Study On Post-Peak Deformational Characteristics Of Rock Mass And Structural Stability Of Tunnel Based On Strain-Softening Model

The post-peak constitutive relationship of rock mass is the basis of stability analysis for surrounding rock mass and bolt support design. The deep study of post-peak constitutive relationship for rock mass may make the stability analysis more precise, reasonable, safe and reliable and has important engineering meaning. At the same time, it also can promote the development of rock mass mechanical theory. Aiming at the deficiency of the existing mechanical models, combinating with the demand of "973program"—acroscopic and mesoscopic mechanical behavior of multiple sets of fractured rock mass in the state of high stress and unloading, Based on elastic and elasto-plastic theory, using the method of theoretical derivation, experiment, probability analysis and numerical analysis et al., the problem of deformational failure mechanism, post-peak stress-strain relationship, and bolt support optimization for rock mass and stability analysis for surrounding rock mass was studied in this thesis. The main work is listed as follows:(1)By comparing the location of mohr-coulomb strength curves of rock and cracks in the coordinate system of σ1-σ3, the strength characteristics, failure mode and optimization method of bolt support angle were studied. The mechanism that the rock mass fails along the structural face under low confining pressure but sometimes switches to fail through the rock under high confining pressure in the triaxial test was disclosed. And the analytical express of critical confining pressure for the switching was obtained. Comparing with the traditional method of Mohr stress circle, the method of Mohr-Coulomb strength curves in the coordinate system of σ1-σ3is simpler and more favorable to study the strength chartacteristics, failure mechanism and other rock mass mechanical problems with the method of mathematical analysis.(2)Based on strain-softening model, the general method of solving post-peak stress-stain relationship was proposed:First, determine the strain-softening parameter and strength criterion. Then establish the relation of strength parameters and strain-softening parameter according to the evolution law and establish the relation of stress and strength parameters according to strength criterion. Finally, the stress-strain relation can be obtained by playing the role of strength parameters as ties. The strain-softening model bases on elasto-plastic theory and takes some thoughts and methods of damage mechanics and breakage mechanics of geological materials into consideration.(3)The influence of elasto-plastic coupling on post-peak stress-strain curves was studied and it shows that elasto-plastic couplings of elastic modulus and poisson’s ratio all have some influence on post-peak stress-strain curves. As for influence extent, the coupling of elastic modulus have more effects on axial strain than that of poisson’s ratio, and the coupling of poisson’s ratio have more effects on hoop strain than that of elastic modulus.(4)The influence of strain-softening parameter on post-peak stress-strain curves was studied. The three types of post-peak stress-strain curves obtained by respectively regarding the plastic shear strain, equivalent plastic strain and major principal plastic strain as strain-softening parameters were studied by comparison.(5)Based on strain-softening model, the general method of solving post-peak stress-strain relation for rock mass with multiple sets or stochastic distribution of penetrative cracks was proposed. The influence of mean space, normal stiffness and shear stiffness of cracks on post-peak stress-strain curves was studied.(6) A more complete method of elasto-plastic analysis was proposed based on strain-softening model by improving the method of elasto-plastic analysis proposed by Lee YK. The proposed method in this thesis may consider the influence of elasto-plastic coupling on stress and displacement, and the step is simpler. On this basis, the influence of evolution for elastic modulus and poisson’s ratio, dilatancy effect and brittleness of rock on yield range, stress and displacement was studied.(7)Considering the confinement effect of liner to surrounding rock mass, the stability problem of surrounding rock mass for a lined circular pressure tunnel was studied based on strain-softening model. First, based on Mohr-Coulomb yield criterion, the method to solve the numerical solution of yield radius, hoop stress and radial displacement for a lined circular pressure tunnel was proposed and it was compared with the ideal elasto-plastic model proposed in paper[166]. In the next place, based on Hoek-Brown yield criterion, the method to solve the numerical solution of yield radius, hoop stress and radial displacement for a lined circular pressure tunnel was proposed and it was compared with the ideal elasto-plastic model and ideal elastic brittle model. The ideal elastic plastic model and ideal elastic brittle model may consider as two extreme cases of strain-softening model. Comparing with the ideal elasto-plastic model and ideal elastic brittle model, the strain-softening model may describe the post-peak strain-softening behavior more precisely, so more precise analysis to stability of surrounding rock mass can be achieved.(8)The switching mechanism from structure control to stress control on mechanical behavior of surrounding rock mass with the increase of excavation depth was disclosed.The analytical solution of switching critical intial stress for structure control and stress control on failure behavior of surrounding rock mass was obtained.