平面应变双轴压缩岩样剪应力异常及破坏过程模拟

Numerical simulation for anomalies of shear stress and failure process of rock under biaxial plane strain compression

利用FLAC,模拟了双轴压缩岩样的破坏过程及剪应力异常.在峰值强度之前及之后,岩石的本构模型分别取为线弹性及莫尔库仑剪破坏与拉破坏复合的应变软化模型.本文仅分析了3个具有较大应力降的周期的剪应力异常及塑性区分布.在第1及第2周期,剪破坏分别发生在试样的两侧及背景空区内部,前兆明显,地震属于断错型.在第3周期,背景空区之外的新剪切破裂带引起断错型地震;空区内部的老破裂带引起走滑型地震,未观测到前兆.在塑性区边缘,剪应力梯度较高.破裂带位置剪应力值反而较低.尽管在应力-应变曲线的软化阶段之前,不同单元的剪应力表现出千姿百态的复杂形态,但尚有规律可循.若几个单元都位于某条破裂带上,则剪应力的变化可能是同步的,或演变规律类似.鉴于原地复发地震类型可能不同,前兆将有差别.

The anomalies of shear stress and failure process of a rock specimen under biaxial plane strain compression are modeled using Fast Lagrangian Analysis of Continua （FLAC）. In the elastic stage, the adopted constitutive relation is linear elasticity. In the strain-softening stage, a composite Mohr-Coulomb criterion with tension cut-off is used and the post-peak constitutive relation is linear. The anomalies of shear stress in three periods with marked stress decrease beyond the peak stresses are analyzed. In the first and second periods, shear failure occurs on the specimen sides and in the gap, respectively, and earthquakes belong to the rupture type with apparent precursors. In the third period, new shear failure occurring outside the gap leads to rupture earthquakes; shear failure in the gap results in strike-slip earthquakes without precursor. In the process of failure, higher shear stress gradients appear in the shear fracture zone boundaries. Shear stress is lower at the zone center, reflecting the strain-softening behavior of rock in the zone. In the short-term stage, the migration is opposite. Though the anomalies of shear stress are especially complex prior to the strain-softening stages of the stress strain curve, the shear stresses of different elements have similar or synchronous evolutionary regularities if the monitored elements are located at the same fracture zones. Precursors of shear stress on the condition of recurrence of earthquakes are possibly different due to different earthquake types.