3-D Discontinuity orientations using combined optical Imaging and LiDAR techniques

The importance of the collection and analysis of data on discontinuities cannot be overemphasized. Problems which include sampling difficulties, risks, limited access to rock faces and exposures, and the delay in data collection has led to a high need for data collection tools and analysis techniques that can overcome these problems. Great developments have been made towards automated measurements using both optical imaging and LiDAR scanning methods but there is still more room for improvement. Discontinuities manifest themselves as „facets‟ that can be measured by LiDAR or fracture „traces‟ that can be measured from optical imaging methods. LiDAR scanning alone cannot measure „traces‟ neither can optical imaging methods measure „facets‟. This is complicated by the fact that both „facets‟ and „traces‟ are often present in the same rock cut, making the selection of an appropriate measuring tool very difficult if not impossible. In this paper, we present our research on the development of robust software to determine 3-D discontinuity orientations from combined LiDAR and optical imaging techniques. Figure 1.1. (a) Example of wedge , (b) planar , and (c) toppling failures along road cuts. discontinuity plane, is assigned to a discontinuity set by using cluster analysis. Cluster analysis techniques are described in detail by Maerz and Zhou [5, 8, 9, 10,11]. The orientations can be and have been traditionally measured using manual compass and clinometer methods. These methods are however slow, tedious and cumbersome, and in some cases dangerous because of potential falling rock, and are often limited to easily accessible locations like the base of the slope. Figure 1.2: Orthogonal nature of joint sets. Measurements of the “cracks” or discontinuities are displayed in Figure 1.3 Figure 1.3: Projections of vectors normal to discontinuity plane on a unit lower hemisphere, clustered into three sets. Once having identified the graphical or computational techniques can be used to determine the kinematic feasibility of failure (Figure 1.4) and standard modeling techniques such as limiting equilibrium analysis can be used to determine if failure will indeed take place (Figure 1.5). Figure 1.4: Planar failure geometry (left) and graphical method of determining if slide failure is kinematically possible [6]. Figure 1.5: Limiting equilibriums analysis applied to planar features (left) and wedge features (right) [6]. 1.3. Surface Expressions of Discontinuities The discontinuities or cracks in the rock mass, when exposed in an outcrop or cut manifest themselves in one of two ways, often in both ways on the same exposure: 1. On flat planar rock cuts, the intersection of the plane of the discontinuity and the planar rock cut results in a visible line (fracture trace) that lies on both planes (Figure 1.6). 2. On rock cuts that are irregular, the actual faces of the discontinuities are exposed. These fracture surfaces can be considered to be like “facets” on a cut precious stone (Figure 1.6). There are emerging techniques to measure joint orientations for each of these situations, however, two completely different techniques are required for the two types of discontinuity expressions. What is worse is that, in at least one of the methods, the mere presence of the opposite type of fracture expression makes the technique unusable. Even though often both expressions are present, there is to date no legitimate way to combine the two techniques. (a) (b) (c)