Study of the costs of cut-and-cover tunnel construction

In urban and semi-urban areas requirements of land conservation and environmental pressures frequently lead to tunnel construction being included at a late stage in the planning of road schemes. This report seeks to provide the planner with guidance to enable an assessment of economic and environmental considerations to be made. Details of the overall cost of the construction and the environmental costs and benefits are given for a total of 14 existing and 13 proposed road improvement schemes incorporating cut-and-cover tunnels in the UK. The factors influencing costs are discussed first in relation to the geometry of the tunnel determined by the number of lanes and verge/hardshoulder requirements. The costs are significantly influenced by site constraints such as proximity of existing buildings and the need to maintain traffic flow. A review is also made of the costs of alternative forms of construction. In addition to the cost of the civil works, the cost of equipping the tunnel with electrical and mechanical services for ventilation, lighting, power and monitoring and safety equipment is discussed. For comparative purposes costs have been updated to 1988 (fourth quarter) rates by use of the Road Construction Price Index. The cost data are displayed in the form of bar charts and scatter diagrams. A typical bar chart giving the breakdown of civil works costs for proposed cut-and-cover tunnels is provided. The environmental benefits are discussed including the reduction in traffic noise and the control of pollution afforded by a tunnel. The report examines conditions at the portals and reviews the data collected from field trials and results from computer analysis carried out to evaluate carbon monoxide levels for specific sites and a typical tunnel situation with varying lengths. The implications of these results on the market value of land released for redevelopment over the tunnel are then considered.