Geologic exploration and risk reduction in underground construction

Geologic uncertainty in underground construction promotes design and construction conservatism and has a significant effect on project cost. Site investigation can reduce this uncertainty and decrease costs by reducing the contingency amounts included in bids. This paper presents research findings that provide a better understanding of how subsurface exploration and improved contractual risk sharing can decrease the cost of underground projects. Issues discussed include: the methodology used by tunneling contractors to estimate geologic profiles given a set of available geologic information; the geologic classification methods used to associate the expected profile with acceptable construction alternatives; the spatial prediction of ground classes and the extents over which different excavation and support methods will be necessary; the factors involved in selecting the initial support, the excavation round length and the estimation of the advance rate; the relationship between exploration, risk allocation, and bidding behavior; the impact of changed conditions clauses in underground construction contracts; the merits of using well-defined geologic conditions as a basis in unit price contracts; and the magnitude of bid contingencies that are actually used in practice.