Swiss technology does a dam good job in the Lake District

Seathwaite Tarn in Cumbria has become the first reservoir dam in the UK to be successfully fitted with a specialist vertical geomembrane to stop it leaking. Project manager Carl Sanders of United Utilities explains

Nestled beneath the imposing peak of Grey Friar, Seathwaite Tarn in the Lake District is a tranquil and beautiful place. You would little guess that it has been famous more than once in its life - and not always for the right reasons.

While building it in the early 1900s, some of the navvies caused a riot in tiny Seathwaite village, ending with one of them shot dead and another injured by the pub landlord. Thankfully, it is for its construction that Seathwaite Tarn is now more famous.

Its unusual curved concrete dam is a marvel of Victorian construction. Some 365m above sea level and over 3km from the nearest road, the 42m structure retains a reservoir with a capacity of 2,945Ml and originally provided drinking water for the burgeoning population of Barrow-in-Furness.

Although a new link-main was built to serve Barrow in the mid 1980s, Seathwaite still forms a critical part of the area's water supply system. Today its water is discharged to Tarn Beck downstream of the dam and abstracted from the River Duddon at a treatment works at Ulpha.

There are in fact, two dams at Seathwaite, separated by a rock knoll. The 'main' dam is 138m long and 9m high and is composite in construction and curved in plan.
It is formed with a concrete gravity section on the right side (north) and an embankment section on the left side (south) with a concrete core. The embankment was built from clayey sand and gravel with some cobbles and boulders. The subsidiary dam, located to the north of the main dam, is a concrete gravity structure that incorporates the auxiliary overflow.

In 2003, a condition survey was undertaken and concluded that the face of the main dam was in a poor condition.

The following issues were identified:
  • Spalling of the face of the dam
  • Lifting at construction joints
  • Map cracking
  • Water could be seen running on the face from the opened construction joints
  • Freeze/thaw and other thermal effects had caused the face to deteriorate
  • Evidence of alkali silica reaction.
In addition to the poor condition of the downstream face, it was noted that there were numerous significant horizontal and vertical cracks on the upstream face, many filled with calcite. Finally, it was found that the wave wall was also in poor condition and it was noted that it does not reach 'high' ground at the left hand end of the dam.

A further statutory inspection was completed by the All Reservoir Panel Engineer in January 2007 during which he recommended that works be carried out to the concrete face of the dam and that additional measures be implemented to prevent further deterioration. United Utilities commissioned MWH to undertake works to improve the condition of the existing concrete dams and to help implement recommendations to ensure that the reservoir would meet the requirements for a Category A reservoir.

The main contractor for the works was civil engineer and construction company Eric Wright.

The Seathwaite Tarn Reservoir project was part of United Utilities' £2.9B programme of investment in water quality and environmental improvement between 2005 and 2010. The chosen solution was to incorporate vertical geomembrane technology to waterproof the back of the dam.

This material was supplied and installed by Swiss waterproofing specialist Carpi. Although used in other parts of the world it had never been used on a dam in the UK.

The downstream face of the main dam was cleaned and loose concrete, efflorescence and other surface deposits were removed. The concrete face was then protected from further deterioration by using a cementitious repair mortar and siloxane waterproofer.

The remedial works on the upstream face included installation of a PVC geocomposite membrane, geonet drainage layer and associated drainage system to the upstream face of the main dam and the auxiliary overflow. The geomembrane is sealed on the concrete face along the foundation line and on the parapet wall 300mm above the crest level to cover the joint between the upstream face and the parapet wall. The geomembrane covers a total surface area of 1,100m2.

A geomembrane drainage system was incorporated into the design to discharge water through three holes in the dam. Joints and cracks were treated with an acrylic resin where the perimeter seal crosses the joint to prevent reservoir water bypassing the perimeter seal.

A reinforced concrete wave wall was constructed to the same dimensions and levels as the existing wall, but extended into the high ground at the left and right ends of the dam. Finally, riprap erosion protection was laid at the left-hand end of the dam to arrest erosion of the embankment. The material for the erosion protection was sourced from the reservoir basin.

As well as giving the right technical requirements, the design solution gave the least environmental impact in terms of the volume of material used and transportation. Because it was such a new method for the UK, United Utilities engaged key stakeholders face-to-face from a very early stage.

This included the Environment Agency, local fisheries and landowners as well as people like the Friends of the Lake District, mountain rescue and the local historical society. To maintain specialist control, United Utilities used a single tender for a specialist contractor and a framework agreement for its main support contractor.

The project was completed under budget and two years ahead of schedule with no lost time or reportable accidents. This was put down to the strategy of bringing the team together at the earliest possible stage, which, in turn, made it possible to make the most of each party's technical input and this gave a strong sense of delivery and unity.

One of the ways the partners worked together to provide a solution was to devise a means of communication and welfare facilities during construction. At two miles from the nearest road, this was quite a challenge. We needed satellite phones for communication and provided self-contained site cabins powered by solar panels.

The membrane itself took just four weeks to install and panel engineer approval was gained in May. However, the entire project took several months as engineers first had to drain down the reservoir to half its normal capacity to expose the back of the dam.

Working closely with the Environment Agency, approximately 20 trout were also rescued from the Tarn Beck, which had to be temporarily diverted to allow works to take place. Contractors also extended the reservoir's wave wall in keeping with the design of the original.

Seathwaite Tarn is a spectacular place and still performs a vital job in keeping the area's water supply topped up with some of the purest drinking water in the world. This cutting edge technology will guarantee at least another half a century of vital service to the people of Cumbria.

It has been a great success and now looks set to pave the way for similar projects in other parts of the country.

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