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Slopes: Not slipping, sliding away

Piling to stabilise a slope on the outskirts of Bath allowed an important commuter route to re-open in November, following a nine-month emergency closure.

The A431 Kelston Road near Bath has been making local headlines this year, after a landslip in February meant part of this important commuter route between Bath and Bristol had to be closed.

With further ground movement likely, constructing a temporary bypass was not deemed feasible or safe. A team of engineers from Bath & North East Somerset Council (B&NES), Skanska and Atkins set to work on a permanent and innovative solution to stabilise this part of the hillside above the River Avon and allow journeys to resume safely along the A431.

Following a prolonged period of heavy rainfall at the beginning of 2014, cracks were found in the road surface along a 75m stretch between Penn Hill Road in Bath and the village of Kelston.

Further inspections revealed some of these were widening rapidly – at one stage by up to 25mm in a week. The road was clearly subsiding and, as it traverses a slope with a history of failures, B&NES decided to close it at the end of February.

“Conditions were so bad that at one point, with the slope continuing to move, there was a risk of the whole road sliding down the hillside,” explains Simon White, business director for Skanska’s B&NES Council contract.

Skanska is the highway term maintenance and improvement contractor for B&NES Council. White adds that a number of different parts of the company, including Cementation Skanska, were involved in designing and constructing the permanent solution to prevent further instability, as well as repairing the road and the damaged retaining wall supporting its southern edge.

Site investigations ran from March to May, with Poole-based contractor ACS putting down 18 boreholes across the failed section – seven along the road, four in the field above and seven in the field below – to characterise the underlying geology.

Ground conditions comprise up to 2m of Made Ground over as much as 7m of colluvium, 9m of weathered Charmouth Mudstone (previously known as Lias Clay) and Charmouth Mudstone, with the groundwater level in the colluvium. Inclinometers were installed in eight of the boreholes, with monitoring continuing throughout the spring and summer.

“Two geotechnical models were developed because the topography and the geology varies from one end of the site to the other,” explains Skanska senior project engineer Darren Wines.

“Results from the site investigation and laboratory testing were used in the back-analysis of the slip, along with the estimated ‘worst credible’ groundwater levels at the time of the failure.

“Analysis determined the failure was probably caused by a translational slip occurring at the softened interface between the colluvium, which is made up of historical slip material, and the weathered Charmouth Mudstone beneath. This was a result of the exceptionally high groundwater level, following the prolonged heavy rain at the start of the year.”

Geotechnical parameters developed during the analysis were used in the design of the permanent solution, which had three main objectives, he explains.

“First, we had to stop the slope moving; we then had to restrain the retaining wall and finally, we had to design a replacement structure for sections of the retaining wall that had been irreparably damaged by the landslip.

“We explored a number of different options during the design phase, including a permanent groundwater control and drainage scheme. However, in the end, because of environmental considerations and B&NES Council’s desire for a robust, maintenance-free long term solution, we opted to use a large diameter rotary bored pile solution.”

The 78, 900mm diameter piles are embedded into stable material below the failure zone in two rows 10m apart. They were placed at centres of up to three pile diameters, to maximise horizontal soil arching between the piles and stop the colluvium moving. Piles in the upper row have caps that are in contact with, and prevent any future movement of, the retaining wall.

“While we could have used minipiling to strengthen the retaining wall, which would have meant a thinner piling platform, large diameter rotary bored piling was faster – a key requirement for this job – and as a result more cost-effective,” Wines adds.

By June, it was decided that movement had slowed sufficiently for stabilisation work to start. Temporary works began in July, with construction of an access road and piling platform downslope of the A431, both built using local quarried stone, which will be removed at the end of the job and recycled for use on future projects.

“The design of the temporary works also presented a challenge,” Wines says. “As well as being on a gentle slope, there is a height difference of about 4m between the eastern and western ends of the site. Maintaining relatively level platforms for the piling rig and the crane was important, as was phasing of the temporary works, to ensure the weight of the platform did not make the relatively weak colluvium unstable.”

In fact, the working platform provided an element of toe weighting to help temporarily stabilise the slope during piling.

“Building the platform with a slight slope minimised earthworks and allowed piling to start as soon as possible,” explains Cementation Skanska’s senior project manager Jonathan Manning.

“This made our job slightly more challenging because we wanted to ensure both the Soilmec SR30 rig and the SC80 crane we planned to use were going to be able to work at an angle without any problems. Our plant department consulted closely with

Soilmec to ensure the correct slope angle was specified and we were able to confirm the machine’s performance would be unaffected – which was borne out during the site works.”

The five-week, £700,000 piling contract began at the end of September, as the key element of the larger £2.6M scheme. The lower row of 48 piles are each 15.1m long, with full length reinforcement cages, installed 1.6m apart. The upper row has a combination 13, 12.5m long piles and 17, 15.5m long piles, all with full length reinforcement and spaced at 2.7m. Pile caps on the upper row of piles are typically 1.8m by 1.2m by 0.5mm, cast up to the repaired retaining wall.

While the slope continued to move during piling – at about 2.5mm a week – this did not affect work either, Manning says. Pile integrity testing was carried out on all of the piles to confirm they were installed to specification.

Foundations were completed in November, with repairs to the retaining wall and the road following on immediately after. Along with replacing the road’s sub-base and resurfacing it, new drainage has also been installed, White says.

To the relief of local residents and commuters the A431 re-opened on 17 November.

B&NES Council’s highways manager Kelvin Packer says disruption caused by the project will be far outweighed by the long term benefits.

“We understand these works have caused inconvenience to local residents and commuters but our first priority has always been safety, which meant closing the road to ensure no-one was at risk. The permanent solution developed by the team will mean residents need not suffer problems in the future,” he said.

PROJECT TIMELINE

February 2014 Cracks appear in the surface of the A431 near Kelston Manor after a prolonged period of wet weather. Inspections reveal serious structural problems due to a landslip and Bath & North East Somerset Council closes the road for safety reasons.

March to May 2014 Site investigations are carried out along the road, as well as above and below the road to locate the slip. Investigations are also used to identify possible temporary diversion routes – subsequently not built due to fears of further movement. Cycle and footpaths built to bypass the failed section.

June 2014 Analysis reveals slip movement has slowed to the extent that stabilisation work can begin.

July 2014 Construction of temporary access road and piling platform starts.

September 2014 Cementation Skanska begins piling work.

November 2014 Piling work completed. Stabilisation and road repairs completed; road re-opens to the public on November 17.

 

 

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