峰前围压卸荷条件下岩石的应力–应变全过程分析和变形局部化研究

地下工程和边坡工程开挖一般处于卸荷条件。岩石在加载和卸荷条件下的力学特性不同,研究岩石在卸荷条件下的强度特性具有重要的理论和现实意义。根据损伤断裂力学知识建立了岩石处于卸荷条件下的全过程应力–应变关系,包括线弹性阶段、非线性强化阶段、应力跌落和应变软化阶段。理论和试验研究发现,岩石卸荷破坏所需要的应力比连续加载破坏时小,且卸荷破坏时的变形比连续加载时大,其主要原因是卸荷时存在裂纹张开,裂纹张开导致了无摩擦滑动和变形模量的减少,岩石的无摩擦滑动必定比摩擦滑动所需的应力小。通过和试验结果的比较验证了该理论的有效性和正确性,该理论对边坡工程和地下工程开挖具有重要的参考价值。

ANALYSIS OF LOCALIZATION OF DEFORMATION AND COMPLETE STRESS-STRAIN RELATION FOR MESOSCOPIC HETEROGENOUS BRITTLE ROCK MATERIALS WHEN AXIAL STRESS IS HELD CONSTANT WHILE LATERAL CONFINEMENT IS REDUCED

Stress redistribution induced by excavation of underground engineering and slope engineering results in the unloading zone in parts of surrounding rock masses. The mechanical behaviors of crack-weakened rock masses under unloading are different from those of crack-weakened rock masses under loading. The pre-existing microcracks will be closed under loading,but they will be open under unloading. The deformation of rock material under unloading is more larger than that under loading. The frictional sliding activation on pre-existing cracks will occur under loading,but the frictionless sliding activation on pre-existing cracks may occur under unloading. Under same condition,the axial stress at which some pre-existing cracks begin to propagate in a stable fashion under unloading is less than that under loading;and the axial stress at which some pre-existing cracks begin to propagate in an unstable fashion under unloading is less than that under loading. The strength of rock material under unloading is less than that under loading. A micromechanics-based model has been proposed for brittle rock material undergoing irreversible changes of microscopic structures due to microcrack growth when axial stress is held constant while lateral confinement is reduced. The basic idea of the present model is to classify the constitution relation of rock material into four stages including some of the stages of linear elasticity,pre-peak nonlinear hardening , rapid stress drop , and strain softening , and to investigate their corresponding micromechanical damage mechanisms individually. Special attention is paid to the transition of structure from rearrangements on microscale to the macroscopic inelastic strain,to the transition from distribution damage to localization of damage and the transition from homogeneous deformation to localization of deformation. The closed-form explicit expression for the complete stress-strain relation of rock materials containing cracks under unloading is obtained. The results show that the complete stress-strain relation and the strength of rock materials under unloading depend on the crack spacing,the fracture toughness of rock materials and orientation of the cracks,the crack half-length and the crack density parameter.