Challenges Assessing Rock Slope Stability Using the Strength Reduction Method with the Hoek–Brown Criterion on the Example of Vals (Tyrol/Austria)

Abstract

To estimate the hazard posed by rock slopes, it is essential to determine the overall stability and potential detachment volume. This is mostly solved using numerical methods together with the strength reduction method (SRM). Many calculation programs do not provide a direct implementation of the Hoek–Brown (HB) criterion. Equivalent Mohr–Coulomb (MC) parameters are often used. Especially for steep rock slopes, the use of equivalent MC parameters with numerical codes and the SRM lead to poor estimates of safety factors. The problem lies in the required and often difficult estimation of a suitable range of minor principal stresses over a ‘slope height’. In the example of the stability analysis of the rock slope Vals in Tyrol/Austria, we show the differences between the application of equivalent MC parameters and a direct application of the HB criterion with apparent MC parameters. The detachment volume and stability are overestimated when applying equivalent MC parameters, as confirmed by calculations with the continuum mechanics code FLAC3D (Itasca Consulting Group). However, the SRM with HB material (i.e., apparent MC parameters) results in a safety factor that cannot be applied to HB parameters. To date, it has not been possible to determine the HB parameters for limit equilibrium via the SRM. This challenge was overcome by fitting an HB envelope to the original HB shear envelope reduced by the safety factor. The envelope is adjusted by two HB variables: GSI and D. This allows to determine the HB parameters at limit equilibrium. It helps to make more realistic predictions about the detachment mechanism and volume.