The principle of rock bridges uncertainty

Summary

Abstract

When attempting to understand rock mass behaviour, we often assume our process is straightforward: collect data, make observations, and synthesize an interpretation. But this assumes we approach the rock mass without preconceptions. In reality, unless we have some a priori framework for what we expect to find, how do we decide which observations matter and which data to collect? This captures an important epistemological insight: observation is never theory-neutral. We always bring conceptual frameworks that guide what we look for and how we interpret what we find. Rock bridges exemplify this problem perfectly. Current practice, particularly Jennings's method based on rock bridge percentage, continues to assume we can measure rock bridge strength pre-failure based on a post-failure interpretation imposed on a pre-failure reality we cannot observe. However, only once a rock mass has failed is it possible, in principle, to define and measure the rock bridges that resisted failure in the first place. Before failure, their location and geometrical nature can only be described probabilistically. This creates the principle of rock bridge uncertainty: once a rock bridge has been identified and its "size" has been determined, the rock mass has already undergone failure. But engineers need to characterize rock bridges before failure occurs. From an engineering design perspective, it is precisely when rock bridges cannot be observed and measured that we claim to quantify their strength. This paradox necessitates abandoning deterministic rock bridge percentage methods in favour of stochastic approaches. We propose a probabilistic framework that can help characterize potential failure mechanisms beyond the classical problem of rock bridge failure and the associated “equivalent discontinuity”. The implications extend beyond technical methodology to engineering practice: by embracing uncertainty rather than ignoring it, we can make better-informed decisions about slope stability, excavation design, and risk management in fractured rock masses