Elsevier

Computers & Geosciences

Volume 89, April 2016, Pages 21-31
Computers & Geosciences

Research paper
Solid images for geostructural mapping and key block modeling of rock discontinuities

https://doi.org/10.1016/j.cageo.2016.01.002Get rights and content

Highlights

  • A combined approach using both 3D point cloud and 2D digital images is discussed.

  • Standalone software helping with the completion of a digital geostructural survey.

  • Computational tools for solid images exploitation are tested on a case study.

  • Results show the value of combining 3D data and 2D imaging for structural analysis.

Abstract

Rock mass characterization is obviously a key element in rock fall hazard analysis. Managing risk and determining the most adapted reinforcement method require a proper understanding of the considered rock mass. Description of discontinuity sets is therefore a crucial first step in the reinforcement work design process. The on-field survey is then followed by a structural modeling in order to extrapolate the data collected at the rock surface to the inner part of the massif. Traditional compass survey and manual observations can be undoubtedly surpassed by dense 3D data such as LiDAR or photogrammetric point clouds. However, although the acquisition phase is quite fast and highly automated, managing, handling and exploiting such great amount of collected data is an arduous task and especially for non specialist users. In this study, we propose a combined approached using both 3D point clouds (from LiDAR or image matching) and 2D digital images, gathered into the concept of ''solid image''. This product is the connection between the advantages of classical true colors 2D digital images, accessibility and interpretability, and the particular strengths of dense 3D point clouds, i.e. geometrical completeness and accuracy. The solid image can be considered as the information support for carrying-out a digital survey at the surface of the outcrop without being affected by traditional deficiencies (lack of data and sampling difficulties due to inaccessible areas, safety risk in steep sectors, etc.). Computational tools presented in this paper have been implemented into one standalone software through a graphical user interface helping operators with the completion of a digital geostructural survey and analysis. 3D coordinates extraction, 3D distances and area measurement, planar best-fit for discontinuity orientation, directional roughness profiles, block size estimation, and other tools have been experimented on a calcareous quarry in the French Alps.

Introduction

Rock mass engineering requires a proper understanding of site geology, rock structure, mechanical and hydrological properties. Rock outcrops consist of intact rock separated and crossed by many discontinuities. Both geometrical and mechanical characterization of intact rock properties is usually performed through laboratory tests including the quantification of compressive and tensile strengths, elastic properties, etc. Beyond these internal characteristics, describing the discontinuity structure is also a crucial input to rock fall risk analysis. According to the rock block theory (Goodman and Shi, 1985), geometrical characteristics of these discontinuities, visible at the surface of the outcrop and extrapolated to the inner part of the massif, leads to the individualization of stones, blocks and masses potentially generating disorders with variable consequences depending on their localization and their fall energy. When determining the most adapted reinforcement method (rock anchors, detection nets, etc.), fracture mapping is therefore a fundamental first step in the design process. Today, cell mapping or scan line survey (Priest and Hudson, 1981) are generally based on manual compass clinometer and tape measuring. Unfortunately, manual field survey methods have several well-known weaknesses (Kemeny and Post, 2003, Slob et al., 2005).

Digital imaging and 3D laser-scanning offer the possibility to mitigate these gaps by providing a complete and accurate 3D geometric description of the surface of the digitized outcrop. Terrestrial laser-scanning technology, also known as LiDAR (for Light Detection And Ranging) and digital photogrammetry are close-range remote sensing technologies which allow rock outcrops to be digitally captured in a very short time and with unprecedented resolution and accuracy (Buckley et al., 2008, Sturzenegger and et Stead, 2009). Resulting 3D models can then be post-processed thanks to automated or semi-automated procedures for rock discontinuity characterization and exploited for the 3D documentation of any part of a rock face. Such dense 3D data, especially LiDAR point clouds, are being used ever more widely, notably for determining discontinuity orientations (Lato et al., 2010, Duan et al., 2011, García-Sellés et al., 2011, Assali et al., 2014, Riquelme et al., 2014) and discontinuity spacing (Riquelme et al., 2015), even for large scale applications (Hilley et al., 2010).

However, although the acquisition phase is highly automated, managing, handling and exploiting such great amount of collected data is an arduous task and especially for non specialist users. The conversion of point clouds data into useful information for the need of rock engineering practices is therefore a crucial issue.

This paper presents our approach to overcome this issue and discusses the solid image principle as a support for geostructral mapping and key block modeling. The concept of solid image is described and its implementation into a standalone software is discussed. Various tools are tested and illustrated thanks to a case study in a limestone quarry.

