Abstract

Ground failure on natural and engineered rock slopes is a geological hazard with potentially fatal consequences to the public or personnel in the mining industry. Aerial reconnaissance with the use of unmanned aerial vehicles (UAV) is rapidly becoming standard practice for geotechnical and engineering geological site investigations, enabling faster and safer data collection on slopes, which are often difficult to access on foot. Data obtained from aerial reconnaissance alongside conventional field investigations assist in the development of an engineering geological model that can form the basis of various stability analyses including kinematic, limit equilibrium and finite element analyses, and even rock fall simulations. This paper presents two case studies in which remote reconnaissance is used as an initial method of site investigation to classify natural and engineered rock slopes. The case studies from San Leo in Italy and an open pit mine in the Caribbean are used to demonstrate the effectiveness of these techniques for developing a preliminary engineering geological model from which stability analyses can be derived to predict future ground behaviour to assist in managing risks associated with the geological hazard.

Introduction

Rock mass characterization represents the first stage in definition of the quality and stability of rock masses. These are based on the engineering characterization of rock outcrops, which is usually carried out through traditional engineering geological surveys. When dealing with high and/or inaccessible slopes, both of natural and man-made origin, rock mass characterization become challenging. With the goal of overcoming such issue, since 2009 [1] suggested to couple traditional field measurements with data from remote sensing techniques, so that to improve the quality and amount of data available for rock mass analyses. Indeed, techniques such as terrestrial laser scanning (TLS) and digital terrestrial photogrammetry (DTP) for rock mass characterisation are increasingly being used in the last decades.

TLS and DTP allow accurate representation of rock outcrops by means of 3D textured point clouds and interpolated models. A limitation of ground-based remote sensing is related to the survey of high slopes and complex morphologies where the site of acquisition, generally at the bottom of the slopes, results in occlusion zones and shadows in the output data [2]. Such limitation can however be overcome using unmanned aerial vehicles (UAV), from which high-detail images can be acquired also in the case of high and steep slopes. UAV-photogrammetry allows to produce 3D data and orthophotos that can be used to define the geometry of slopes and some characteristics of discontinuities. Although the use of UAV data in rock slope analyses have been discussed by many authors [3, 4, 5, 6, 7]; however, their use as tool for incorporation in empirical rock mass classification methods, such as the Geological Strength Index (GSI) [8] and Q-slope [9], has not been discussed in depth.

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