The detailed design of the Medway Tunnel project Available to Purchase

Immersed tube tunnels may be considered to be particularly suited to a design-and-construct form of contract, in that there is considerable influence on the permanent structure by the temporary works required for the float-out and immersion of the tunnel elements. The scope of the reference design produced by the engineer for the Medway Tunnel project, which allowed tenders to be sought, afforded flexibility for the tenderers in their options for constructing the immersed tunnel, the cut-and-cover tunnels, and their approaches. This paper discusses the designs carried out by Mott MacDonald for the various forms of construction employed on the project by the contractor. These works include the cut-and-cover tunnels which were constructed using permanent diaphragm walls or with cast-in-place concrete within a temporary combi-pile wall arrangement, the piled slabs and gravity structures which were employed on the open approaches, the innovative sand-placing method used for the embankment built on very soft ground, and the immersed tunnel itself. The paper also addresses the coordination required in the design-and-construct process between the designer and contractor, and also between the civil and mechanical and electrical works. Finally, the paper discusses some of the technical complications encountered during the construction of the project, and how the design-and-construct form of contract was beneficial in achieving a rapid and satisfactory solution.