Energy Evolution Mechanism During Rock Deformation And Failure

Rock mass engineering disaster such as coal bump and rock burst results fromnonlinear evolution and catastrophe of energy. Research on rock deformation andfailure from the point of energy evolution can break through the limit of stress-strainanalysis, and offer a new view and method to know deep the mechanical behavior ofrock mass. Aim at the energy evolution mechanism during rock deformation andfailure, four aspects i.e. energy transformation, energy evolution and allocation,nonlinear characteristic and mesoscopic characteristic of energy evolution werediscussed. The main progresses are as follows.(1) Energy transformations during rock deformation and failure were analyzed.The energy transforming process can be divided as four steps i.e. energy input, energyaccumulation, energy dissipation and energy release roughly. A part of inputted energyis transformed into elastic energy, while another part is transformed into other energydissipated. The relationship between the number of rock fragments and dissipatedenergy as well as the one between the velocity of rock fragments and elastic energywere established respectively, and the results show that the dissipated energy decidesthe rock fragmentation, while the residual elastic energy after failure decides therupture intensity. There are two main mechanism of energy drives rock fracture i.e.energy dissipation reduces the failure resistance and energy accumulation enhancesthe driving force of rock failure.(2) Energy evolution and allocation pattern during rock deformation and failurewere obtained. Firstly, the accumulation energy limit, the residual elastic energydensity and the maximum dissipated energy density are put forward. Secondly, theelastic energy shows a slow-fast-slow growth pattern with the accumulation energylimit of0.21MJ/m3under uniaxial compression, while the dissipated energy growsgently at first and has a sharp increase near failure with the amplification of85%; theratio of inputted energy transforming into elastic energy is about60%~82%at thewhole loading process, and it drops slightly near failure. Thirdly, the effects of loadingrate, confining pressure, lithology, and water environment on energy evolution andallocation pattern are investigated. Lastly, the difference of energy evolution amongthree typical mining ways is got, and the maximum elastic energy density in front coalmass of coal mining working face without pillars is1.5times as that in caving mining,while2.5times as that in protective layer mining. The energy release rate increases orderly along protective layer mining, caving mining and coal mining without pillars.(3) Bifurcation and chaos characteristics of rock energy evolution were revealed.The self-repression model reflecting energy transformation was constructed, and theevolution equation of rock energy with stress was obtained and proved. The model issuitable for energy evolution before peak strength. Rock energy evolution has thecharacteristics of bifurcation and chaos, and rock system turns into period doublingbifurcation region when the axial stress reaches92%of peak strength, while it turnsinto chaos state when the axial stress reaches97.5%of peak strength. The energyiteration growth factor μ is put forward, and it can describe the iteration growth effectof rock energy. According to the nonlinear evolution of μ, the process of rockdeformation and failure can be divided four stages i.e.0<μ≤1,1<μ≤3,3<μ≤3.5699and3.5699<μ≤4, and they represent the phase of energy decrement, energyaccumulation, energy dissipation and energy release leading respectively.(4) Mesoscopic characteristics of rock energy evolution were investigated. Theinternal relation among mesoscopic characteristics on geometry and strength, energyevolution characteristic and mesoscopic fracture characteristic is discovered. On theone hand, the effect of average strength, homogeneity, characteristic scale of rockelement and fissure distribution on rock energy evolution was studied, and therelationship between mesoscopic characteristic and dissipated energy was set up.These show that the homogeneity of rock element decides the pattern of energydissipation, while the average strength of rock element and the critical dissipatedenergy decide the value of energy dissipation. On the other hand, the effect of rockenergy characteristic on mesoscopic rupture pattern was discussed. The relationshipbetween effective impact energy index and fractal dimension of fracture surface wasset up, which shows a threshold of fractal dimension exists, and it is positivecorrelation between effective impact energy index and fractal dimension of fracturesurface when the fractal dimension is less than the threshold, while inverse correlationwhen the fractal dimension is more than it. The relationship between effective impactenergy index and micro-rupture evolution is set up too, which shows micro-ruptureevolution has property of catastrophe when the effective impact energy index is large,while it has property of gradualness when the effective impact energy index is small.