In the context of the Kennet & Avon canal's entire 140km length, the 10km Avon valley section is a problem hot-spot. It accounts for £12M of a total £29.24M budget allocated for refurbishment works, designed to extend the life of the canal by 200 years. The steep-sided Avon valley is cut through oolitic limestone and, since it opened in 1810, springs have constantly penetrated the canal's puddled clay lining.
While they flow, the springs help keep water levels high. But during summer droughts the springs dry up, allowing canal water to escape through the holes. And there has always been the risk that slippage of the banks would lead to major breaches.
Kennet & Avon Canal Partnership project manager John Laverick says that in the 1960s the canal was derelict. 'Navigation was possible until the 1950s, but under the 1968 Transport Act the canal was classified as a drainage ditch.'
Laverick says voluntary restoration work in the 1970s and 1980s was crucial in preventing the 'drainage ditch' becoming a forgotten, vegetated indentation on the landscape. Volunteers have been central to the campaign for full-scale refurbishment and are represented through the Kennet & Avon Canal Trust in the client partnership, with the Association of Canal Enterprises, British Waterways and seven local authorities.
Contribution in kind - voluntary labour - is counted towards the 14.5% match funding requirement.
Restoration of the canal started in October 1997 and is going ahead with 85.5% funding from the Heritage Lottery Fund. The client, Kennet & Avon Canal Partnership, and designer British Waterways Technical Services, must make good more than 100 years of neglect. They must also try to cancel out flaws which are a relic of under-funding and over-tight schedules when the canal was built.
The Napoleonic wars erupted when the Kennet & Avon was under construction, sucking funds out of the project. Chief engineer at the time, John Rennie, is reported to have been unhappy about the quality of work and materials. Excavation into the hillside should have been deeper, the clay lining was thinner than the 3ft (900mm) specified, and cheap stone was used for construction of walls and the valley's two aqueducts.
A transport revolution then hit the canal. The Great Western Railway built a London-Bristol line that slashed freight transport times. To reduce competition from the canal further still, GWR bought it in 1846 and slowly ran it down.
The long period of decline saw the canal develop a rich ecosystem and it is this that helped the canal partnership clinch Heritage Lottery funding. When restored, the Kennet & Avon will be a 'linear nature park', says Laverick. Keeping the canal nature-friendly has been an important factor in carrying out the relining. The Heritage Lottery Fund has appointed monitors from Ove Arup, English Heritage, English Nature and the Countryside Commission.
Work in the Avon valley started in 1997 and will not be completed until the end of 2002. The slow pace of progress has been dictated by the desire to keep the ecosystem as intact as possible, and allow it to re-establish after works have been carried out.
The canal is being dredged in maximum lengths of 3km, and contractors must wait a year after completing work on and re-filling a section before dredging and dewatering the next.
Plant and the materials used to make the Kennet & Avon watertight have changed dramatically in two centuries. A flock of sheep was driven back and forth to compact its puddled clay lining when the navigation was built. Today, canal-side sheep are cropping grass, oblivious to the dumpers and tracked excavators busy relining it.
After dredging and dewatering, 300-600mm of the puddled clay lining is removed and a 250mm crushed stone base layer installed. On this is laid a PVC liner sandwiched between two geotextile layers, capped by a 125mm slab of mass concrete, to protect against barge poles and fish hooks.
The PVC liner is heat welded to the PVC water-band cast into a mass concrete wall being installed along the canal's down-hill bank to resist slippage. Where the gradient of the hillside is less severe and the canal wider, a cheaper geotextile-bentonite mat sandwich is being used.
The 1.5m2 section wall is built in 12m lengths, the join between each length sealed with a vertical PVC water-band. If hillside slippage does occur, the 12m sections should be able to accommodate the movement without failing structurally.
Where moorings are being provided the retaining wall is raised 400mm above water level. 'In heritage terms, putting in a mooring is equivalent to building a multi-storey car-park in the centre of Bath,' says monitor Simon Birkbeck of Arup.
The visible course of concrete contains Thames Valley flint aggregates and is rusticated to give it the appearance of stone. It is then shot- blasted to provide a good key for lichens and moss.
Everywhere else, though, the wall is all but invisible. With mature vegetation planted above, there is little to tell the new lined sections of canal from those sections awaiting work.
Along its length the up-hill bank is being kept soft to encourage growth of reeds and other aquatic plants. The geotextile and PVC liner is simply wrapped around its contours. Reeds and bank-side vegetation removed during the works are stored in temporary lagoons and later re-instated.
Project engineer Peter Evans of contractor Alun Griffiths, currently working on a 2.2km, £1.9M section, says the work is technically straightforward.
But he adds: 'The project is a logistics nightmare. Work is being slowed by heavy rain that has turned the site into a swamp. It would be a nice job to do in the summer.' But Evans has to hand the section over by Easter so that boaters can enjoy a summer on the water. Griffiths is working on a framework contract and is set to work another three years on the canal. Work will resume in October.
Evans says the logistics of organising land access is also proving difficult. Next year, he will be working on a section closer to Bath, and must negotiate with individual householders, as well as farmers, whose land will be affected by the project. Evans says that relining of the entire valley could be done in 18 months if environmental conservation, start-stop construction and tricky access were not issues.
Relining the canal is not only aimed at securing its future as a navigable waterway. It will also contribute dramatically to water conservation - an increasingly important issue since the Environment Agency imposed a limit on abstracting water from the nearby River Avon. The canal partnership will improve efficiency further by installing backpumps to recycle the 273,000 litres lost each time a lock is emptied.
Meanwhile, financial analyst Coopers & Lybrand has calculated the rejuvenated canal will support three new marinas and spin-off tourism will create 2,600 jobs, putting £28M into the local economy annually.