Stabilisation of a busy rail embankment in Streatham is nearing completion and work should ensure local homes will only be disturbed for minor maintenance in the future
For almost a year Bam Nuttall has been getting up close and personal with residents in Streatham’s Edgington Road while working to stabilise the Streatham South Junction rail embankment. While part of the embankment faces onto the street, the failing slope also extended behind the houses making access challenging and limiting the techniques available to resolve the issue.
Network Rail’s consultant Tony Gee and Partners worked with Nuttall and specialist geotechnical contractor Bam Ritchies to develop a solution that would not only stabilise the slope but also minimise ongoing maintenance. Residents are likely to be relieved with this achievement after a year of piling platforms taking up road space and pedestrian site access passing bedroom windows, not to mention the piling and ground anchoring work itself.
The embankment was built in 1862 using end-tipped London Clay fill over natural London Clay and is around 6.5m high. Ground movement first started to affect the embankment in 2015 and speed restriction have been imposed at various times as a result.
Tony Gee and Partners senior engineer Sam Doe adds: “There were two existing failures and the ongoing movement had been dealt with at track level with placing of ballast and tamping. Investigations showed that there were pockets of ballast up to 1.5m think.”
Network Rail route asset manager (geotechnics, drainage and off track) Derek Butcher explains that normally ballast is only 300mm to 400mm in thickness.
“As well as disturbing the track level, the cables running through the site were also slipping down the slope and points had suffered from differential movement,” says Butcher.
An inclinometer installed at the site of the worst ground movement during a ground investigation in 2015 helped Tony Gee to identify the nature of the ground movement as a deep seated failure at 5m depth.
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The embankment carries the up and down line from Portsmouth but is close to the junction with other rail lines and is considered a critical part of the network.
“The work is focused on stabilising the embankment but there was a lot of enabling works required,” says Bam Nuttall site agent Mark Cornish.
Access to the site is restricted with residential properties and a road at the foot of the embankment. Bam Nuttall’s site office and part of the piling platform has taken up half of the road, as well as land formerly used by a builder’s yard.
Work on site started in January this year and Nuttall is aiming to complete the work by the end of the year.
One of the biggest challenges of getting the stabilisation work underway was installation of the piling platform to access the 120m long section.
“There are three parts to the piling platform,” says Cornish. “At the bridge end of the embankment, the platform is formed by Legato blocks and polystyrene fill to create a retaining wall that avoids placing excess load on the embankment.
“The middle section has a two layer approach with an upper and lower platform using a reinforced earth solution from Maccaferri with more polystyrene blocks.
“The platform at the country end of the site, where there is more space, is formed from conventional granular fill.”
Cornish says that the polystyrene fill was fiddly to install compared to conventional granular fill due to the detailing of fitting it with the earth reinforced structure and the Legato blocks, as well as the need to protect is from potential hydrocarbon damage using membranes.
According to Bam Ritchies geotechnical manager Andrew O’Donovan, the main challenge for the construction was finding a solution that worked with the access available. Ritchies first became involved in the discussions for the project in autumn last year with a real focus on the buildability.
“We needed to consider the stability of the slope in general, as well as its stability in the temporary case with the earthworks for the piling platform and the rig itself,” says O’Donovan.
At its narrowest point, the platform was just 3.5m wide which, along with the weight considerations, precluded the use of most equipment.
“It meant we needed to have confidence in the ground investigation as there were not many options to start with and very little alternative to bring different equipment in if we encountered issues,” he says.
According to Doe, a number of alternative designs were considered including sheet piling, soil nailing, toe walls and slope regrading. “Sustainability and cost were considered,” he says. “Network Rail settled on the most robust solutions to ensure long term costs and minimise maintenance.”
The result was the use of a Tescar piling rig with a Kelly bar arrangement to construct a piled retaining wall tied back with ground anchors but only one rig could be used due to the access challenges.
The piled retaining wall, constructed by Ritchies, is formed by 83 450mm diameter piles installed to 10m depth at 1.2m to 1.5m centres, which are connected by a capping beam. The capping beam was cast with sleeves through which Ritchies is installing 36 25m long multistage strand anchors at 35° from the horizontal.
“The anchors have a long free length in order to reach the undisturbed London Clay,” says Doe. “The use of multistage strand anchors allowed the number of anchors needed as they are fixed at several points but it did call for a redesign of the capping beam to cope with the additional load.”
According to Cornish, the piling went well despite some water being encountered during part of the work. “Ritchies was using a Kelly bar technique and the water-bearing section meant a change to a sectional CFA-type piling approach was needed,” he said. “The piles still stood open so no casing was needed. The water was about 5m from track level and we believe it may have been from an old culvert.”
O’Donovan says that the final stage of work involves installing 21 soil nails to secure the section of ground above the bridge wing wall between the existing and the stabilising structure. These will be 11m long, 150mm diameter R32-360 self-drilling nails with Greenax netting.
In addition to ending the risk of slope failure at the site, the work will also improve ease of ongoing maintenance for the whole railway. The previous pedestrian steps are planned to be replaced with a ramp or better steps and the former builders yard is likely to be retained by Network Rail to further aide access.
While the work has meant almost a year of disruption for residents on Edgington Road, the end it=s now in sight and they look set to benefit from less disruption in the future, as do rail passengers.