A New Method to Assess the Possibility of Brittle Failure of Rock Induced by Deep Excavations

Abstract

Spalling and rockburst are severe criticalities that can emerge while excavating deep tunnels in rock masses under heavy natural stress states. Here, rock brittle failure can induce massive releases of the energy stored during the excavation and dangerous projections of rock blocks into the opening. The prediction of rock brittle failure is therefore crucial and, for this purpose, different empirical brittleness indexes have been proposed in the past. However, many of them provide predictions that is often not consistent and/or truthful, as they do not consider the stress and energy variations induced in the rock mass by the excavation. This paper presents an innovative method to distinguish between ductile and brittle failure of rock around deep tunnels. The method is based on two mechanical models of rock damage that were formulated to describe brittle and ductile failure mechanisms within the rock mass, as induced by the stress release during the excavation. These models are integrated into the definition of a new brittleness index, named tunnel brittleness index (TBI). TBI quantifies the outcome of the competition between the two failure mechanisms, estimating the susceptibility of the rock mass to brittle failure. The effectiveness and the application of TBI are shown with reference to a real case study. Specifically, TBI appears as a promising and useful tool for engineers dealing with deep tunnel projects that may be employed for predicting brittle collapses in the early stages of the design, which would be crucial in the preliminary choice of excavation techniques and machinery, and the support systems.