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Search for the seal

How do you fix Britain's biggest leak?

'Easy: just wallpaper over the cracks with 25,000m 2of PVC carpet, ' says Yorkshire Water. Not so easy, says Diarmaid Fleming after a trip to the Pennines. Pictures by David Jones.

Water engineers are used to leaks, but when the seepage comes from a breach of one of the UK's largest rock fill dams, the engineering challenges involved in stemming the flow are far from run of the mill.

Perched high up in the Pennines in Yorkshire, Winscar Dam is the scene of an emergency contract to seal Britain's biggest leak, which if unplugged could have bled the reservoir of enough water to quench the needs of a sizeable town every day.

It all began last January, when a routine inspection of the 26-year-old dam discovered the emergence of vast quantities of water downstream.

'We could see all was not well because of the sheer volume of water coming out of the ground.

There is always seepage through and under the dam, but this was too big a flow to be taken by the drainage system, ' says Yorkshire Water Services project manager Jim Claydon.

'Exit velocities were very slow and there was no threat to stability. But we were losing around 6,000m 3day - about a quarter the yield of the reservoir and enough to supply 12,000 families.'

Built between 1972 and 1975, the 50m high and 520m long dam created a new 8Mm 3capacity reservoir to supply the Denby Dale area in Yorkshire. It actually submerged an older structure, the Dunford Dam, built in the 1860s as an ornamental lake.

The new dam was the first in the UK to use a membrane of asphaltic concrete, laid in two layers to a total thickness of 120mm. Design life was 30 years, but the membrane was obviously the first suspect as the cause of the problem.

Finding the source of the leak was not an easy task however.

Inspection of a concrete structure 21,000m 2in area is challenge enough, but when most of the structure is under water, the task becomes considerably more difficult, costly, and dangerous.

Emptying the reservoir to enable a complete inspection of the upstream dam face was decided to be the only option, but that brought its own problems.

'Water control was a major problem, and we had to find ways of controlling the silt, ' says Matthew Hill of TEAM, a joint venture of consultants Montgomery Watson Harza and Arup appointed to investigate the leaks at the beginning of the year. Releasing such a vast quantity of water could have serious downstream effects, with the principal conduit, the River Don, a prized trout, salmon and grayling river.

The solution was provided by a combination of pumping and bringing back into commission the old submerged Dunford Dam which helped create a stilling pond. 'There were two reasons for water control - one was to protect the works and the second was to minimise the transportation of silt downstream. We used sets of floating pumps to ensure that water was pumped from the top down, leaving the silt to settle at the bottom, ' says contractor Morrison Construction contracts manager Guy Wilson. Drainage was carried out after discussions with the Environment Agency and the Salmon & Trout Association. The reservoir could not be emptied entirely, but the old dam, dwarfed by its newer replacement, holds the remaining 400,000m 3of water, and also keeps the structure needing repair in the dry.

Speaking at the waterless toe of the dam, Claydon says that emptying a reservoir to work on a dam is a unique and unusual experience. 'It was quite surreal.

I worked on the original design and construction of the dam in the 1970s, and didn't expect to be standing here again where normally there would be 45m of water, ' he says. As the water level was lowered the leakage reduced significantly at around half depth. Inspection revealed minor defects, cracks and blisters but a definitive location of the leak has still not been identified.

'We think the defects are due to separation of the two layers of asphaltic concrete, ' says Claydon. 'We decided against a patch-and-repair job: the dam was due for major works in a few years so we brought forward money from our capital budget to do it now.'

Two options for repair were looked at - one was to replace the membrane with asphaltic concrete, the other covering with a geomembrane. 'Resurfacing would have had much more environmental impact and would have cost more. It would mean scraping off 12,000t of material which would have to be disposed of and then replaced, ' says Claydon.

Dam engineering is a closeknit world, and news of a successful project to seal a leaking dam in the Czech Republic was not long in reaching Yorkshire.

Claydon and Wilson soon found themselves at the Moravka Dam where the application of a PVC geomembrane patented by Italian manufacturer Carpi had been successful.

The decision was made to use it at Winscar, its first use in the UK. Another first was the choice of a green shade to blend in with the reservoir's surroundings in the Peak Park after discussions with planners. The material is only 2.5mm thick, but highly robust with a geotextile membrane underneath to allow for any drainage and protect against puncture.

The sight of the vast nowgreen dam is spectacular, like a giant card table or billiard baize on its side. 'The geomembrane is laid in sheets - a bit like rolling out wallpaper, ' says Claydon.

Special 'tensioning profile' channels are resin anchored into the dam at 5.7m centres. These clasp the edges of the PVC rolls laid down the slope of the dam, while the channel holds a closing flap which is then heat welded to seal the joint, leaving just a bulge indicating the hidden channel underneath. The sheets are also sealed watertight at the crest and toe of the dam.

Czech and Italian workers from Carpi, and Yorkshire labour from Chesterfield firm Alps, performed what must be akin to the most arduous carpet-fitting job around, laying 25,000m 2of material in total, allowing for overlaps. The 1.7 slope meant workers effectively abseiled, and safety considerations forbade anyone operating below another worker on the dam.

Watertight performance tolerances are required with a maximum leakage rate of only 1 litre per second predicted. State-of-the-art monitoring equipment is being installed by German subcontractor Keppelmeyer which laid 10km of fibre optic cable underneath the membrane.

This uses a laser light source to give a continuous temperature measurement along the length of the cable. Any leak will change the temperature regime which the monitoring system will be able to locate, allowing repairs if required. Other work to repair the dam's grout curtain is being carried out by Skanska Cementation.

Sealing the dam is due to be completed shortly to take earliest advantage of the wet Pennine winter and allow reimpounding of the dam to begin. Refilling will take two years.

Claydon says co-operation using the expertise of all parties to the NEC project on the £3.2M contract has been essential to getting the work done quickly and successfully. Completion is expected in the early spring.

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