Acoustic Emission Of Rock Deformation And Failure Under Mining Stress Path

As coal mines gradually shift to deep mining,engineering disasters occur frequently.In order to protect personal safety and reduce property losses,in addition to studying the inherent mechanical characteristics of engineering rock itself,it is necessary to consider a variety of human and natural factors on engineering rock Influence,such as stress field and geological environment caused by mining disturbance.Before excavation of the roadway or coal seam,the surrounding rock of the coal seam and roof and floor is in a stable and balanced three-way stress state.With the excavation of roadways and the mining of coal seams,the surrounding rocks are often subjected to complex loading and unloading processes,and different driving and mining methods produce different stress paths and mining influences,which in turn cause the deformation,strength and destruction of the surrounding rocks different.At the same time,acoustic emission is one of the mature methods to characterize the deformation and failure of rocks.Studying the acoustic emission characteristics of rocks under mining stress paths is an effective way to reveal the mechanism of rock destruction under mining disturbance.In this paper,TAW-2000 testing machine and acoustic emission monitoring system were used to conduct the acoustic emission test of red sandstone under conventional uniaxial and triaxial compression and different mining stress paths.The conventional loading and different mining stress paths were analyzed by acoustic emission parameters.The damage evolution law of the lower red sandstone;combined with the fractal theory,the temporal and spatial evolution law of the red sandstone under uniaxial and triaxial compression and different stress paths is analyzed.Through the research of this paper,some progress has been made in the following aspects,and the main results are as follows:(1)Based on the results of the acoustic emission experiment,the evolution law of the acoustic emission time series during the whole process of rock deformation and failure is analyzed.Conventional uniaxial compression has gone through three stages(rising period-quiet period-active period),and the quiet period can be used as a precursor of rock deformation and failure under uniaxial compression;triaxial compression has gone through four stages(compacting stage,elastic stage,Crack growth stage and failure stage),the confining pressure has an inhibitory effect on the acoustic emission activity,the greater the confining pressure,the greater the inhibitory effect,the steeper the curve shape of the ring count and time,the more lagging the stress level at the acceleration inflection point,The rapid increase in the cumulative ring count of acoustic emission can be regarded as a precursor to the overall destruction of the rock.The evolution of the time series of acoustic emission under different mining stress paths is divided into three stages.The sharp increase in ring count with time and the sharp increase in ring count rate in the second loading and unloading phase(phase III)can be regarded as precursors of rock failure and deformation information.(2)The evolution process of the ratio of the rise time and amplitude ratio RA value and the average frequency AF value in the experimental acoustic emission parameters of uniaxial compression,triaxial compression and different mining conditions is analyzed,and the damage mode of rock is revealed.The uniaxial compression is mainly based on tensile failure,and the sandstone under triaxial compression and different mining stress paths mainly exhibits shear failure.The degree of damage decreases in the order of no coal pillar mining,top coal mining and protective layer mining.(3)Through the G-P algorithm combined with matlab language to write a program to obtain the correlation fractal dimension of the ring count rate time series,research and analysis to get the change law ofD_t.The yardstick r has a linear relationship with the logarithm of the correlation function within a certain scale,and the correlation coefficient is large,indicating that the time series of acoustic emission is a fractal distribution.Under different stress paths,the overall evolution trend of the correlation dimensionD_t showed a trend of increasing first and then accelerating and decreasing.When the peak stress was approached,D_t showed a significant acceleration and decline.This stage roughly corresponds to the accelerated rupture period of the rock,which indicates the formation of macro cracks and the critical state of rock instability.(4)Through acoustic emission localization technology,the AE space fractal dimensionD_s is calculated based on the theoretical calculation of the column covering method,and the variation law of the correlation dimensionD_s of the spatial positioning point under different stress paths is analyzed.The spatial positioning of acoustic emission is also a fractal distribution.The general evolution trend of spatial positioning of rock acoustic emission is basically the same.The spatial fractal dimensionD_s shows a downward trend.D_s is in the descending phase when it is near the peak stress,and there are obvious acceleration decline phenomena,indicating the critical state of rock instability.When the peak stress is approaching,_sD has an obvious accelerated decline stage._sD without coal pillar mining has the fastest decline rate,followed by top coal caving.