In Norway 18 subsea road tunnels have successfully been constructed during the last 22 years. Four tunnels are presently under construction, among them the Oslofjord and Froya tunnels. Like the Bjoroy tunnel, that were taken into use in 1996, these tunnels have encountered weakness zones of exceptionally poor rock mass quality. For the Bjoroy tunnel the location and complexity of the zone were unforeseen, in the Oslofjord tunnel only the complexity. In the Froya tunnel the pre-investigations had revealed exceptionally poor rock before construction started. The paper describes the decision process and actions that were taken from the time when the zones were detected in the tunnel up until permanent support of the zones. The level and types of pre-investigations are discussed as well as the design of minimum rock cover at critical parts of the tunnel. The methods and costs for crossing of the zones are presented. Finally, it is discussed how systematic and thorough pre-investigations and risk analyses can be used to handle or avoid similar situations as in the Bjoroy and Oslofjord tunels.
展开▼
