In tunnel excavations, the optimization between time and level of detail for the collection of geological information is crucial for safety and cost. The 3D-laser-scanning technique can be used as a complementary or sometimes alternative method to traditional geological mapping because it produces a large amount of highly accurate geometrical data in a short time. The 3Dlaser- scanning technique allows for the geometrical data from the tunnel to be analyzed with the desired level of accuracy away from the tunnel front. This decreases the time spent in hazardous conditions by the geologists. In this study fracture mapping carried out based on laser-scanning data is compared to traditional geological mapping of the same area. It was found that the same fracture sets could be identified.
High risk working environment
Long time spent at the excavation face
Usually non repeatable
Imprecise reference system
Usually two-dimensional (except for fracture orientations)
Low data density.
To improve data quality during fracture mapping of blasted tunnel walls, a study was performed in the TASQ Tunnel at the Äspö Hard Rock Laboratory, Sweden, on behalf of SKB, the Swedish Nuclear Fuel and Waste Management Co. This study aims to overcome some of the shortcomings of the traditional geological fracture mapping methods, such as:The state-of-the-art technique of 3D-laser scanning was applied by using a Leica HDS4500 system . This technique shortens the time spent by the geologist in hazardous environments and limits the interference of the mapping with the scheduled works in the tunnel. Moreover, it produces large amount of data that can be stored as documentation and processed later in time by different operators. The object of the study is the TASQ Tunnel (Tunnel Äspö Site designation Q) excavated for the purpose of investigating the effect of high induced stresses on the stability of large boreholes [2-4]. 80 m of the TASQ Tunnel were scanned by using a Leica HDS4500 system. 10 m of the scanned section was selected for a detailed investigation of the fracture spatial position and orientation (Fig. 1).
Fig. 1. TASQ Tunnel: the theoretical profile of the tunnel compared to geometry measured by Leica HDS4500 laserscanning system.(available in full paper)
Fig. 2. Presentation of raw data by 3D-laser scanning by means of: a) 3D-point cloud; b) 3D-laser intensity image and; c) 2D-laser intensity image.(available in full paper)
2. 3D-LASER-SCANNING TECHNIQUE
3D-laser scanning allows for fast and accurate record of rock faces without physical contact. The Leica HDS4500 system makes use of a phase-based laser technique , for high-speed (up to 625 000 points/s), high-accuracy (3 to 5 mm) and eye-safe scanning in outdoor or indoor medium-range applications (scanning range between 0.1 and 54 m). One scanning station can survey a large surface of the object as the measurement field overviews 360° in azimuth and 310° in elevation. Each scanning station takes about 1 to 13 minutes depending on the point density (point distance down to 1 mm) and can be combined with other scanning stations by means of common reference points.