Abstract

Rock mass characterization plays an essential role in rock engineering activities. Presently several approaches for digitally measuring rock mass discontinuities are available. This study summarizes some of the main technological approaches, together with comparative quantitative evaluations of discontinuity measurements. For the steeply inclined Engelswand outcrop in Tyrol, Austria, an Unmanned Aerial Vehicle was used to acquire a series of photographic images in order to generate a true color georeferenced point cloud and derivative 3D models. Discontinuity orientations were measured by manual, assisted and automated digital approaches. Orientations of individual discontinuity planes and the statistics of the overall discontinuity structure were evaluated. The manual and assisted measurement methods provide compatible results and are considered highly relevant in the context of rock engineering. While automated measurement methods are able to provide an overall indication of the dominating discontinuity pattern, discrimination of individual discontinuity sets is not feasible. While this is a major limiting factor in terms of extracting useful data for rock engineering practice, a dual digital measurement approach is considered prudent when large data sets are being evaluated. This involves conducting rapid automated measurements to obtain an overall representation of the rock mass structure, followed by supplementary manual or assisted measurements to more precisely extract pertinent structural data.

Introduction

Characterizing rock mass discontinuity properties such as orientation, spacing, persistence, wall strength, infillings, roughness and aperture is a fundamental task in rock engineering practice [1]. To an ever-increasing extent, classical analog methods for characterizing these properties are being superseded or supplemented with remote digital measurement techniques. The digital methods tend to increase objectivity and repeatability, and are particularly attractive when accessing the rock mass entails safety risk for field personnel. However, there are a number of different approaches and techniques for performing digital measurements, the intricacies of which should be well understood by the rock engineering practitioner.

The purpose of this study is to make a quantitative comparison of discontinuity orientations obtained using different digital measurement methods. Specifically, an Unmanned Aerial Vehicle (UAV) was deployed to capture photographic imagery of the Engelswand outcrop in Tyrol, Austria. The imagery was combined to generate true color high resolution 3D point clouds using the Structure-from-motion (SfM) method [2]. As enumerated herein, the discontinuity orientations were then extracted from the point cloud using different manual, assisted and automated techniques. The discontinuity measurement approaches investigated include: (i) manual methods (RiSCAN PRO v 2.1.2 [3]), (ii) semi-automated / assisted methods (open-source CloudCompare v 2.11.3 – Compass Plugin [4], ShapeMetriX UAV (SMX) v 4.0.5 [5], 3DM Analyst v 2.6.2 [6]) and (iii) automated methods (open-source CloudCompare v 2.11.3 – Facets Plugin [4], 3DM Analyst v 2.6.2 [6]). Prior studies have shown strong correlations between digital point cloud measurement results and traditional measurements made with a geologic compass [7][8][9]). Considering that safe access to the Engelswand impedes traditional analog measurements, the intent of this study is to focus on digital methods only.

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