Abstract - The percolation threshold of a fracture system represents the fracture density at which the system becomes macroscopically connected, and is an important system parameter as many physical properties of a fractured rock-mass are a direct function of the proximity of the system to the threshold. By plotting on a ternary diagram the relative frequencies of fracture intersections, fracture abutments/splays and isolated fracture tips, the connectivity of a particular fracture system may be quantified, and the style of connectivity characterised. Percolation thresholds (critical density and critical connectivity) of fracture systems are known only for fracture network models with one connectivity style (intersections) and one spatial arrangement (random). This contribution presents analytical and numerical results defining the critical connectivity and critical density of fracture systems with realistic connectivity styles (e.g. joint abutments, fault splays) and non-random spatial arrangements. The results, which are verified by comparing predicted with observed macroscopic connectivities in natural fracture maps, indicate that the critical density of geological fracture systems are generally between one third and one half of the critical density of a traditional random continuum network model, and vary systematically as a function of fracture clustering.
Abstract of talk given to:
Irish Geological Research Meeting, Coleraine, February 2001