Hoek-Brown parameters for predicting the depth of brittle failure around tunnels

A review of underground openings, excavated in varying rock mass types and conditions, indicates that the initiation of brittle failure occurs when the damage index, Di, expressed as the ratio of the maximum tangential boundary stress to the laboratory unconfined compressive strength exceeds approximate to 0.4. When the damage index exceeds this value, the depth of brittle failure around a tunnel can be estimated by using a strength envelope based solely on cohesion, which in terms of the Hoek-Brown parameters implies that m = 0. It is proposed that in the brittle failure process peak cohesion and friction are not mobilized together, and that around underground openings the brittle failure process is dominated by a loss of the intrinsic cohesion of the rock mass such that the frictional strength component can be ignored for estimating the depth of brittle failure, an essential component in designing support for the opening. Case histories were analyzed using the Hoek-Brown failure criterion, with traditional frictional parameters, and with the proposed brittle rock mass parameters: m = 0 and s = 0.11. The analyses show that use of a rock mass failure criteria with frictional parameters (m > 0) significantly underpredicts the depth of brittle failure while use of the brittle parameters provides good agreement with field observations. Analyses using the brittle parameters also show that in intermediate stress environments, where stress-induced brittle failure is localized, a tunnel with a flat roof is more stable than a tunnel with an arched roof. This is consistent with field observations. Hence, the Hoek-Brown brittle parameters can be used to estimate the depth of brittle failure around tunnels, the support demand-loads caused by stress-induced failure, and the optimum geometry of the opening.