Absolute displacement monitoring has become common practice on many tunnel sites. Those data in combination with the information on geological and boundary conditions have a tremendous potential to enhance the insight into mechanical processes around the tunnel and the ground-support interaction. Different methods of data evaluation are used for specific purposes. Displacement history plots can be used to evaluate the stabilization process or to predict the final displacements rather than to predict geotechnical conditions ahead of the face. For such a request the use of more advanced evaluation methods are required. To describe the spatial and transient development of displacements during tunnel excavation, analytical functions are used. The daily evaluation process will be enormously improved by comparing the currently monitored displacements to the expected displacement characteristic, determined using the analytical function. Any deviation from the normal behaviour can be identified in time. Critical displacement trends can be automatically detected as well as typical trends used to predict the ground conditions ahead of the face.
The introduction of absolute displacement monitoring using geodetic methods has significantly improved the basic concept of the observational approach for tunnel construction. Rabensteiner (1996) already described the scope of optical displacement monitoring and the procedure to determine reliable 3D coordinates of monitoring points installed at the circumference of the tunnel. The huge amount of displacement data which are commonly produced on a daily basis require appropriate evaluation methods in order not to get lost in data tables, as well as to extract as much information as possible. In the mid of the last decade, several authors have given an overview of useful evaluation methods to improve the understanding of geomechanical processes during the tunnel excavation (Schubert & Steindorfer 1996, Vavrovsky & Schubert 1994, 1995).