Section snippets

Definition

Bornaz and Dequal (2004) introduced the notion of ''solid image'' as the enrichment of a classical 2D digital image with the corresponding 3D geometrical information, e.g. a laser-scanning or photogrammetric point cloud. For all practical purposes, it is widely accepted that the geometrical data is not basically stored into different layers-for the coordinates components -, but are preferentially supplied thanks to a single range matrix-or depth map layer-for maximizing computing capacity (see

Implemented software for creating and managing a solid image sequence

A specific software package has been developed in a combination of C++ and R1 code to create, manage and exploit solid image sequences for fracture mapping and key block modeling purposes. Implemented tools have been included in the Gaia-GeoRoc software. Gaia-GeoRoc was developed by the authors since 2012 and is intended for processing 3D point clouds and calibrated images for rock mass

Structural mapping

The complete procedure, from the camera calibration to the solid image exploitation has been performed on different sites and especially on a limestone quarry located near the town of Saint–Jeoire, in Haute–Savoie, France (Assali et al., 2014). The results obtained with the solid image approach could therefore be compared with the already established structural statement.

The laser scanning survey has been performed with a Leica HDS7000 device and a spatial density of 1600 pts/m2, i.e. with

Conclusion

This paper describes a computational approach for 3D mapping and geostructural survey and analysis thanks to the combination of 3D point clouds and 2D digital images. Based on the concept of solid image, a new software-GAIA-GeoRoc-has been developed and validated on a typical field case study. In comparison to classical survey data which are affected by many deficiencies (lack of data and sampling difficulties due to inaccessible areas, safety risk in steep sectors, etc.), the reliability of

Acknowledgments

This project was carried out as part of a doctoral research project supported by SNCF (French National Railway Company),for improving the risk management methodologies related to linear outcrops along the railway network. The authors would like to thank SNCF's Engineering and Research & Innovation Departments, as well as the engineering company IMSRN (Ground Movement and Natural Hazards Engineering) in Montbonnot-Saint Martin (Isère, France), INSA de Strasbourg, and the University of Savoie for

Cited by (23)

  • Towards semi-automatic discontinuity characterization in rock tunnel faces using 3D point clouds

    2021, Engineering Geology
    Citation Excerpt :

    Discontinuities have a vital impact on the mechanical and hydrological characteristics of rock mass exposures (Bieniawski, 1973; Goodman, 1989), as they control the complexity, heterogeneity, and anisotropy of rock masses, especially in underground environments (Fabuel-Perez et al., 2009; Hodgetts, 2013; Jimenez-Rodriguez and Sitar, 2006b; Rarity et al., 2014). Tunnels under construction often encounter dense and complex discontinuities in the rock mass, which can lead to tunnel collapse resulting in huge economic and production losses (Assali et al., 2014, 2016; Senent et al., 2013;(Cai et al., 2021)). Thus, the discontinuities existing in a rock tunnel face are of significant concern for constructors and engineers before the next stage of excavation (Wang et al., 2019; Zhang et al., 2021; Zhao et al., 2020).

  • A fully automatic-image-based approach to quantifying the geological strength index of underground rock mass

    2021, International Journal of Rock Mechanics and Mining Sciences
    Citation Excerpt :

    By using practical techniques, rock engineers can remotely acquire a digitized representation of a rock exposure that records complete and accurate data on the discontinuity geometry in both a short period of time and a risk-free field environment. An increasing number of studies have been devoted to the automatic identification of discontinuities from the 3D point clouds captured from either LiDAR or SP.14–19 Nevertheless, the emergence of three shortcomings hinders further adoption of these techniques, especially in coal mine roadway driving.

  • Advances in statistical mechanics of rock masses and its engineering applications

    2021, Journal of Rock Mechanics and Geotechnical Engineering
  • Extraction and statistics of discontinuity orientation and trace length from typical fractured rock mass: A case study of the Xinchang underground research laboratory site, China

    2020, Engineering Geology
    Citation Excerpt :

    Traditionally, discontinuity orientation has usually been measured with a geological compass on the exposed outcrop surface, and the size (trace length and spacing) is measured with a measuring tape one discontinuity at a time (Slob et al., 2005). Although this method can obtain accurate parameters, it is less efficient because of the limitations in terrain conditions and the long time requirement for work on site (Assali et al., 2016; Haneberg, 2008). Fortunately, benefiting from the development of photogrammetry and digital image technology, the use of these techniques to acquire discontinuity information of outcrops or exposed rock surfaces based on the point cloud model has attracted considerable research attention.

View all citing articles on Scopus
View full text