A case study of monitoring tunnel wall displacement using laser scanning technology

Abstract

Displacement monitoring is a common practice in rock engineering to observe the evolution in time of the rock mass behaviour and to predict potential stability problems that may occur in the future. This typically involves repeated measurements of the relative or absolute displacement of a limited number of reference points within the rock mass or on the excavation surface at different times. The use of 3D laser scanners allows coping with practical constraints encountered in rock engineering since it provides quickly a realistic and permanent representation of excavation surfaces that can be used for topographic surveying, rock mass characterization and documentation purposes. Additionally, these systems do not require the installation of physical targets on the rock surface and have therefore the potential to be employed on a regular basis as an efficient and unique tool to record input data required for various engineering analyses. The paper presents a case study where the Imager 5003 laser scanner of Zoller+Frohlich was employed to measure surface displacements in an experimental tunnel throughout and after its excavation. The Imager 5003 3D laser scanner is particularly well adapted to applications in underground excavations because of its scanning principle (panorama scanner), the 3D-accuracy of single points (few millimetres), and its high performance (up to 625,000 points per second). The reliability of the method has been assessed by comparing displacement measurements provided by the laser scanner with those calculated from total station surveying of markers installed on the tunnel surfaces. The determination of absolute displacements has been made possible by referencing the data obtained from the laser scanner and the total station to an established coordinate system. The main objective of the work is to assess the potential of this technique in deriving time lapse surface displacement maps of rock outcrops with a high spatial resolution. The developed field methodology, processing algorithms and preliminary results are presented in this paper.