage evaluation method theoretically.Based on the micro-seismic data of engineering site,the rationality of this method was verified.Combined with the mine earthquake location,wave velocity tomography and multi-parameter evaluation indexes,a comprehensive evaluation method for the coal-rock mass damage in Tianchen Coal Mine is formed.The main research content and results of this thesis are as follows:(1)The rock mechanical property test under multistage stress disturbance was carried out,the rock stress-strain curve,mechanical parameters and fracture characteristics were analyzed,which determined the damage deterioration law of rock mechanical properties under the influence of various disturbance factors and the main control factors affecting the macro fracture mode.The confining pressure is the main controlling factor that determines the macroscopic failure mode of the sample.Under the same confining pressure,the change of stress state will increase the generation of secondary section,resulting in more complex crack propagation network.The increase of confining pressure will reduce the degradation rate of rock mechanical properties,while the increase of disturbance amplitude,disturbance time,and disturbance initial stress will accelerate the degradation of rock mechanical properties.(2)Combined with laboratory test and PFC3D numerical simulation,the mesoscopic damage mechanism of each disturbance factor was further revealed.In multistage stress disturbance test,during the stress disturbance stage,rapid loading and unloading will gradually destroy the rock particles and the cement between the particles.After that,the rock particles destroyed in the constant pressure stage are gradually compacted,making the rock exhibit creep characteristics.The increase of confining pressure will limit the propagation of microcracks to reduce the damage accumulation rate in rock.The increase of disturbance time,amplitude and initial stress can promote the development and propagation of cracks in each disturbance stage,accelerate the expansion of damage area and the formation of damage zone in rock,which will accelerate the accumulation of rock damage to induce failure.(3)Establishing parallel bonding stress corrosion numerical model in PFC3D,combined with the acoustic emission(AE)source phased location based on energy classification and micro-crack classification method based on K-means clustering algorithm,the crack development characteristics and propagation laws of sandstone under multistage stress disturbance were determined.The crack development in the rock presents a developmental evolution process of"local convergence of microcracks forming damage area--continuous development of microcracks expanding damage area--connection of damage area forming fracture zone--connection of fracture zone forming macroscopic failure surface".Microcrack development is the fundamental cause of rock damage,and the aggregation mode and propagation direction of densely distributed high-energy AE sources are the main controlling factors affecting the macroscopic crack propagation direction.The tensile crack is distributed in the whole loading process,while the shear crack is mainly distributed in the failure and post-peak stage.The increase of confining pressure,amplitude,disturbance time and initial stress will increase the total number of cracks in rock,but the increase of confining pressure will increase the proportion of shear cracks,while the increase of disturbance amplitude,disturbance time and initial stress will increase the proportion of tensile cracks.(4)Based on the crack development law,a multi-parameter evaluation index of rock damage was constructed to characterize the crack development,and a comprehensive evaluation method of rock damage based on entropy weight method was proposed.AE b value,Q_t value,S value andΔF value are well coupled with the crack development characteristics of rock different damage stages.AE b values and Q_tvalue show a trend of"high value fluctuation--decrease--continuous low value",and AE S value andΔF value show a trend of"low value fluctuation--increase--continuous high value",which corresponds to the damage state of"low--medium--high"of rock damage.Based on this,the entropy weight method was used to assign weights to four indexes,and the rock damage comprehensive evaluation index was introduced to determine the rock damage grade.The obtained evaluation results were in good agreement with the reality.(5)Based on the rock damage comprehensive evaluation method established in theory,combined with micro-seismic location and wave velocity tomography,a comprehensive evaluation method of coal-rock mass damage in Tianchen Coal mine was established.During mining in working face,the damage area and stress concentration area are determined by micro-seismic location and wave velocity tomography.The precursor information of damage evaluation index and the weight coefficient are optimized in real time by combining the micro-seismic database and the micro-seismic data of the current working face damage area.This realizes the recognition of stress concentration area,the location of damage area and the determination of damage grade in working face.The research results of this thesis reveal the damage and fracture mechanism of each multistage stress disturbance factor,ascertain the crack development law of rock under multistage stress disturbance,and establish a reasonable and effective engineering rock mass damage evaluation method,which has important theoretical significance and application value for maintaining the stability of underground engineering surrounding rock and establishing a monitoring and early warning system for evaluating surrounding rock damage.This thesis has 125 figures,57 tables and 210 references. |
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