Studies of Seismicity Generated by Unstable Failures around Circular Excavations

摘要

Stress perturbation in highly stressed ground may lead to induced seismicity caused by compressive failure or sudden shear slip along discontinuities. This paper presents a numerical methodology for studying these forms of unstable failures by calculating the unstable excess energy which is released by their occurrence. An explicit finite difference model is used to demonstrate several cases of underground openings being formed in highly stressed ground. Energy changes calculated by the models are first validated from analytical solutions. The unstable excess energy released during a brittle failure or slip along a discontinuity is compared with the theoretical approach. Effects of material brittleness in more complex models are then explored as well as the strength and failure characteristics of a neighboring fault. Parametric studies are performed to identify the circumstances under which seismicity may occur. Unstable failure conditions resulting in seismicity emerge in some of the models and excess energy is produced which cannot be stored or consumed in the system. This excess energy is calculated for each model and is used to assess the relative magnitudes of instability for the given conditions. The results of the models demonstrate that a reduction in strength of the rock during failure can lead to sudden slip of the fault and can trigger a self-propagating failure, although in-situ stresses and the stiffness of the system can totally alter the results. The energy based methods of analysis outlined in this paper provide a rational means of studying induced seismicity in potentially complex combinations of rock mass properties, excavation geometries, and failure characteristics of faults or bedding plane discontinuities.