Quality control of a sub-sea tunnel project in complex ground conditions

Sub-sea rock tunnels are often constructed in complex and difficult ground conditions. Major instability, uncontrolled water inflow and excessive cost are the potential consequences of insufficient pre-investigation. To ensure a satisfactory decision basis for planning and construction, comprehensive pre-investigations are required, and the interpretation and evaluation of the investigation results are crucial. Feasibility, cost and risk evaluation are key words here. This paper is discussing these issues based on quality control recently carried out for the 5.2 km long and 160 m deep Frøya sub-sea road tunnel, currently under construction on the West coast of Norway. Due to the very complex rock conditions, more comprehensive rock support than for the majority of Norwegian sub-sea tunnels is expected to be required. To ensure safe tunnelling, the following factors are considered crucial: · High quality of planning and tender documents. · Experienced personal of all parties involved in ground investigation, planning and construction. · Continuous following up by experienced engineering geologists. · Quality assurance by panel of experts.