Application of a new brittleness index to estimate the proneness to brittle failure of rock around a deep tunnel

A major problem that may arise during the excavation of deep tunnels is the incidence of brittle failure of rock, induced by the stress release in a particularly heavy natural state of stress. The severe energy release is often associated to rapid fracturing and to projection of rock blocks inside the opening, phenomena commonly referred to as spalling or rockburst, which endanger personnel and equipment. Thus, the proper prediction of the occurrence of brittle failure is paramount in underground excavations. This paper presents the application of a new rock brittleness index based on the response of two mechanical models of rock damage, which allows the estimation of the proneness to brittle failure of rock around deep tunnels. For this purpose, the competition between ductile and brittle failure is analyzed. The calculation and usage of the index is described considering a real case study of brittle failure in a deep tunnel.