Strength of most rock masses is anisotropic. This means that strength is often not uniform in all directions but depends on layering and structural fabric of the rocks. Three components of strength are rock mass, geological defects and intact rock or rock mass 'bridges' between defects. The Hoek-Brown criterion applies to those segments of failure path that are not co-aligned with dominant defect orientations. Conversely, the Hoek-Brown criterion is less representative where the failure path is co-aligned with such fabric. In the latter case, strength is better estimated by the Step-Path method. This approach requires slope face mapping data for several input parameters; drill core does not provide the necessary data. Conventional Step-Path methods use Monte-Carlo simulation of large numbers (> 10,000) of paths (Baczynski 2000). Any statistical model (normal, lognormal, exponential) can be considered for input parameters; many are lognormally distributed. A less rigorous Simplified Step-Path method is described in this paper. However, this approach has a limitation; statistical variability for all inputs is approximated by normal distributions. The Rosenblueth method of statistical moments, and Sampling Theory, are used to develop statistical strength models for any user-nominated Step-Path traverse length. Analysis is Excel spreadsheet based. Case study examples are presented. Step-Path computed strength may be 30 to 45% less than Hoek-Brown values. This strength reduction significantly decreases Factors of Safety and increases stability risks for rock slopes.
Examples of statistical models for geological defect attributes are presented. Whilst data collection errors can occur for all attributes, the factors specifically influencing defect length measurements are examined in more detail. Mapped lengths may underestimate actual lengths by 60 to 80%. Hoek-Brown and Step-Path methods are used to estimate shear strength of failure paths through rock slopes. Both methods are compared. For the inputs considered, Step-Path method may yield shear strength results that are up to 40% lower than Hoek-Brown results. Inputs influencing Step-Path results are examined. Implications of Hoek-Brown and Step-Path results for pit slope design Factors of Safety are discussed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.