Equivalent Geological Strength Index (GSI) approach with application to rock mass slope stability

Abstract

A considerable amount of slope stability analysis has been observed in jointed rock masses in which the GSI (Geological Strength Index) estimated at the outcropping level is considered input data to define the rock mass strength. However, this procedure is unsuitable when the rock outcrop scale and the slope scale are significantly different (e.g. open-pit slopes), resulting in an overestimated rock mass strength. For this reason, and in the absence of criteria to modify the GSI based on the scale effects, in this research, a new GSI version is proposed, called GSIe or “equivalent GSI”. To define an expression for obtaining the GSIe in terms of the rock mass properties, comparative stability analyses were conducted in a series of hypothetical slopes using two approaches: the first considers the rock mass as a discontinuous medium of rock blocks separated by discontinuities; the second considers the rock mass as an equivalent continuous medium characterized by an equivalent GSI. For the adequate equivalent GSI value, evaluated in each analyzed slope, the safety factor and the failure surface are similar in both approaches. In conformity with the results, a GSIe formulation in terms of the slope height, the spacing, the intact rock strength, the persistence, and the joint conditions has been proposed. Finally, the formulation was validated by applying it in five cases of mining slopes where the failure occurred.