ABSTRACT

Within any underground construction, digital information is significant for geotechnical assessments and further monitoring. This paper presents a new method for tunnel digital geological mapping and documentation by using 3D terrestrial laser scanning (TLS) data. 3D terrestrial laser scanner is a non-contact close range sensing system, which can capture three dimensional data and images in a short time. TLS can be used as a tool that allows the users to undertake high accuracy mapping and surveying from some distance. Laser scanning data is possible to set up a database that store in 2D images and 3D models of real tunnel with local or global co-ordinate system.

This study was performed in three tunnels in Stockholm, Sweden. Data acquired by Leica HDS4500 terrestrial laser scanning system. Utilization of the laser intensity image for geological mapping in a tunnel can digitally documentation of real tunnel and extraction of geological information, such as fractures, lithological classification and water leakage. Image processing methods have been tested with the aim to increase the possibility to extract useful geological information. Different types of water leakage in the tunnel are identified on laser images, and a method to quantitatively detect the water flow has been studied. Using of laser 3D point cloud to generate the best-fit plane of fractures and then calculating orientation of fractures have been used in the tunnels. The results show a digital geological map based on an unfolded image of the tunnel, plus trace line and orientation of fractures, rock type classification and water leakage etc. It is concluded that laser scanning technique for tunnel geological mapping is rapid, digitize, cost effective and more accurate, which offers some advantages over traditional methods.

1 INTRODUCTION

Geological mapping in a tunnel is an essential work for tunnel stability analysis and reinforcement. The traditional method is to use a geological compass with inclinometer to measure geometrical parameters of fractures, such as orientation (dip angle and direction) on the rock surface and to make a detailed geological map manually. Manual measurements give no digital document from the inside of a tunnel, so it is difficult to use the result for further geotechnical assessments. To address these issues, a new method has been developed for tunnel digital geological mapping by using 3D TLS data and data processing techniques.

The Leica HDS4500 TLS system see Fig. 1, is an optical-wavelength system measuring the range and direction between sensor and object surface as well as the reflectance of the object surface, which can provide both range and visual data. TLS data can display as a 2D grey-scale image and 3D point cloud of coordinates from the inside of the tunnel. The data is obtained as a series of scenes; the scans can be registered within a reference coordinate system via control points (CP) and then merged into a single data cloud. This method offers a way to digitally record the object surface in three dimensions and in visual format as a database for applications [1].

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