Groundforce Shorco has supplied a deep excavation support solution on a site in south London for main contractor Mace.
Palmerston Court, an ambitious residential and commercial development by private student accommodation specialist Urbanest, is among the latest schemes to get underway in Battersea.
Located on the south side of Battersea Park Road, opposite the redeveloped Battersea Power Station complex, it is designed to accommodate over 850 student residents.
One of the key challenges on the project was the potential damage to the Thames Water Assets within close proximity to the basement excavation, which Mace managed successfully.
As with most high-rise developments, the foundations and basement structure of Palmerston Court is a significant undertaking in itself.
The contractor responsible for this is Getjar, a member of the Masterson Holdings Group known primarily as a specialist in groundworks and reinforced concrete frame construction.
As well as the reinforced concrete package, Getjar’s contract (for tier 1 contractor Mace) involves construction of a deep basement measuring approximately 36m by 46m and descending two levels, or about eight metres, below ground.
Ground conditions are predictably poor, bearing in mind the development’s close proximity to the River Thames, and a robust support solution is essential during construction of the basement excavations.
To address this requirement, Getjar employed Groundforce Shorco, one of only a handful of equipment hire companies in the UK that can offer a heavy duty, high-capacity modular system of ground support.
The advantage of the proprietary system supplied by Groundforce Shorco, is that all the components are available off-the-shelf and can be combined in any configuration to suit the application. Most importantly, the modular design enables very quick and simple assembly and disassembly.
The basement propping arrangement was designed to limit capping beam deflections to less than 10mm. Regular monitoring validated the propping design against the agreed movement criteria imposed by the Highway Authority and interested third parties.
The Groundforce system comprises a suite of hydraulically-extendible props and a range of extension tubes in a variety of lengths to allow each prop to be tailored to the optimum length. The hydraulic system allows an adjustment of around 500mm so that each prop assembly can be easily installed and then pre-loaded hydraulically within the excavation.
Typically, the props brace one side of the excavation against the opposite side and bear against an in-situ concrete capping beam or one of Groundforce’s proprietary waling beams, which are also modular in design and hydraulically adjustable.
For this project, Groundforce supplied its MP 250 and MP 375 hydraulic props with maximum load capacities of 250kN/m and 375kN/m respectively. The standard extension tube is nominally 610mm in diameter, but for this application, Groundforce also supplied its 1,220mm-diameter “Super Tubes” and a new product, an intermediate extension tube measuring 813mm in diameter.
Tom Hughes, major projects manager (south) for Groundforce Shorco, explains why the 813mm tubes were required:
“The 813mm section was specified as the loadings were such that for the 610mm section they would be too high and for utilisation of the 1,220mm sections they would have been low, such that it would have been less commercially viable.”
The 1,220mm Super Tubes were developed to enable props to reach long spans, where the standard 600mm tubes would have proved too flexible. Over very long spans, it is sometimes possible to use an intermediate vertical support to prevent the prop deflecting under gravity. But this complicates the design and can interfere with other operations inside the excavation.
The 1,220mm Super Tubes have the necessary stiffness to accommodate long spans, but their size makes them less convenient to handle and transport.
“The 813mm tubes were developed as an option somewhere in between the 610mm and 1,220mm sections,” says Hughes. “They’re relatively light, they’re easy to handle and quick to install and they offer reduced transport costs compared to the larger sections.
“This is the first time that the 813mm diameter section has been used on a Groundforce project as an extension piece to the hydraulic rams,” he adds.
The current excavation at Palmerston Court is being supported by a total of 16 Groundforce props. Seven of these are fitted with the new 813mm extension tubes and five with the 1,220mm Super Tubes. Only four use the standard 600mm extension tubes.
The longest span for any of the props within this excavation is in excess of 33m. For this, the 1,220mm diameter Super Tubes have been used.
The excavation is lined with a secant pile retaining wall and the Groundforce props are installed at two levels within the deep excavation. The top level props bear against the insitu concrete capping beam, while Groundforce’s largest perimeter waling beam system, the Super Mega Brace, is used for the lower level due to the high loadings inside the excavation.
“The lower level of props have to resist a load in excess of 200kN/m. The site has a main road adjacent to one side, and railway lines adjacent to two of the other three sides,” explains Hughes.
According to Hughes, Groundforce has worked with Getjar on previous projects, but had not worked with them for some time when the original enquiry for Palmerston Court was received in May 2020.
The contractor is already one of the leading reinforced concrete frame and groundworks specialists in London and the south-east, and is steadily developing its portfolio to include complementary services including demolition and piling.
“We have a good reputation in the concrete frame market and we are building on that now,” says Niel Murdoch, Getjar’s project manager on the Palmerston Court project.
“We did all the secant piling work on this project – it’s an important project for us because it’s one of the first contracts undertaken by our new piling division,” he adds.
“We’ve worked very closely with Groundforce on this job and I’m really happy with the results we’ve achieved. The Groundforce design is a very cost-effective and practical solution.”
Mace’s senior construction manager, Sophie Drury, was delighted with the performance of the installed props with no red trigger level breached, providing programme certainty and no need for the implementation of any emergency measures. “The environmental benefits of re-using the props and walings rather than being scrapped at the end of the project was extremely satisfying” said Sophie.
Tom Hughes is equally pleased with the way the teams have worked together: “The enquiry was originally received in May 2020 and while the drawings were suitably detailed to provide a prop arrangement, there was no loading information and the scheme was developed using assumed loads.
“These assumed loads later proved to be quite accurate, but the layout changed significantly over the different iterations. We hadn’t worked with Getjar that recently, so clearly it was a well-engineered, economically viable solution that was provided,” he says.