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Water special: Sinking solution

An unusual technique has been adopted as part of a major upgrade to York’s clean water supply. Margo Cole reports from Acomb Landing.

Acomb Landing, just north west of York city centre, is the oldest water treatment works in Yorkshire Water’s portfolio. It has been in continuous operation since its slow sand filters were constructed in 1846, and is now one of two works that provides all of York’s clean water.

During its 150 year history various new buildings have been added, taking capacity up to the current level of 23M.l per day, but that is no longer enough, and a major upgrade of the works is under way. This will increase capacity to 36M.l and improve levels of treatment.

“The existing works relies on slow sand filters,” explains John Dixon, project manager for contractor ETM, which is responsible for the £13M upgrade under its “large schemes” framework with Yorkshire Water.

“They work well, but take a lot of maintenance, because you have to clean them by taking the sand out, washing it and putting it back.”

“It was cheaper to build a completely new works off line”

John Dixon, ETM

They also take up a lot of space, making it difficult to fit in the new facilities required to increase capacity at the works.

Capacity expansion

“We needed to turn this into a large capacity works, and one that complies with current asset standards,” explains Dixon. “One choice was to upgrade this works by replacing lots of things and providing new parts ad hoc, but it was actually a lot cheaper to build a completely new works off line.”

The current footprint of the works, with its massive filter beds, takes up hundreds of square metres, but modern treatment technologies mean the entire new works - other than storage reservoirs - will fit inside a new building measuring just 50m by 50m.

Fact file

Client: Yorkshire Water
Contractor: ETM (a JV of Aecom Design Build and Galliford Try)
Auger boring subcontractor: Perco
Contract value: £13M (as part of its large schemes framework)
Start: December 2010
Completion: September 2012

When it comes to the construction programme, the critical path is all about the main process building, because of the time needed to install and commission all the specialist process and M&E equipment. But, alongside it, ETM has also had to solve a tricky civil engineering challenge in order to construct the intake that brings water into the works from the River Ouse on the eastern edge of the site.

A new intake is needed to reduce future maintenance and to comply with current legislation - including aspects of the European Water Framework Directive designed to prevent fish becoming trapped in the works.

“At the moment there is quite a coarse screen on the intake,” explains Dixon. “We either need to provide better screening or put in a return system.” The Ouse is home to a considerable fish population, including lamprey and eel, so the new intake will have screen with 2mm holes in it to stop them getting in.

“We wanted to stick a mini-digger in, but the machine couldn’t sit on it - it just sank”

John Dixon, ETM

This intake is being built very close to the original, but is slightly deeper and further out into the river. It will be linked to a new pumping station via 22m of pipeline, which must be installed beneath a concrete flood wall built some years back to protect the treatment works from floods. The wall’s construction consists of reinforced
concrete cast onto sheet piles sunk into the underlying clay.

ETM considered various options to install this new pipeline, including tunnelling and cut and cover.

Tunnelling options

“Open cutting was an option, but we would have had to pile the whole length right up to river - which would have been a massive flood risk for a long duration - it’s a big open trench you’re putting in,” says Dixon.

“Also, there are services within the [river] embankment, including the [electricity] main that feeds Boroughbridge, and an old 800mm diameter sewer that would have to be supported. That would be quite complex.”

After considering all the options, ETM decided the best solution would be to excavate two cofferdams - one for the intake and one for the new pumping station - and auger bore between them. It is a technique that is often used to install services under motorways, and for sewer construction, but not for water mains, and they are rarely laid as deep as the 8m required at Acomb Landing.

The new intake and fish screen are being built inside a 10m x 7m x 7m deep sheetpiled cofferdam, with the piles extending 16m into the underlying clay. The cofferdam for the pumping station, built just behind the flood wall, measures 11.6m x 6.2m x 9m deep.

“These are quite big items of civil engineering that you don’t do every day,” says Dixon. “We have gone down 9m into really bad ground, so the excavation was quite difficult”

As well as a very high water table, the contractor had to contend with a high water and sand content in the clay, which made it difficult to put any kind of excavation equipment into the shaft. “We wanted to stick a mini digger in, but the machine couldn’t sit on it - it just sank,” explains Dixon.

The solution was to dig as deep as possible using a grab from the surface, and then finish the excavation using a machine sitting on a platform suspended from the top of the cofferdam by chains.

Once the excavation was finished, ETM poured a permanent concrete base in both cofferdams. Inside the intake excavation, two H-piles were driven in below the base, and topped with a concrete capping beam to form a foundation for the intake pipe.

On the other side - in the excavation for the pumping station - blinding concrete went in, followed by a permanent 0.5m thick concrete base and a temporary concrete thrust wall for the auger boring.

Dry January

Specialist contractor Perco carried out the boring over a mercifully dry period of weather in early January, using a three phase operation. First, a pilot hole was drilled to establish line and level. This was followed by an intermediate augur, which pushed the pilot tube out as it went through and created an enlarged bore.

Finally, a 750mm diameter augur was drilled through, again pushing the smaller sections out as it went. As it pushed its way through the ground, 1.5m long mild steel sleeve sections were welded together behind the augur to form the permanent support for the bore.

The 600mm diameter polyethylene water mains pipes fit inside this sleeve, and the annulus between the steel and the polyethylene is grouted up.

With the auger boring successfully completed, construction of the intake and pumping station structures is now continuing alongside the main treatment works.

Although the new works has a relatively small footprint, it still has to be squeezed in among all the existing buildings, reservoirs and tanks, as they have to remain fully operational until the new facility is finished and can be brought on line. This has not been an easy task, as space is limited, and the site is tightly bounded on one side by the main north-south railway line between London and Edinburgh.

Work is now well on the way towards completion and ETM can look forward to later this year when water will flow through the new works in earnest.

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