Sheet Piling (UK) Ltd worked on behalf of Costain Atkins Black and Veatch (CABV) JV and the contractor's client, Thames Water, to deliver a watertight solution under emergency conditions at Staines Reservoir near Heathrow Airport, in the UK.

This is the second such reservoir repair job that Sheet Piling has handled for CABV JV, with the first having been at Island Barns Reservoir, where it demonstrated the value of a contiguous sheet pile wall in 2017. It is now working to the same successful sheet piling design and installation blueprint, to carry out the rapidly required remedial works in Staines.

Jon Green, Thames Water's reservoir safety manager, said: "It's a legal requirement that Thames Water keeps the 120-year-old reservoir in the best possible condition so it can continue to store and supply water to millions of Londoners.

"This work will strengthen and reinforce the dam wall, extending the life of the reservoir well into the next century. We are using innovative techniques to keep noise and disruption to a minimum and have adapted to the coronavirus outbreak with great flexibility and dedication.

"The construction team put in various measures to demonstrate that the site could be run in accordance with government advice. This included a constant cleaning regime, re-organising briefing and break times to allow social distancing and buying in all food to prepare on site."

How sheet piles assist emergency reservoir repair works

Sheet Piling's sheet pile cut-off wall will prevent any more leakage of water through the reservoir's core. It forms an impermeable steel sheet pile barrier and consist of 18m long ArcelorMittal PU32 sheet piles. Using its Giken F201 silent piling press, Sheet Piling is installing the sheet piles through the existing reservoir embankment's core into two sections of the dam wall.

The approach will be to feed sheet piles from a purposely designed and constructed floating barge, which Sheet Piling has built from Unifloat Pontoons. This gives the business a means to get both sheet piling equipment and the sheet piles themselves across the water and then use the barge as a lifting platform from which the steel ArcelorMittal sheet piles can be fed to the walking Giken press.

A floating sheet pile installation solution

Getting plant on to the water requires the building of a temporary jetty, in this case, a rectangular cofferdam of sheet piles, filled with graded granular material. This enables Sheet Piling to get an 80t crane on to the floating structure and float it out to the required installation location where it is assisting the work of the low-impact Giken press on the dam's crest.

The process impacts on the existing and somewhat fragile structure to a far lesser degree than the installation of a cement-bentonite cut-off wall would do. That would require extensive enabling works - not required by this specified steel sheet piling solution, which is focused on protecting the dam from further damage.

Minimising environmental impacts

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A shallow trench for the walking press-in sheet piling machinery helps reduce the environmental impact of remedial work at Staines Reservoir

Sheet Piling is also, as always, mindful of environmental impacts and its piling works will not pollute the water with slurry, which could arise through the creation of spoils and arisings in a cement wall project. All that is needed is a shallow trench in which the walking press-in sheet piling machinery can operate.

Once the sheet pile wall installation is completed, the operator will fully reinstate the clay core, after conducting pitch, tenacity and elongation tests.

Victorian infrastructure repairs

The steel sheet piles' design-life is around 100 years, so the repairs will last a long into the future, ensuring the water supply and leisure facility is safeguarded from any leakage in the area of the works.

Talking about the project, Andrew Cotton, managing director at Sheet Piling (UK) Ltd, said: "It is satisfying to know that we have gained a reputation for outstanding expertise in this type of reservoir wall repair project and that hardwearing steel sheet pile cut-off walls are being recognised for their effectiveness in dam repair work.

"While this emergency call-up was not expected, we do believe we will handle much more of this type of reservoir repair work in the next few years, as once solid Victorian infrastructure starts to come to the end of its life."

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