Challenges in the characterisation of intact rock bridges in rock slopes

Abstract

Intact rock bridges have been recognised as of critical importance in the stability of rock slopes but still remain a poorly understood and challenging engineering problem both in respect to their measurement and their incorporation into design analyses. The objective of this paper is not to provide conclusive answers as to the definition and measurement of rock bridges, and the manner that they can be accounted for in rock slope stability analysis. Rather, we highlight important and as yet unanswered questions to allow review of fundamental rock bridge concepts and then proceed to provide new insight into how rock bridges may be incorporated into slope stability analysis. Early definitions of what constitute a rock bridge are somewhat limited, and the authors suggest that they could be improved by including important constraints such as “block forming potential” and “block kinematics”. In this context, this paper introduces an improved terminology (RBij) commonly used by the discrete fracture network (DFN) community to define rock bridge intensity relative to the sampling region. In rock engineering, the measurement of rock bridges is exacerbated by the fact that rock bridges are not visible unless the rock mass is exposed by human activities or by natural events such as rockfalls. This constitutes a major problem for engineering design scenarios, since it would not be possible to validate any assumption made with respect to extent of rock bridges without performing some form of field testing or back-analysis. The results of a of the Finite-Discrete numerical analysis are presented to support our conclusions with respect to the limitations current methods used to characterise rock bridge strength. The role of scale effects, block theory, kinematics and what we term “negative rock bridges” in stability analysis is discussed.