Spatial-temporal Evolution Characteristics Of Acoustic Emission In Rockburst Process Of Granite Under True-triaxial Loading Condition

Currently,with the development of deep underground engineering,rockburst,an engineering geological disaster,is also frequently encountered.This disaster constantly threats constructors' lives and safeties,and seriously delays the schedule of engineering.Rockburst is the ejection damage of adjoining rock caused by the excavation unloading of deep rock mass under high crustal stress.Occurrence mechanism of rockburst is complicated.At present,the issue about how to forecast rockburst accurately is still a worldwide hotspot and difficult problem.As a kind of non-destructive testing,acoustic emission can monitor the process of rock failure which is an effective method for forecasting rockburst.In this paper,we carry out acoustic emission research on granite rockburst process under true-triaxial loading conditions by using a true-triaxial rockburst test system.By analyzing evolution characteristics of acoustic emission characteristic parameters,frequency spectrum,wavelet energy and 3D spatial-temporal acoustic emission events,the spatial-temporal evolution characteristics of acoustic emission in rockburst process of granite under true-triaxial loading conditions are revealed,and the main conclusions are as follows:(1)After analyzing the evolution characteristics of acoustic emission characteristic parameters,we find that in the early stage of rockburst development,both the count-hits ratio and the accumulation of absolute energy are low,and RA value is basically under 50 ms/v.Immediately before rockburst,the count-hits ratio shows a continuous sharp increase,the accumulation of absolute energy shows a laddering increase,and acoustic emission signals with high RA value and high amplitude increase.During rockburst,the count-hits ratio sharply decreases from high value,accumulation of absolute energy reaches the maximum,and the distribution of RA value becomes wide.(2)After studying the evolution characteristics of frequency spectrum and wavelet energy,we find that in the early stage of rockburst development,the dominant frequency is mostly concentrated in low frequency bands and these signals are with low amplitude,the advantage frequency band whose energy proportion is the maximum based on wavelet analysis is less than 125 kHz.Immediately before rockburst,AE signals with high frequency and intermediate frequency begin to teem,the discrete dominant frequency turns to the continuous one,and the proportion of the intermediate and high advantage frequency band increases significantly,especially,the proportion of the advantage frequency band 250 kHz?500 kHz increases most obviously.During rockburst,the acoustic emission is characterized by low frequency and high amplitude,and the advantage frequency band turns to low frequency band which is below 125 kHz.(3)After analyzing the spatial-temporal evolution characteristics of acoustic emission events,we find that in the early stage of rockburst development,the scattered AE events turn to concentrate,and new AE events migrate gradually away from the free face.Immediately before rockburst,the nucleation zone of AE events forms away from the free face and the number of AE events increases near the free face.During rockburst,The acoustic emission is active near the free face and the nucleation zone of AE events can be clearly observed,which indicates that the spalling or slabbing occurs near the free face and the macro shear failure occurs away from the free face.