The observational approach in rock and underground engineering often is interpreted as “design as you go”, implying that there is no or only a very basic design available prior to start of construction. Spectacular cost- and time overruns are the consequence of a lack in preparation of the observational approach. Appropriate preparation includes a thorough investigation and ground characterization as a basis for selection of the alignment, assessment of possible failure modes, and assignment of appropriate construction methods to each section of the tunnel. With a serious observational approach, the design is continuously reviewed during construction to account for the uncertainties, and updated if required. To optimally adjust the construction to the actual ground conditions, a sound preparation of the project is required. Even if the preparations prior to construction are perfect qualified personnel is required on site to successfully implement the observational approach. They need to have a sound understanding of kinematical and mechanical processes to be able to make the optimal adjustments to the real ground conditions. Rock mass and site specific criteria for assigning construction methods to the ground conditions and requirements have to be developed. Advanced tools for data collection and interpretation assist in making the right decisions.


A sound and economical tunnel design depends on a realistic geological model (Riedmueller & Schubert 2001), a quality rock mass characterization, and the assessment of influencing factors such as primary stresses, groundwater, and kinematics. Despite this requirement it is still current practice to base the tunnel design primarily on experience, basic empirical calculations, and standardized rock mass classification systems (Bieniawski 1974, 1989, Barton et al. 1974, Barton 1998). Additionally, the on site decisions on excavation and support modifications are frequently based more on intuition than on analyses.

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