Finite element check of GSI adjustment equation reconciling Hoek-Brown and Step-Path strengths

Rock mass strength is impacted by the coalignment of geological defects and intact rock (IR) bridges. The greater the alignment of geological defects and smaller the rock bridges, the weaker the rock mass strength. Current versions of the 1980s Hoek-Brown Method (HBM) for rock mass strength considers rock mass blockiness and defect condition via a Geological Strength Index (GSI) chart. This chart was refined several times, but these revisions still do not directly consider coaligned defects and bridges. Step-Path Method (SPM) evolved in mid-1970s to assess strength implications of coaligned defects and IR bridges. Early SPM models were simple; but progressively evolved to consider more complex situations. In late 1980s, computer software was written to assess rock mass shear strength by Monte-Carlo statistical simulation of defect and IR bridge along failure paths through slopes. Since mid-2000s, EXCEL spreadsheets have aided this type of assessment. SPM often computes a lower rock mass strength than HBM. Based on 230+ case studies an equation was proposed in 2019 to adjust the GSI index in HBM based on relative occurrences of coaligned defects and IR bridges along failure paths. The general equation is GSI _Design = {GSI _{General Rock Mass} – [0.4 x (% coaligned defects)] + [1.2 x (% IR bridges)]}. With the above adjusted GSI, HBM computes near-same rock mass strength as SPM. Appropriateness of above GSI adjustment equation with respect coaligned defects is checked by two-dimensional (2D) jointed finite element (FE) modelling. Results are presented and discussed. The method is presented for alternative strength inputs into jointed rock masses to account for the presence of co-aligned defects dipping into the excavated slope.