Influence of tunnel wall roughness and localized stress concentrations on the initiation of brittle spalling

Abstract

Brittle failure of massive hard rocks occurs when the stress near the excavation reaches the in situ strength. After reviewing published papers, we found that two approaches, one using UCS field ci ≈ 0.45 UCSlab and the other using UCS field cd ≈ 0.8 UCSlab, have been used to estimate the in situ strength of massive rock, where UCSfield and UCSlab are the field and laboratory uniaxial compressive strengths, respectively, and the superscripts denote UCSfield defined by the crack damage stress or crack initiation stress. In this study, a continuum model considering the “as built profile” was constructed to simulate brittle rock failure in two orthogonal tunnels in the 420 level of Atomic Energy of Canada Limited’s (AECL’s) Underground Research Laboratory. Results obtained from the simulation with in situ strength at UCS field cd seem to capture actual rock conditions better than that with the in situ strength at UCS field ci . Opening wall roughness has a great influence on the brittle failure of massive hard rocks. It was found that compared with the smooth boundary condition, stress can concentrate around the rough boundary and the induced tensile stress driven by compressive stress loading can promote rock spalling failure. If the opening roughness is overlooked, the field uniaxial compressive strength of UCSfield will be underestimated.