Autor: Hrvoje Lukačić, mag.ing.geol., mag.ing.min.
Successfully implemented interventions in rock masses largely depend on the quality and manner of conducting research work. Mechanical properties of rock mass depend on the properties of intact rock and the properties of discontinuity (ISRM, 1978). For this reason, knowledge of the geometric features of discontinuities and the definition of representative crack systems is of utmost importance for the definition of the engineering geological model and the analysis of possible types of instability.
When defining a discontinuity fracture system, it is necessary to collect a large amount of data on discontinuity orientation. Engineering geologists usually collect this data manually in the field using a geological compass.
Figure 1 Rock mass with different number of discontinuity sets (Pollak, 2007)
This method of data collection results in a limited number of data. The development and implementation of remote sensing methods (laser scanning and photogrammetry) in engineering geology have created the conditions for collecting and analyzing a large amount of structural data, manual and semi-automatic methods, thus defining an objective structural model of rock mass (Riquelme, 2015). The application of remote sensing methods reduces the time required to stay in the field and increases the time available for mapping the rock mass in the cabinet. The provision of additional time for mapping structures increases the amount of data collected and, thus, greater objectivity (Buyer, 2018). To date, many remote methods for data collection have been developed, with the primary goal of obtaining a 3D model of rock mass or 3D point cloud (Figure 2), which serves as a basis for engineering geological mapping of the rock mass.
Figure 2 3D Point Cloud of rock mass obtained by terrestrial laser scanning of a rock slope (Đikić, 2016)
Using the CloudCompare computer software and the Compass tool integrated, it is possible to simulate geological compass measurements and read the orientation of discontinuities on the 3D point cloud.
Figure 3 Measurement of discontinuity orientation on a 3D Point Cloud (Lukačić, 2020)
An identical procedure can be performed using a semi-automatic method for identifying discontinuity orientations from a 3D digital model using open-source software Discontinuity Set Extractor (DSE) developed in the Matlab programming language (Figure 4). The primary purpose of this software is to identify planes and associated sets of discontinuities and determine their orientation from 3D point clouds obtained by laser scanning or digital photogrammetry (Riquelme et al., 2014).
Figure 4 Display of identified discontinuity sets on a 3D Point Cloud of rock mass
The performed analyzes have shown that the application of remote sensing methods in engineering geology enables the collection of a large number of representative data on the geometric features of discontinuities. However, algorithms for semi-automatic discontinuity identification are not at the stage of development to be completely independent, and there is still a need for an engineering geologist to validate the results and assess whether they reflect the actual situation in the field to avoid erroneous conclusions about rock mass condition.
References:
Buyer, A. (2018.): Contributions to Block Failure Analyses using Digital Joint Network Characterization, PhD. Thesis, Institute of Rock Mechanics and Tunnelling, Graz University of Technology, Graz, 123 str.
Đikić, Z. (2016.): Primjena tehnologije oblaka točaka za projektiranje sanacije stijenske kosine Špičunak, diplomski rad, Građevinski fakultet, Rijeka, 105 str.
ISRM (1978.): Commission on standardization of laboratory and field tests Suggested methods for the quantitative description of discontinuities in rock masses. International Journal of Rock Mechanics and Mining Science & Geomechanics Abstracts 15: 319-368.
Lukačić, H. (2020.): Inženjerskogeološko kartiranje stijenske mase na zasjeku Špičunak (Gorski kotar) primjenom daljinskih istraživanja, diplomski rad, Rudarsko-geološko-naftni fakultet, Zagreb, 102 str.
Pollak, D., 2007. Utjecaj trošenja karbonatnih stijenskih masa na njihova inženjerskogeološka svojstva, doktorska disertacija, Rudarsko-geološko-naftni fakultet, Zagreb, 299 str.
Riquelme, A. J., Abellán, A., Tomás, R. i Jaboyedoff, M. (2014): A new approach for semi-automatic rock mass joints recognition from 3D point clouds. Computers and Geosciences 68: 38–52.
Hrvoje Lukačić, mag. ing. geol., mag. ing. min. is an assistant at the Department of Geology and Geological Engineering, Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb. He is a member of the Croatian landslide group and the Internationl Society for Rock Mechanics and Rock Engineering.
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