锦屏大理岩加、卸载应力路径下力学性质试验研究

 地下岩体开挖卸荷应力路径不同于加载应力路径，由此引起的岩体强度、变形特征和破坏机制也不尽相同。针对锦屏二级水电站引水隧洞群围岩赋存于高地应力环境的特点，对其中3# 引水隧洞大理岩开展单轴加、卸载以及三轴压缩和高应力条件下的峰前、峰后卸围压等4种不同应力路径力学试验，得到了的应力–应变全过程曲线、变形破坏特征和主要力学参数的变化规律。试验研究结果表明：(1) 建立在岩样单轴逐级等量加、卸载应力路径下的回滞环面积递减，尤以屈服阶段的卸载对应变影响最大；(2) 不同围压下岩样三轴压缩全过程试验结果表明，当围压达到40 MPa时，应变软化特性转化为理想塑性，可以认为该值为锦屏大理岩脆－延转化点；(3) 对比以上不同应力路径下的强度准则方程以及峰前、峰后黏聚力和内摩擦角，相同初始应力条件下，岩石卸载破坏所需应力变化量比三轴压缩破坏情况下对应的应力变化量小，说明岩石卸载更容易导致破坏；(4) 在变形破坏机制方面，由于峰后比峰前卸围压塑性变形大，岩样塑性变形已吸收较多的弹性变形能，其脆性特性受到抑制，因而不像峰前卸围压破坏具有突发性，岩样由张性破坏过渡到张剪性破坏；(5) 根据大理岩岩样加、卸载破坏断口SEM扫描结果，从细观角度验证了脆性岩石在不同路径下微观剪断裂破坏机制。总之，以上研究结果揭示了锦屏大理岩加、卸载应力路径下力学特性差异，对解决工程实际问题具有重要的参考价值。

EXPERIMENTAL STUDY ON MECHANICAL PROPERTIES OF JINPING MARBLE UNDER LOADING AND UNLOADING STRESS PATHS

Unloading stress path resulted from underground rock mass excavation is completely different from loading stress path，for which results in difference from that of rock mass strength，deformation property and failure mechanism. The surrounding rock mass of diversion tunnel group of Jinping II Hydropower Station lies in the condition of high geostresses. Aiming at this characteristics，four various stress paths are chosen as test schemes corresponding as the uniaxial loading and unloading test，triaxial compressive test，pre-peaking and post-peaking unloading the confining pressure test under high stress conditions are carried out on the marble specimens from the diversion tunnel #3. The complete stress-strain curve，deformation properties，failure characteristics and change rule of major mechanical parameters are obtained under respective stress path. Some instructive conclusions are drawn from above experimental research as follows：(1) the turning rings for axial repeated unloading vary with gradual higher stress level in terms of area and the unloading stress at yielding stage takes the greatest effect on deformation；(2) the triaxial compressive tests performed under different confining pressures discover that strain softening attribute of marble begins to transferred to ideal plasticity when confining pressure reaches above the value of 40 MPa；so it can be considered as the transition point of brittleness and ductility；(3) by compared with the strength criteria equations under different stress paths，and regular peaking strength parameters，under the same initial stress conditions，the changed magnitude of stress required for rock failure is comparably less than that of triaxial compressive process，which reveals that marble sample is more prone to be subject to failure under unloading stress path；the pre-peaking confining stress unloading path strength are the most sensitive to confining pressure；(4) in terms of deformation property and failure characteristics of marble，its brittleness is constrained by plastic deformation；plastic deformation under post-peaking unloading path is greater than that of pre-peaking；and it has absorbed considerably elastic deformational energy after reaching peaking strength；therefore，post-peaking failure has no outburst property under this stress path；the rock failure mode transforms from tensile failure to the combination of tensile and shear failure；and (5) according to the results of SEM test performed on fractures of marble specimens in case of loading and unloading paths，fracture analysis of marble specimens under different stress paths validates the microcosmic tension and shear failure mechanisms. In conclusion，the above research reveals the differences in mechanical properties of Jinping marble under loading and unloading paths，and it can offer valuable references in solving underground rock engineering problems.