北山深部花岗岩常规三轴压缩条件下的强度参数演化及能量耗散

Evolution of strength parameters and energy dissipation of Beishan deep granite under conventional triaxial compression

岩石在荷载作用下的变形破坏实质是能量驱动的渐进损伤演化过程,是裂纹闭合、发育、扩展和贯通的综合表现。从能量耗散角度分析岩石破坏机制能更好地认识其破坏的复杂过程。为探究北山深部花岗岩破裂过程中的强度参数演化和能量耗散规律,对北山预选区深度为550~560 m的花岗岩开展常规三轴力学特性试验和声发射特征试验。基于不同围压下的应力–应变曲线和声发射特征,以归零化后的裂隙体积应变(ε_(vmax)^(c)-ε_(v)^(c))为损伤变量,建立北山深部花岗岩黏聚力、内摩擦角和岩石性质参数随损伤变量变化的演化模型,并从能量角度构建花岗岩破裂过程中的能量耗散围压效应方程。研究结果表明:(1)应力–应变峰前曲线大致分为裂隙压密阶段、弹性变形阶段、裂隙稳定扩展阶段和裂隙非稳定扩展阶段,各阶段分界点的特征应力与围压大致呈线性关系;(2)Mohr-Coulomb强度准则参数c,φ和Hoek-Brown强度准则参数mi随损伤变量ε_(vmax)^(c)-ε_(v)^(c)的增加先快速增长后缓慢降低最终趋于稳定;(3)岩石压缩过程中的声发射振铃计数率与能量累计数具有良好的对应关系,振铃计数率高频段伴随能量累计数阶跃增长,能量累计数曲线可分为初始平稳段、第一次快速增长段、平缓增长段、第二次快速增长段和稳定段5个阶段;(4)弹性应变能各特征值随围压的增大逐渐增大,耗散能各特征值随着围压的增大先平稳变化然后阶跃式增长最终趋于稳定。

Rock deformation and failure are essentially a damaged evolution process,which includes crack closure,crack development,crack propagation and crack penetration driven by energy.The failure mechanism and complex process can preferably be analyzed from the view of energy dissipation.The conventional triaxial loading tests and acoustic emission characteristics tests on Beishan granite obtained from 550‐560 m depth were conducted to investigate the laws of strength parameter evolution and energy dissipation.Based on the stress-strain curves and acoustic emission characteristics under different confining pressures,a mathematic model describing the relationships of cohesion,internal friction angle and rock property parameters with the damage variable,namely the normalized crack volume strain ε_(vmax)^(c)-ε_(v)^(c),of Beishan deep granite was established.Besides,the energy dissipation confining pressure effect equation during the process of granite fracture was established from the point of energy dissipation.Results show that:(1)the pre-peak diagram of the stress-strain curve can be divided into four different stages including crack compaction,elastic deformation,crack expansion and crack unstable growth,and the characteristic stresses at the boundary points of the stages have a linear relationship with the confining pressure,(2)with increasing the damage variable,the strength parameters c andφof Mohr-Coulomb criterion and mi of Hoek-Brown criterion increase rapidly,then decrease slowly and finally tend to be stable,(3)there is a corresponding relationship between the AE ringing count rate and the energy cumulative number during the rock compression process,and the high frequency points of the ringing count rate are accompanied by the step growth of the energy cumulative number.The curve of the energy cumulative number can be divided into five stages such as initial stable,first rapid growth,gentle growth,second rapid growth and stable,and(4)with increasing the confining pressure,the eigenvalues of the elastic strain energy gradually increase,while the eigenvalues of the dissipated energy first change steadily,then increase by step growth and finally tend to be stable.