Knowledge of the temporal and spatial evolutions of displacements occurring in the vicinity of underground openings is essential to understanding the response of a given rock mass to its excavation. The study presented in this paper aims at assessing the potential of laser scanning in accurately mapping surface displacements in underground excavations. The laser scanner Imager 5003 of Zoller+Froehlich was employed to scan the surfaces of a tunnel located in the Mont Terri Rock Laboratory (Switzerland). The reliability of the approach was assessed by comparing measurements provided by the laser scanner with those obtained through the surveying of targets using a total station. The developed field methodology and data processing are presented in the first part of the paper and are followed by a description of the results obtained for two investigation areas. The advantages and limitations of laser scanning in mapping surface displacements are discussed.
Collecting data concurrently with construction work is often necessary to correct or refine the design developed during the preliminary stages of an excavation project. The information gathered during construction allows for a more precise ground characterization and hence for an adjustment of the construction method to the observed ground behaviour (Schubert 2006). In addition to time and access constraints related to excavation work, near-face data collection must tackle stability problems occurring in freshly excavated sections of openings. Several tools and methodologies addressing these issues have been developed in the last years (e.g. Gaich et al. 2004, Feng & Rosshoff 2004, Lemy & Hadjigeorgiou 2004a). A thorough understanding of rock mass behaviour requires quality data about the evolution of the displacement field occurring around openings. The analysis of such information also facilitates the prediction of adverse ground conditions lying ahead of the face.