Monster tunnel at Loch Ness

What is probably the UK's largest hard-rock tunnelling project to date will soon be up for grabs, as interest is sought for the construction of a hydroelectric power station in Glendoe, Scotland, that includes 17km of tunnelling.

This month the client, Scottish and Southern Energy (SSE), is simultaneously conducting a short tender process to find a suitable consultant for the preparation of tender documents, and inviting notes of interest from contractors.

The project has already received interest from British, European and US companies, and when T&TI asked Mike Seaton, project manager for SSE, who had already expressed an interest, he said: "ask me who I haven't talked to."

A spokesperson for UK-based Cementation Skanska confirmed that it had formed a Joint Venture with Morgan Est in order to bid for what is estimated to be a US$140M design and build contract. "We want to make sure that we put together a competitive bid and make it through to the final bidding stage," he said.

Mike Seaton said that SSE intends to whittle down the field to two or three parties by Spring 2004, each submitting detailed designs. The contract should be awarded by the Autumn 2004, ready for a Spring 2005 start – although actual tunnelling will not start until later, allowing for more detailed design.

A single contract will be awarded for the construction of the entire project, which includes the tunnels, the dam, and the power station (probably an underground cavern-type structure).

A 1,000m long, 35m high dam will create a new reservoir at Glen Tarff, which will be situated 600m above the power station, to be built on the south-eastern corner of Loch Ness. The dam will be the longest in SSE's portfolio, and the drop to the head will be the biggest in the UK. The installed capacity will be between 50MW and 100MW.

There are two tunnels in the contract, both 8.5km long and 4m diameter. The first is a high-pressure surge tunnel from the reservoir to the power station. The second is a water transfer tunnel.

The tunnels will run up to 300m underground and pass through schist, quartz and some granite. Seaton said that the tunnelling method had not been finally agreed upon, but it is likely that one tunnel in built with a TBM, and the other with drill and blast. "The feeling is that if there is too much quartz, it could stop a TBM," Seaton said.

The project will be fully financed by SSE as part of its programme to increase the amount of energy created from renewable resources. It also has the full support of the Scottish Executive.