ABSTRACT

Aterrestrial laser scanner surveywas carried out in a marble quarry on differently oriented faces, either excavated or along the intact slope. A quantitative analysis devoted to the estimation of the discontinuities orientation, which is one of the peculiar parameters relevant to the stability of rock masses, was performed. Laser data belonging to each recognized discontinuity were extracted and processed by applying a Total Least Squares adjustment method for estimating representative plane. The results of this approach allowed to describe how the variance model is conditioned by the constraints of the problem. The laser survey, which revealed the presence of four sets of discontinuities, was compared to a traditional compass survey demonstrating the usefulness of the laser data and the good reliability of the applied numerical method.

1 INTRODUCTION

Topographical surveys represent a good alternative to traditional compass surveys for instancewhen very high rock faces or not easily accessible areas are present, as it often occurs in quarries. Indirect measurements, such as those collected by means of laser scanner and photogrammetric surveys, do not require to access potentially instable areas for positioning instruments on the exposed faces. These methods permit to describe the rock mass morphology at higher resolution than normally, providing an almost continuous data coverage on the whole exposed surface. Recent literature extensively reports on applications of 3D laser scanner surveying techniques to the study of rock faces, emphasizing their appeal and also their limits. The more straightforward application of such surveys in Rock Mechanics is the estimation of discontinuities orientation (Lemy & Hadjigeorgiou 2004, Voyat 2005). An interesting aspect regards the possibility of reconstructing the orientation of exposed surfaces by automated analytical procedure (Slob et al. 2002, Slob et al. 2004). Comparisons with traditional surveys carried out by compass indicate that the method is reliable even if some difficulties are reported, such as failures in the extraction of sub-horizontal joints (Voyat 2005). Numerous are the possible applications of laser measurements, other than orientation: estimation of typical geometrical characteristics usually sampled at exposed rock faces, such as spacing, length and persistence of traces (Feng & Roshoff 2004); slope monitoring by comparing laser scanner surveys at different times; morphometric analysis of the 3D data allowing the reconstruction of profiles, useful for estimating run out of blocks detached from rock slopes or lost volumes in case of rock falls. Most applications have provided results that are as accurate as those from topographical surveys (Bornaz 2005). Other achievements can be obtained from further numerical elaboration of the raw data aiming to describe roughness and waviness of discontinuities. An interesting method recently developed is based on fractals which produces a description of scales of roughness both along scanlines and on 2D surfaces (Fardin et al. 2004). In ourwork a terrestrial laser scanner surveywas carried out in a marble quarry on differently oriented faces. Laser point cloudswere filtered, combined and interpolated to reconstruct a unique digital model of the surveyed surface.

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