Brittle rock property and damage index assessment for predicting brittle failure in excavations

The brittle rock properties for Korean Peninsula granite and gneiss were experimentally investigated. The brittle behavior of these rock types was numerically analyzed using the cohesion weakening frictional strengthening (CWFS) model. The monitored depth and the extent of brittle failure at the site were compared using numerical analyses and applying an elastic model, elasto-plastic model, elastic-brittle model and the CWFS model. The variations of axial and lateral strains with the applied axial stress were recorded during the test, with the volumetric strain and the crack volume strain calculated. Based on the rock properties determined from testing, numerical analyses were conducted on deep excavations to assess the cover depth of brittle failure. The results show that the modified method gives results closer to the numerical analysis than the original approach. The damage-controlled test shows that the intrinsic cohesion reduces while friction is mobilized during a rock damage occurrence, as shown by the cohesion weakening frictional strengthening (CWFS) model.