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UNDERGROUND CONSTRUCTION - Ayrshire's River Nith doesn't know whether it's coming or going. Just when it's getting settled into a new course, it gets rerouted again. Damon Schunmann reports.

Scotland's House of Water is an area of natural beauty, a profitable open-cast coal pit and the home of an important salmon and sea trout river. Scottish Coal must juggle all three considerations to satisfy the various parties interested in the Ayrshire site.

The River Nith was first diverted four years ago to allow access to underlying coal. Now Scottish Coal wants to get to a 250,000m 2patch beneath its present course, between two previously worked areas.

But there's more to moving 3.8km of river than making the water follow a different route.

Scottish Coal projects manager David Booth says: 'Twenty per cent of the job is river diversion and the rest is creating a habitat, and that's what we will be measured on. If the salmon don't come back we have failed.'

Scottish Coal is doing the work itself as it has the specialist skills and equipment on site and the job must be finished by mid-September when the salmon return to spawn. By then, 540,000m 3of material will have been laid to line the riverbed and build up the bankside.

The base of the riverbed will consist of a 2m deep clay liner reinforced with geogrid. Within this will sit a 4-5m wide cutting to accommodate the flow. This will be lined with a 500mm deep layer of graded gravels and a top 250mm layer of large gravels and cobbles.

Although the riverbanks will be 4-5m apart, bulldozers are laying the clay liner much wider. Booth says: 'To follow the new river course exactly would have been impossibly slow so we've put the in clay in straight lines typically 50m wide about the centre line of the new river channel.'

To reinforce the base, Tensar SS40 geogrid with a strength of 40kN/m is being laid 300mm from the bottom of the clay and in many places a second layer is added 300mm from the top.

Tensar's area civil engineer Jonathan Cook says: 'We realised it needed to act like a concrete beam with the forces acting around the neutral axis. It's designed as a strain controller as what we don't want to happen is cracks to appear and the water to drain away.'

Tensar has supplied about 120,000m 2of grid and Cook adds: 'We've used SS40 as it's the stiffest grid we do, so the strain is picked up very quickly before it stretches.'

Sandstone boulders of 500mm to 1,000mm are being laid at the outside edges of the channel for erosion protection and to encourage salmon to come back. 'Pools and shallows are also being constructed in the line of the new river for fish to live in, ' says Booth.

Site workers will secure coir matting to the banks to allow vegetation to take hold and to stop plant growth from being washed away.

'We could have used geogrid but we wanted something that would biodegrade, ' Booth explains. This process will take about 10 of the 15 years that Scottish Coal has agreed with East Ayrshire council to manage the site.

The clay liner is being laid on top of the 80m thick mudstone, shale and sandstone overburden that has been used to backfill the underlying worked-out pit and compacted in 2m deep layers.

Booth says: 'What [consultant] Halcrow is concerned about is the potential for localised areas of differential settlement where the worked-out pit's safety benches are.'

Clay has been levelled by bulldozers to compact specification and to a permeability level of 1x10 -9 m/s, but Booth says they have actually achieved 1x10 -10 m/s.

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