Geotechnical risk management for tunnelling beneath open water

Tunnel construction beneath open bodies of water presents certain types of risks not encountered when tunneling beneath “dry land”. If major problems develop with tunnel equipment, ground control, or ground water control, construction personnel are exposed to risk of catastrophic flooding in the tunnel and recovery operations from the surface are generally not possible. Under these construction conditions, management of geotechnical risk during construction is of paramount importance. This paper addresses some of the more critical risks that must be considered and evaluated for such tunnel construction, starting with the planning process, and continuing through design and construction. Due to the increased difficulty and expense of geotechnical site exploration under water, the extent of the site investigation is sometimes decreased rather than increased, increasing the geotechnical uncertainty, requiring more detailed planning of construction methods and sequences, and appropriate redundancies and contingencies. The paper discusses the investigation, planning and construction processes for both soft ground and hard rock tunnels, and both conventional and machine excavation.