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  Site Investigations Reduce Cost Overruns in Tunnelling Projects

  Geology plays a dominant role in designing and constructing a tunnel. The geological and geotechnical studies allow to specify the conditions in which will be realized the works. However, the rock mass is complex and the knowledge of the geologic context is always imperfect. This context can be very different from one site to another and the techniques of construction to be implemented to support the ground are very varied. This variability is thus translated by a very strong uncertainty on the cost of the work when the geological, geotechnical and hydro-geological conditions are underestimated. In the upstream phases of the studies, often not much information is available and the uncertainties on the geologic context are often very important. To succeed in determining a reliable cost of the works, it is necessary to lead different stages of geological site investigations. Anyone involved in design and construction of underground projects will understand that site investigation is required, in order to arrive at an effective design and construction. However, a recurring question of the clients concerns the amount to be spent for appropriate results and the most adequate moment to realize site investigation. It is true that the geological site investigations can have an important cost but this one is however disproportionate with the future cost of the works. This article attempts to review how much site investigations are usually carried out for underground works. The analysis begins from a review of experience feedback made mainly in France and Europe but also from recommendations or existing international publications. Finally, this article seeks to demonstrate that there is indeed an economic interest to carry out reconnaissance, if taking into account the overall cost of the investment.

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