A large percentage of the risk associated with underground construction is in the uncertainty of geologic conditions. Applying the science of geology in identifying these risks is a crucial part of planning a tunnel. Techniques were used on the MetroWest Water Supply Tunnel to identify geologic conditions that could impact construction of a bedrock tunnel. Directional core drilling techniques, phased explorations, continuous mapping, and the contractual breakout of tunnel reaches based on geology are among the approaches used.

With the completion of tunnel excavation, it is now possible to reflect on the effectiveness of these methods. The high cost of more intensive and specialized explorations must be weighed against the uncertainty of the project geology and the potential for construction problems. On the MetroWest tunnel, the decision to spend additional time and money in the exploration phase on tunnel sections where the geology was suspect proved fruitful in that potentially higher construction costs were avoided. Suggestions for further improvements in identifying the risk of uncertain geologic conditions on future tunnels will be made.