Extraction of rock mass structural data from high resolution laser scanning products

A detailed reconstruction of the 3D structural setting of a rock mass is a necessary starting point for analyzing engineering geology problems involving rock masses. Traditional geomechanical surveys are performed in situ, either in one dimension (scanline method) or two dimensions (window method), and require direct access to the rock face for the collection of the relevant parameters. In order to perform correct analyses from a statistical point of view, one should investigate a portion of the rock face as wide as possible. However, for logistic and safety reasons, this is not often feasible, making thus impossible to fully reconstruct the 3D variability of the rock mass. ISRM (1978) selected the following ten parameters for the quantitative description of discontinuities in rock masses: orientation, spacing, persistence, roughness, wall strength, aperture, filling, seepage, number of sets, block size. The main product of a long range laser scanning survey is a high resolution point cloud, obtained by measuring with high accuracy (millimetric or centimetric) the distance of a mesh of points on the object, following a regular pattern with polar coordinates. With the aim of extracting the most relevant rock mass geomechanical characteristics hidden in the point cloud, two main different levels of automation con be conceived: - manual: by inspecting the point cloud or the derived surface, fitting local planes, taking measurements, drawing polylines of interest etc. This procedure, however, follows a non-systematic approach, is time-consuming and tends to neglect the smallest features. It is a subjective or biased analysis, as only those discontinuities which appear to be important are investigated. The success of this approach depends on the quality of digital data and on the skill and experience of the interpreter; - automatic / semi-automatic: by selecting a specific algorithm for the segmentation of the original data in clusters of points belonging to the same discontinuity. This can be defined as an objective or random analysis, since all detectable discontinuities within the surveyed area are sampled. Since raw data can contain up to tens of millions of points, the adopted algorithm should be optimized to make computational time acceptable. A Matlab tool called DiAna (Discontinuity Analysis), for the 3D geo-structural analysis of rock mass discontinuities on high resolution laser scanning data is here presented. The proposed algorithm is based on the definition of least squares fitting planes on clusters of points selected by moving in the space a searching cube with variable dimensions. If the associated standard deviation is below a defined threshold, the cluster is considered valid. By applying geometric criteria it is possible to join all the clusters lying on the same surface; in this way discontinuity planes can be reconstructed, and rock mass geometrical properties are calculated. One of the main outcomes of the described procedure is the definition of surface roughness at different scales. Therefore, the presented approach is able to semi-automatically retrieve some relevant rock mass parameters, namely orientation, number of sets, spacing/frequency (and derived RQD), persistence, block size and scale dependent roughness. Some examples of the proposed method have demonstrated its ability to investigate rock masses characterized by irregular block shapes, and suggest applications in the field of engineering geology and emergency management, when it is often advisable to minimize survey time in dangerous environments and, at the same time, it is necessary to gather all the required information as fast as possible.