Application of discrete fracture networks (DFN) in the stability analysis of Delabole Slate Quarry, Cornwall, UK

Abstract

The failure mechanism of rock slopes is mainly controlled by the strength and orientation of discontinuities within the rock mass. A realistic representation of the joint network within the rock mass is therefore an essential component of stability analysis of rock structures (e.g. rock slopes, tunnels etc.). Discontinuity persistence and connectivity are significant parameters which control the stability of rock slopes. A small percentage of rock bridges on the discontinuity surface can significantly increase its strength and prevent slope failure. Discontinuities within the rock mass are rarely fully connected. In practice, however, discontinuities are often assumed fully persistent due to the difficulties both in mapping and simulation of non-persistence. Discrete fracture networks (DFN) provide a rigorous and convenient tool for the simulation of joint systems within a rock mass. Utilizing statistical methods, DFNs consider the stochastic nature of some key parameters (e.g. persistence and orientation) within numerical models. Discrete fracture network engineering is increasingly used due to recent developments in discontinuity data acquisition techniques (e.g. ground-based digital photogrammetry and laser scanning). Recent development in geomechanical modelling codes and increased computing power have also allowed to either import DFN’s into models or to generate DFN’s within the numerical modelling code itself (e.g. 3DEC). This paper describes the use of photogrammetry at the Delabole slate quarry in Cornwall, UK for remotely acquiring key discontinuity parameter data (orientation, intensity and length) and its subsequent use in developing statistically validated discrete fracture network parameters. The 3D distinct element code, 3DEC, is used for the DFN generation and subsequent stability analysis. Several realizations of the 3DEC-DFN models are run to investigate the stochastic nature of discontinuities within the quarry and their potential influence on the stability of the pit. Finally the simulation results are used to determine the slope instability mechanisms and determine the most likely areas of potential instability.