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Project profile | ground remediation

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A major ground stabilisation project was required when a pumping station building in Renfrew, just along the River Clyde from Glasgow, started sinking.

After undertaking urgent works in early 2013, geotechnical specialists Donaldson Associates worked with Morrison Construction and Scottish Water to design a bold, long-term solution that relied on the back-up and mitigation expertise of the UK arm of Finland-headquartered Uretek.

Donaldson principal engineer Michael Purkis recalls finding a 500mm drop in the ground around a pumping plant.

“It became apparent the station was pumping the ground out from around itself,” he says. “There was a pipe break that meant Clyde alluvial soils were being drawn into the pipe network. After our initial works to clear this sand, we were left with a pumping station that had to stay operational, and lots of very loose soil.”

Uretek top right cropped

Uretek top right cropped

Main work site

The problem worsened over time.

“We had continued settlement for a period of two years. The only feasible solution was to densify the ground.”

That meant dewatering the soil to a depth of 10m, to induce the full anticipated settlement in a quicker and more controlled way and allow work to level off the site.

A system of 14 ejector wells, each 17m deep, was installed on a 20m diameter ring. A great deal of planning and modelling was required, with manholes and other infrastructure critical to the successful running of the pumping station at risk of movement. There was also potential for damage to two houses at the end of a nearby cul-de-sac.

Expansive resins

Bringing in Uretek – which specialises in injecting expansive resins to stabilise the soils and lift the structure – gave Donaldson the confidence it needed to proceed.

“We had Uretek on standby during the riskiest part of the dewatering process, so it could remediate the houses if they got to a pre-ordained degree of settlement,” says Purkis.

“Although we had a great deal of uncertainty in our works we knew we could mitigate the risk through Uretek being on standby.”

One advantage of the remediation system offered by Uretek – which uses 12mm diameter tubes, and which can be accurate to 250 microns – is that it does not affect building insurance policies.

Underpinning ruled out

“Had we underpinned the house then we would have had to go through lengthy reviews and approvals to get works accepted by insurers,” says Purkis.

As it turned out, careful works and even more careful monitoring showed there was no need for intervention on the manholes or houses.

However, while Uretek was on site, another issue emerged.

“Our dewatering system was being overwhelmed by recharge flows coming into the ejector system along existing pumping station pipe bedding,” says Purkis.

Dewatering aims

So although it wasn’t causing problems to the built environment, the dewatering project was not achieving its original aims. Uretek was able to adapt its role and solve this unforeseen problem.

“Water was coming down gravel channels used for drainage pipes, back into the dewatered ground,  so we sealed the surrounds of the pipes to stop that,” says Uretek engineering manager Dan Hadfield.

This was done using injections of a carefully specified resin through small pipes inserted into the gravel pipe bedding. As well as being an impermeable geopolymer itself, the resin helped consolidate the surrounding material, again reducing water flow.

Significant settlement

Elsewhere on the site, the surface above one of these pipe runs settled significantly.

“We did a number of probe tests to confirm the depth and extent of the problem then injected one of our faster-reacting materials into the problem zones to control the flow of water and remove the presence of softer soils,” says Hadfield.

The overall project Donaldson was working on was completed successfully.

“We achieved 60mm settlement, which was close to our target, levelled the ground and reinstated surfaces such as the roads,” says Purkis. “The movement on the houses was closely  monitored and was contained within predetermined limits, with remediation intervention proving unnecessary; indeed, when we turned the dewatering off, the houses recovered a few millimetres.”

Key role

Uretek played a key role in making the scheme work, he says.

“Without Uretek, we would have had to manage the uncertainty better, isolating the dewatering system from the houses. We would have needed cutoff walls and it would have made it a far bigger civil engineering job with much higher risk and cost to the client.

“Scottish Water might even have preferred to rethink the location of the pumping station, which would have meant significant changes to its network.”

Hadfield says Uretek is increasingly working in this way – being brought in early to help design, shape and protect projects, rather than being called in, in an emergency when something has gone wrong.

Early involvement

“By bringing us in early enough we can make a very difficult project more achievable,” he says. “

“Anywhere you are expecting a building to move downwards it is prudent to have us involved.”

Uretek recently completed a time-saving job on the Crossrail project.

“We can treat soil so that piles are not needed for foundations in some projects. We did a job on a workshop in Ilford for Crossrail where a new roof and crane were being added – the piling option was four months but we did it in 12 days.”

Elsewhere the company was called in to a drain collapse affecting the A74 that runs between Celtic Park and the Chris Hoy Velodrome in Glasgow.

Uretek treated the ground below a newly installed slab so that it could take the increased load of the strengthened infrastructure.

A74v2

A74v2

A74 trench

“The work site was 50m long by 5m wide, so our trucks parked on the side and we only had to get hoses, drills and manpower down into the trench,” says Hadfield. “We used our PowerPile solution to increase the strength of the soil to make it suitable for our usual injection technique.”

The firm has a supplier that makes a family of resins exclusively for Uretek, worldwide.

“Material selection changes expansion rate, density and speed of reaction,” says Hadfield. “To lift a heavy structure you need a very expansive resin.”

Uretek works in several sectors on various types of project.

“We add value to many schemes through the speed and delivery of our solutions. Airports, roads and railways are increasingly using Uretek to deliver fast solutions, reducing project costs.”

Produced in association with Uretek

 

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