Study On Marble Failure Acoustic Emission Characteristics And Its Constitutive Model Under Different Loading And Unloading Stress Paths

There has initial damage in rock mass during its formation and long geologicalprocesses，which is a typical non-homogeneous material. Rock engineering excavationis a complex process of loading and unloading. In the rock excavation process，internaldamage continues to accumulate，micro-cracks are expanded and aggregated，and theseultimately connect to cause rock failure. Rock failure mechanism and its failureprecursor has important guiding significance to design rock excavation. Based on theresults of the marble conventional triaxial loading and unloading tests，variations of thecharacteristic stress are proposed in rock deformation process. The characteristics ofrock acoustic emission parameters during failure process under various confiningpressures are analyzed. A damage variable is defined in terms of acoustic emissionparameter. A damage constitutive model is established according to the deterioratinglaw of damage variable. The main work can be summarized as follows：(1) Based on the results of the marble conventional triaxial loading and unloading tests，variations of the characteristic stress，dilatancy angle ψ are proposed in rockdeformation process of loading and unloading stress paths. With the increase of theconfining pressure hoop strain and volumetric strain corresponding to the compactionstress became smaller under the same unload rate. But the crack initiation stress andexpansion stress became larger. The hoop strain corresponding to expansion stress werestabled at the range of (-0.0004±0.0001). The dilatancy angle ψ increased rapidly afterthe unloading point，then it slowly decreased and substantially unchanged. Thedilatancy angle ψ monotonically increased in conventional triaxial test.(2) The extreme-point symmetric mode decomposition method is used to denoisethe acoustic emission data of marble unloading confining pressure failure process. Thecharacteristics of acoustic emission variation are analyzed. The acoustic emissionfrequency trends to fluctuate. It increases suddenly before peak strength and decreasessignificantly near the peak strength. The fractal dimension of acoustic emission presentsa phase change during marble unloading confining pressure failure process. It increasesat the initial stage and then decreases. The fractal dimension increases sharply after80%peak strength and decreases significantly before rock failure. There is relation betweenthe frequency spectrum and the fractal dimension. The sudden rise of the acousticemission frequency and the fractal dimension is the rock failure precursors. (3) Initial yield surface and subsequent yield surface conform to Mogi-Coulombfunctional form in unloading stress path. The cohesive force c decreases and internalfriction angle increases with the increase of the plastic parameter psbefore the peakstrength. A tunnel excavation process was analyzed, which used the traditional methodand the bond weakening-friction strengthen method. Development of tunnel rockdeformation，stress and plastic zone were compared in two models. A damage variableis defined in terms of acoustic emission parameter. The damage constitutive model isestablished according to the deteriorating law of damage variable, which considers theeffect of confining pressure change. The theoretical model is in good agreement withtriaxial loading and unloading test data.