LiDAR and optical imaging for 3-D fracture orientations

Data on discontinuities are always necessary for design, characterization and analysis of rock structures. The time honored method of manual measurements with Brunton compass is both time consuming and often inconvenient given issues such as restricted access to measurement areas, introduction of erroneous data due to sampling difficulties and human bias, considerable safety risks since measurements are sometimes carried at the base of existing slopes or during quarrying, tunneling or mining operations or along busy highways and difficulty to have direct access to rock faces. Discontinuities manifest themselves in rock cuts as „facets‟ that can be measured by LIDAR or fracture „traces‟ that can be measured, at least in 2-D by optical imaging methods. Unfortunately LiDAR scanning cannot measure „traces‟ nor can optical imaging measure „facets‟. To overcome all these problems, the need to combine both LiDAR data and optical imaging is necessary. 1.0 Introduction: Naturally there are breaks or cracks in every rock mass [1]. Discontinuity is the most general term which suggests a break in the continuity of a rock fabric with no implied genetic origin (Fig. 1a). Discontinuity can be defined as a significant mechanical break or fracture of negligible tensile strength, it has a low shear strength and high fluid conductivity when compared to the rock itself [2]. Discontinuity influences all the engineering properties and behavior of rock [3]. When dealing with discontinuous rock masses, the properties of the discontinuities become a prime importance since that determines to a large extent the mechanical behavior of the rock mass [4]. The presence of discontinuities in a rock mass can affect engineering designs and projects, which include the stability of slopes in a rock mass, the stability and behavior of excavations in a rock mass and the behavior of foundations in a rock mass. The presence of discontinuities also affects rock properties such as the strength of the rock and the hydraulic conductivity of the rock which is responsible for the transportation of groundwater and contaminants [5]. Thus, the importance of the analysis of discontinuities in of a rock mass cannot be overemphasized. Discontinuity properties can be grouped as geometric or non-geometric. Geometric properties include position, orientation, persistence and open size. Non-geometric properties include wall strength, roughness, filling and water conductivity. The most important discontinuity property is orientation. Orientation influences the potential of the rock mass to move, the direction of movement and the volume of material to be moved [6]. Orientation is so important that it is ultimately used in every kind of analysis, either numerical or non numerical modeling. Fig. 1.0a: A rock cut showing discontinuities Discontinuities in rocks manifest themselves in two different ways; as facets or fracture traces. Facets are defined as the actual discontinuity surfaces that are exposed in the rock cut while fracture traces are NSF GRANT # 0856420 ARRACMMI, GEOMECHANICS