Abstract

Significantly different inputs are considered in deriving rock mass shear strength via Hoek-Brown Method (HBM) and Step-Path Method (SPM) approaches. The HBM cannot effectively assess the directional strength in rocks. Conversely, the key objective of the SPM is to consider the relative strength impact of the geological defects that are near co-aligned with part(s) or full length of critical failure paths through rock masses. Defect attributes considered in SPM are their orientation, relative occurrence, likelihood of cut-off by other defects, presence of intact rock and/or rock mass bridges between non cut-off defects and shear strength.

Where applicable by the structural conditions in the rock mass, the SPM strength may be 40–50% less than the equivalent HBM strength. However, SPM approach does require a more comprehensive understanding of the structural conditions in the rock mass and a greater analysis effort to develop the shear strength parameters.

In the SPM, the rock mechanics problem is partitioned into structural domains. Each domain is associated with a specific geological defect set that is approximately co-aligned with the anticipated critical failure path through this domain. General HBM conditions apply in those domains where no defect set is co-aligned with the failure path. In domains where the SPM is relevant, the conventional Hoek-Brown Geological Strength Index (GSI) chart values are adjusted for probability of occurrence, cut-off and length of bridges between non cut-off defects of those defect set(s) that impact rock mass strength outcomes.

The logic, methodology and step-by-step procedure for adjusting the Hoek-Brown GSI chart to derive SPM outcomes are described in detail. Example adjustments are presented to better illustrate the involved process.

1.
Introduction

The Hoek-Brown Method (HBM) for shear strength of rock masses was first published by Hoek and Brown (1980). HBM considers the intensity of rock mass ‘structure’ (blockiness) and ‘condition’ of geological defect surfaces. The same strength is computed for all directions through the rock mass.

Contrary to this initial HBM non-directional strength premise, failure paths are often structurally controlled and coincide with geological defects co-aligned with path directions. Some paths may be almost entirely defined by geological defects with only minor shearing through intact rock and/or rock mass ‘bridges’ between defects. To address HBM directional strength limitation, several authors revised the Geological Strength Index (GSI) chart to consider foliated rocks. These revisions are mostly qualitative; not quantitative.

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