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Destination Dark Peak

Gaining access to three Georgian reservoirs in the Peak District became as much part of the project as the tricky repair work itself.

High up on the wind blasted, boggy plateau of the Dark Peaks in the Peak District, British Waterways has been quietly, stoically, working away on Swellands, Black Moss and Redbrook reservoirs.

The three structures are part of a Georgian network of reservoirs designed to feed into the Huddersfield Narrow Canal, itself a landmark engineering project which still holds the title of the highest, longest and deepest canal tunnel in Britain. Work on Swellands and Black Moss involves refurbishing draw off pipes and valves, while repairing the embankment is the main job at Redbrook. Refurbishing and maintaining the structures has been a huge logistic and environmental challenge.

At Black Moss, the original timber draw off pipe had decayed and is now being replaced with a new 750mm diameter plastic pipe. This is being installed using open cut construction.

At Swellands, work is proving to be more complex since it is more than three times deeper than Black Moss so more water is being pumped out and silt, which had accumulated over the years, is being excavated to reach the draw off pipe. The draw off pipe is made of cast iron and has valves downstream of the dam. This creates a permanently pressurised pipe through the core of the dam. Refurbishment work involves lining this pipe and installing a new valve.Construction could only take place when the ominous Dark Peaks climate allowed. Unfortunately, this summer's heavy rain stalled road access work as the freshly laid aggregate washed away in some areas.

Accessing the original draw off pipe on the Swellands reservoir requires the construction of a cofferdam once the level of the reservoir has been sufficiently lowered. This proved particularly problematic during June and July's monsoon. Pumps were used to control high inflow, while several more were used to keep the water level below the cofferdam while repairs were carried out.But on reaching the base of the cofferdam, Holt discovered problems.

"We found a cast iron pipe through the core of the dam with a culvert leading to the core on both the upstream and downstream shoulders of the dam. The culvert, however, was much narrower than expected at less than 600mm in diameter."

His solution was to install a hot cure liner designed to be fully structural through the original cast iron pipe. The narrower-than-anticipated bore of the cast iron pipe however required a rethink. Following significant input from Morrison Construction, it was decided that slip-lining the pipe and the culvert with a rigid High Density Polythene liner was the easiest and most cost-effective solution, even though this would limit the draw down rate.

Again, Holt and his colleagues are taking a common sense approach to the situation: "A Closed Circuit TV survey of the pipe identified bends and imperfections in the existing pipe as a result of long term settlement and decay as well as the original manufacturing process."To reduce the risks associated with placing the new pipe, calculations were undertakenby British Waterways. These ensured that the reservoir could be drawn down in an appropriate timescale while minimising the diameter of the liner", says Holt.

The result is a 250mm external diameter liner which is being pulled though the 300mm nominal internal diameter cast iron pipe, and the annulus grouted up.

In the next few weeks, the valves will be fitted and the restoration will begin.

But the project does not end there. Plans for the reinstatement of the bog and the re-use of road aggregate as a buttress on the Redbrook reservoir embankment will also be carrie out, in keeping with the project's high regard for maintaining the environment as well as the reservoirs.Getting bogged down by the weather

While weather was always going to be high on the agenda while working high in the Peak District, access to the site proved the most significant challenge for British Waterways and its contracting partner Morrison Construction: there were no roads to Swellands and Black Moss reservoirs. The A62 was the closest approved road for construction vehicles but was still 2km from the remote Swellands reservoir.

Coupled with this was the high level of environmental sensitivity surrounding the reservoirs which are classified sites of special scientific interest (SSSI).

But the unique nature of the bog habitat and its importance to wildlife and nesting birds in particular meant that building a permanent access road was out of the question.

The solution was both ingenious and ecologically astute. It was decided that the access would follow one of the original channels cut to feed into the reservoirs thereby reducing disruption to the flora and fauna of the area as well as limiting the impact of any settlement.

With this in mind, creating access began in a narrow window between September 2006 (after the bird the nesting season) and before winter roared over the Pennines.

The 5m wide road has been laid over existing vegetation, the root-mass of which gives some strength to the peat. Stone varying from 250mm to 450mm has been laid on a high strength geotextile membrane. The relatively thin construction aimed to minimise settlement and facilitate a more effective reinstatement once maintenance work was completed. Aggregate used to surface the access road was also given due care and attention; run of the mill hardcore could not be used. The alkalinity of limestone could be detrimental and recycled aggregate was avoided due to concerns over pollution from oil or other hazardous substances. This meant that stone had to be freshly quarried and sourced relatively locally so that its pH was similar to the typically acidic bog. British Waterways has planned their maintenance of these reservoirs such that the bulk of the aggregate used can be reused in repairs on Redbrook reservoir's embankment, due to be carried out in 2008.

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