Stability of underground excavations is of utmost concern in both mining and civil engineering projects. The brittle Hoek-Brown parameters (m = 0 and s = 0.11) introduced by Martin et al. (1999) were found to be very useful in assessing the stability conditions of underground excavations. However, observations in granodiorite at the Underground Research Laboratory (URL), in quartzite at Thompson Mine, and in stopes in sulphide ore bodies reveal that the use of the current brittle Hoek-Brown parameters over-predicts the depth of failure in some rock types. These observations lead to a conclusion that the brittle Hoek-Brown cohesive parameter s is not constant at 0.11 for all rocks but varies depending on rock type.

The objectives of this thesis are to identify the factors that cause brittleness in rocks and to develop a brittle rock rating system for selected rocks, such that the rating system can be used to get the appropriate brittle Hoek-Brown parameter s for different rock types. Several factors account for the differences in rock brittleness, namely m ineralogy, texture (grain size), foliation conditions relative to loading direction, sample disturbance (stress relief fracturing), porosity, inherent inter and intra mineral grain fractures, and total metal content in terms of sulphide and other metallic ores.

A numerical code, RFPA20 (Tang, 1997) was used to quantitatively determine the effect of material heterogeneity on brittleness of rocks. A flowchart and procedure are developed to rate the brittleness of selected rocks based on grain size, mineralogy and foliation conditions. The parameter developed for rating rock brittleness is termed Brittleness Rating Index (BRI). BRI accounts for the role of texture, foliation and mineralogical composition in rock brittleness. A relationship is developed between BRI and stress level (SL). Stress level is defined as maximum tangential stress at the excavation wall normalized by intact rock uniaxial compressive strength. On the other hand, strength ratio (SR) is defined as the stress level at which depth of failure is zero.

BRI is related to the Hoek-Brown brittle rating parameter s and provides a means for determining rock specific brittle parameter s for input into numerical codes for assessment of underground excavations performance. The numerical code Phase20 (RocScience, 1999) was used to validate the rating system and the brittle parameter s determined by analyzing case histories. These analyses show that the brittle rating system is promising as it gives more reliable depth-of-failure predictions for underground excavations in different rock types, when compared with the original brittle Hoek-Brown parameter s (s = 0.11 for all rock types). The procedure developed in the thesis is only applicable to massive hard brittle rocks close to the boundaries of underground excavations where loading conditions are close to uniaxial compression.

It is recommended that further work be done on the effect of external factors such as sample disturbance and other inherent properties such as porosity, metal content, inter and intra mineral fractures and grain shape.