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Invert convert

Tunnels - Falkirk interchange

A special conversion to a road planer saved time and money on a tunnel project as Rodney Byles discovers.

Contracted to remove exceptionally hard sandstone in the invert of the UK's first canal tunnel to be built in over 100 years, specialist planing contractor Tetlaw Contracting Company converted one of its fleet of road planers into a surface miner. It proved to be a cheaper, faster and safer method of excavation than conventional hydraulic breakers.

Tetlaw worked round the clock, rapidly milling out a 2m deep stepped channel along the centre line of the 147m long Roughcastle tunnel in Falkirk, Scotland. Client and tunnelling contractor, a joint venture of Spray Concrete and Joseph Gallagher then followed on, removing the remaining material from the base of the tunnel walls with breakers and completing the sprayed reinforced concrete final lining.

'There is no way that we could have taken out the rock with breakers at the speed and accuracy of Tetlaw's Wirtgen 2100 DC in that confined space, ' said the joint venture's deputy project manager Richard Dexter. 'The Wirtgen milled out two-thirds of the rock in the invert in just five days. This cut two weeks off our original seven week programme and provided a cost saving of about 10% to 15% on that section.

The machine also produced about 3,500t of a Type 1 well graded, consistent spoil, which was used as sub-base fill.'

The Roughcastle tunnel is an integral part of the new ú17.5M ($24.9M) Falkirk canal interchange being built by joint venture of Morrison Construction and Bachy Soletanche. The 2km interchange, with its locks, tunnel, impressive aqueduct and spectacular rotating boat lift, rejoins the 56km long Forth and Clyde Canal with the 50km Edinburgh and Glasgow Union Canal.

The tunnel, designed by consultant High-Point Rendel, is basically a 7.5m diameter horseshoe shape with a flat, level, invert and sprayed reinforced concrete final lining. The joint venture drove the crown in twoside-by-side headings, staggered by 20m, using a Schaeff ITC112 tunnelling excavator fitted with a breaker and bucket. They had intended using breakers and excavators to muck out the invert in 2m sections. But Dexter, familiar with road planers, contacted Wirtgen and sent rock samples to the German manufacturer's headquarters to check if it could be milled instead.

Wirtgen was confident that a Surface Miner, which uses a much heavier duty cutting drum with fewer and far stronger picks could do the job. But there are no purpose-built Wirtgen Surface Miners available. However, Tetlaw has converted one of its 450kW Wirtgen 2100DC road planers, replacing the standard cutter drum with a Surface Miner drum module. The company had used the conversion on several contracts, including the Channel Tunnel Rail Link.

Tetlaw started cutting out the invert with an initial pass to remove the undulations left from mucking out the crown. The machine worked in a series of cuts, turning outside the portals for the return runs. The spoil was discharged from the conveyor into dumpers for placing as subbase outside the tunnel.

Tetlaw's team lowered the invert floor by 2m from an initial 5.5m wide swath in steps down to a final width of 4m in the bottom. The operators were able to adjust the height of the cutting drum to tune and optimise the performance of the tungsten carbide tipped picks to match the varying strength of the rock.

Cutting out a trough down the middle of the invert also improved safety and increased the stability of the walls while the tunnelling contractor followed on breaking out the remainder of the rock and completing the sprayed concrete lining. 'The central channel has given us three unconfined faces along each end of the walls, making it easier and faster to break out the rock in a staggered operation, ' said Spray Concrete Joseph Gallagher joint venture project manager Stuart Manning. 'We've been able to triple production to 3m/shift.'

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