Characterizing block geometry in jointed rockmasses

Abstract

The intersection of discontinuities in a jointed rockmass creates in situ blocks of variable three-dimensional (3D) geometry. The size and shape of rock blocks in a rockmass assembly have a dominant influence on the engineering properties of a rockmass, and control, for example the failure geometry of a rock face and the optimum associated support and surface restraint strategy. The distribution of block volumes within a rockmass can be effectively analyzed and communicated using a cumulative volume distribution curve. Integrated with this analysis, a methodology has been developed to characterize the shape and volumetric shape distribution of rock blocks in 3D models of jointed rockmasses. The proposed Block Shape Characterization Method takes into account two factors: the first, α, describes the shortening of the minor principal axis of the block while β describes the elongation of the major axis. When merged with the block volume distribution, the Block Shape Characterization Method can effectively describe and classify both the size and shape distributions of any jointed rockmass and is not limited to orthogonal blocks or hexahedra. This methodology can classify any polyhedra and has been developed and calibrated based on synthetic joint data and simulated block assemblies. A demonstration is given of its effectiveness in characterizing block geometries in the field using mapped fracture data.