Study On Mechanical And Acoustic Response And Instability Precursor Determination Of Thermally Damaged Coal Measure Sandstone During Loading

Mine fire is one of the important factors that seriously affect the safety production of underground mines.The high temperature it produces will restrict the safe and stable operation of mine roadway during its service period.Therefore,focusing on the key scientific problems such as mechanical and acoustic response and instability early warning of coal measure sandstone after high temperature thermal damage,this paper takes the coal measure sandstone in Southwest China as the research object,and comprehensively uses the interdisciplinary research methods such as thermal analysis kinetics,thermoelasticity,rock mechanics,statistics,fractal geometry and mathematical elasticity theory,and based on the combination of indoor experiment,theoretical derivation and numerical calculation,the coal measure sandstone affected by different temperature is studied.The main research work is summarized as follows:(1)The sandstone underwent severe physical/chemical changes subjected to high temperature,and the thermal stress promoted the obvious thermal fracture of the rock.The multi-scale pore fissure evolution characteristics of coal measure sandstone after heat treatment at different temperatures are analyzed by means of NMR,MIP and SEM,and the physical/chemical changes of sandstone under the influence of temperature are clarified.Based on the theory of TAK,the three-dimensional diffusion mechanism equation suitable for the solid-phase thermal decomposition of sandstone is explored,the kinetic parameters of rock thermal decomposition are obtained,the thermal stability characteristics of rock minerals are analyzed,and the phenomenon of thermal reaction kinetics of rock is revealed.Starting from the meso structure of rock,according to the random heterogeneity of meso rock particles,a thermoelastic model of random medium is constructed based on elasticity and Weibull distribution theory,and the thermal fracture mechanism of rock samples under temperature was revealed.(2)Temperature has a significant effect on the strength,brittleness and failure characteristics of rock,which shows two mechanisms of mechanical properties: hightemperature strengthening at 25-400 ℃ and high-temperature damage at over 400℃.Uniaxial compression tests were carried out on sandstone after different temperatures treatment,the relationship between rock strength parameters and temperature were analyzed.An evaluation method of rock brittleness based on axial stress-volumetric strain curve was proposed,and the brittleness characteristics of sandstone samples after thermal damage at different temperatures were quantitatively characterized by this method.Based on CT,3D reconstruction and digital image processing technology,the spatial distribution and correlation dimension of rock fracture cracks were analyzed.Finally,combined with the results of uniaxial compression test and microscopic morphology observation,two different damage mechanisms of rock strength at different temperatures in the range of25 ～ 1000 ℃ were revealed.(3)There are significant differences in cyclic load deformation and permeability characteristics of coal measure sandstone under different temperatures.The mechanical properties and deformation characteristics of heat damaged sandstone were analyzed based on cyclic loading and unloading test,and the influence of stress history on the permeability of high temperature heat damaged rock was expounded.The energy calculation equation considering osmotic pressure was constructed,and the energy evolution mechanism of heat damaged sandstone during cyclic loading and unloading was analyzed.Based on two different plastic deformation mechanisms(pore collapse and plastic shear)and permeability evolution characteristics,the trigger mechanism of plastic deformation mechanism of thermally damaged sandstone samples was discussed,and the influence of different temperature on bearing deformation and permeability of sandstone was revealed.(4)Statistical parameters of acoustic signal in thermal damage process of coal measure sandstone can reveal the deformation and failure characteristics of rock,and the nonlinear system characterization parameters can be used as early warning information of rock instability.The statistical law of AE energy during sandstone failure process was analyzed,and the self-organization of AE energy were investigated.It is revealed that the power index obtained by maximum likelihood estimation(MLE)has the obvious advantages of high reliability and strong stability of calculation results.By using the AF and RA,two different crack propagation modes of rock tension and shear were effectively identified,and based on the KED,the high-density concentration area of AF and RA in the AF-RA coordinate system was visually analyzed,and the crack propagation mechanism in the bearing process of high-temperature heat damaged sandstone ws revealed.Based on the phase space reconstruction theory and G-P algorithm,the characterization parameters of rock nonlinear system were calculated,the evolution process of thermal damage sandstone failure process nonlinear system was explored,and the early warning information of rock instability was obtained.(5)The stress analytical solution of roadway surrounding rock is derived,the disaster mechanism of thermal damage roadway surrounding rock under loading is revealed,and the comprehensive judgment method of thermal damage roadway surrounding rock instability warning information is put forward,and the high precision warning of surrounding rock instability is realized.Based on the indoor biaxial compression test,the catastrophic instability process of roadway surrounding rock affected by fire high temperature is simulated,and the deformation and failure characteristics of roadway surrounding rock affected by different temperature are analyzed.The section shape mapping function is determined by complex function theory,conformal transformation and PSO algorithm,and the analytical solution of surrounding rock stress of straight wall arch roadway is deduced.From this,the stress distribution characteristics of surrounding rock in the bearing process are calculated.Finally,combined with SEM,CT scanning and KDE visual analysis,the catastrophe mechanism of heat damaged surrounding rock mass under stress is revealed.Based on the avalanche effect of surrounding rock fracture,the AE correlation dimension,b value and the critical value of the non fixed critical point of the MLE curve were calculated and analyzed,and the precursory information of rock mass instability around the roadway was obtained.On this basis,the feasibility of multi index surrounding rock instability early warning system is preliminarily discussed and verified,and the high-precision early warning of surrounding rock instability is realized.This study deepens the understanding of rock mechanical behavior under the influence of high temperature,disaster characteristics and instability early warning,and provides support for stability analysis and instability warning of roadway surrounding rock after mine fire in China.During the research period,a total of 12 academic papers were published,of which 8 academic papers as the first author were indexed by SCI,and14 national invention patents were authorized.