Study On Mechanical Properties And Failure Precursor Characteristics Of Gas-bearing Coal Subjected To True Triaxial Loading-unloading

Underground mining activities in coal mines will cause stress unloading of coal and rock,resulting in static loading-unloading changes in three-dimensional stress.The unloading of stress will further lead to deformation and failure of coal and rock,thereby inducing the occurrence of coal and rock gas dynamic disasters.Therefore,studying the failure precursor characteristics and mechanical properties of gas-bearing coal subjected to true triaxial loading-unloading conditions is particularly important for understanding the occurrence and development of coal and rock gas dynamic disasters.In this paper,using the self-developed true triaxial deformation and failure test system,taking gas-bearing coal as the research object,the conventional triaxial loading tests and true triaxial loading-unloading tests are carried out under different conditions,the mechanical characteristics of gas-bearing coal under different conditions are discussed,and the precursor information of instability and failure of gas-bearing coal under different loading-unloading conditions is analyzed.The main research results are as follows:(1)In order to study the deformation and failure characteristics of gas-bearing coal under different confining stresses and different gas pressures,analyze the influence of different loading conditions on the deformation and failure characteristics,and determine the initial unloading point of the true triaxial loading-unloading test,the conventional triaxial loading test was carried out.(2)The true triaxial loading-unloading test of gas-bearing coal under different loading-unloading conditions was carried out,and the mechanical properties,energy evolution and acoustic emission characteristics were studied.Under true triaxial loading-unloading conditions,in the direction perpendicular to the minimum principal stress,the cracks in the gas-bearing coal gradually develop to form the main failure surface,and the failure form is tensile-shear composite model.As the distance from the unloading surface becomes smaller,the crack gradually turns from a shear crack to a split tensile crack.A spall structure is formed near the unloading surface,and most of the damage near the unloading surface occurs in the form of splitting into plates and bending of the plates.Under true triaxial loading-unloading conditions,the energy change curve and the stress-strain curve during the failure process of gas-bearing coal correspond well,which can be used as an important reference basis for analyzing the failure process of gas-bearing coal.The AE time series of gas-bearing coal subjected to true triaxial loading-unloading conditions can be divided into three stage,which are basically the same as the failure process of the sample.(3)Based on rock mechanics strength principle,statistical damage theory and Terzaghi effective stress principle,considering the effects of gas pressure and triaxial stress,a constitutive model of gas-bearing coal subjected to true triaxial stress conditions is established.The stress-strain theoretical curve and the experimental curve under different loading-unloading conditions are compared,and it is found that the two have good similarities.According to the elastoplastic fracture mechanics,considering the effects of free gas and adsorbed gas,the coal strength criterion is derived,and the relationship between crack propagation length and three-dimensional stress is analyzed.According to the theory of elastic mechanics,a buckling model for the failure of the spall structure is established,and the critical load conditions for the failure of the spall structure are calculated.(4)Based on the multi-fractal theory,the multi-fractal characteristics of AE time series during the true triaxial loading-unloading failure process is calculated,and the change process of AE multi-fractal parameters under different loading-unloading conditions is analyzed.At the same time,the change process of the multi-fractal parameters is compared with the damage evolution process of the sample,and the precursor characteristics that characterize the failure of the sample are obtained.The dynamic variations of ?? and ?f can reflect stress condition and damage degree of coal samples.The dynamic variation of ?? has a good correspondence with the damage evolution of coal samples.The gradual decrease of ?? corresponds to the slow increase of damage,while the rapid jumps of ?? corresponds to the rapid increase of damage.Meanwhile,the mutation point of damage curve shares the same change trend as that of?? value,which suggests that the change of ?? can reflect the damage variation process of coal samples,which can be used as a precursor information for predicting coal and rock rupture.