The installation of Demings Moss service reservoir

The use of a revolutionary 'intelligent membrane' at Demings Moss service reservoir on the Cumbrian Fells above Haweswater, allowed the new construction to be overlaid with a restored peat bog!

The successful use of a revolutionary 'intelligent membrane' at Demings Moss service reservoir.

If conventional waterproof membranes puncture when they are buried beneath other materials, failure is usually evident from the leakage that occurs.

Hitherto, it has been impossible to locate the exact position of the rupture without significant excavation.

The 'Hylam IQ' system was developed to provide a method by which integrity can be guaranteed both at the time of installation and at any time in the future.

The system works on simple well-established technologies but provides a novel electronic means of leak detection.

Development involved production of a sophisticated waterproof laminate which overlays an electronically sensitive network of sensors.

The precise position of any rupture can be reliably determined from ground level without exhaustive excavation.

DRC Polymers had spent several years bringing the two technologies together in a practical form.

The Haweswater AMP3 water quality improvement scheme required a new 9.0-megalitre twin-compartment service reservoir to be constructed, high on the Cumbrian fells.

The site is in an area of outstanding natural beauty in the Lake District National Park.

The GBP30 million project centred on building a water treatment plant at Watchgate.

From here water would be pumped to a new service reservoir at Demings Moss and then gravity fed down to Bellfield, Swindale, Barkwood Springs, Haweswater and Braysteads for downstream distribution.

In addition to the construction of both the water treatment plant and the service reservoir, the scheme involved laying over 60 kilometres of mains.

Demings Moss, where the service reservoir is located, is an established peat bog, some 460 meters above sea level.

This presented some serious environmental challenges.

Both English Nature and the Lake District National Park were involved in the final designs and landscape requirements.

The scheme specified that the reservoir was to be a standard twin-compartment structure built to current standards but the location dictated that the finished structure would need to blend with the existing landscape and be sympathetic to the location.

In order to achieve these objectives, it was necessary to remove the peat bog and store most of it under controlled conditions for the duration of the project.

After excavating the site to accommodate the reservoir underground, a roof had to support the re-establishment of the peat bog.

Under normal conditions a new service reservoir would not require any additional waterproofing.

However, as this structure had to support an inherently acidic peat bog, a liquid coating was originally specified to protect the concrete roof against acidic corrosion.

The project started in good weather but by the time the contractors were ready for waterproofing, winter had begun to set in.

It became impossible to find a 'weather window' in which the liquid coating could be applied with any degree of confidence.

Several alternatives, including conventional loose laid membrane, were considered to surmount this problem.

Despite the pressing need to put the reservoir in service and the cost penalties of failing to complete the project on time, conventional membranes were not judged to be an acceptable alternative, as their integrity was impossible to guarantee.

The search began for an effective solution.

The MWH Service Reservoir team in discussions with Brookes formulated a concept to improve Client confidence in loose-laid membranes.

Brookes immediately went to DRC Polymer Products, who applied their innovation and expertise in product development, to solve the problem and offer an 'intelligent' membrane to the Client.

At the MWH Service Reservoir team instigation and with the help of Brookes Specialist Contractors, DRC set up a live demonstration of Hylam IQ for MWH and United Utilities.

After two separate, independent demonstrations, technical approval for the membrane was obtained.

With January approaching and no prospect of completing the project with the original materials in sight, Brookes Specialist Contractors reached agreement with main contractor Murphy to install the intelligent membrane at Demings Moss.

Little could be done over the Christmas period but DRC Polymers put materials into production without delay.

Brookes mobilised on January 24th, completing the installation of the electronic sensors the same day.

After accurately recording the location of each sensor, the Hylam IQ membrane was installed and temporarily ballasted by a team of five.

3000 square metres of membrane were required and although this work was scheduled to take three days, in the event delays with releasing the second compartment caused interruptions.

Even by upland standards the weather was unkind, with gales and snow presenting every obstacle to smooth progress.

The reservoir vertical walls required lining prior to backfilling with both natural stone and clay in these extremely testing conditions.

Technicians from both DRC Polymers and its technology partner Sensor worked alongside the installation team from Brookes, providing assurance of integrity as the membrane was installed.

In order to provide the best environment for the peat bog, it was necessary to install both a deck drain and bentonite matting.

Although providing a healthy environment for the peat, this system posed a threat to the detection system.

The air space created could cause a break in continuity so a further layer of conductive geotextile was required below the deck drain.

The Hylam IQ system was fully installed and the initial certificate of integrity was issued to the contractor before the end of February.

Work backfilling the original excavation could then begin.

The secondary geotextile, deck drain and bentonite matting were also hung down the vertical walls of the reservoir.

Although this provided a degree of protection, it was thought prudent to install protection boards before backfilling with rock.

Simple plywood was chosen on the grounds that it would ultimately biodegrade as compaction takes place around the reservoir.

The toe drains were put in place and the inspection chambers built as the backfilling began.

To the credit of the contractors, only one incidence of mechanical damage to the membranes was reported.

In repair and maintenance situations, the ability to both easily locate and easily repair damage to buried waterproofing offers a huge cost advantage over alternative systems.

Being thermoplastic, Hylam IQ can easily be repaired using conventional welding techniques When roof level was reached, the integrity of the waterproof system was once again confirmed before specialist landscape contractors undertook a final capping with sphagnum moss.

The outcome of full scale interrogative testing undertaken on April 26th was entirely satisfactory.

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