Modeling violent rock failures in tunneling and shaft boring based on energy balance calculations

Abstract

Excavation in highly stressed grounds may lead to violent compressive failures of rock or violent slip along discontinuities in the vicinity of the underground space. Violent failures, usually with anomalous seismic signatures, are characterized with radiating excess energy during the failure, endangering the safety and productivity of the operation. This paper adopts a numerical methodology, developed in the Universal Distinct Element Code (UDEC) for quantitatively studying the occurrence and intensity of violent failures, and advances it for respective analyses in tunneling and shaft-boring operations. The methodology enables estimating the radiated energy as a measure for evaluating the occurrence and relative intensity of violent failures. We first validate the radiated energy calculation in the models against analytical solutions for the formation of a circular opening, analogous to the geometry of tunnels and shafts. After reviewing the concept of violent failures by ideal models, we explore the effects of different factors on the occurrence and intensity of violent failures caused by an excavation into brittle rock or next to a preexisting fault. Limited cases studied in this paper demonstrate that different combinations of the rock elastic modulus and post-failure response of rock and discontinuity can result in a violent failure with various intensities. Larger excavations under higher deviatoric stresses can intensify the potential violent failure mostly through expanding the rock failure zone or the slip area on the activated fault. However, as opposed to the radiated energy, using the failure zone volume and fault slip area alone may lead to underestimating or overestimating the intensity of possible violent failures in a modeling practice. The energy-based analyses outlined in this paper provide a rational yet simple means for quantitatively studying violent failures in potentially complex geological conditions and excavation layouts.