News

Dec 30, 2023

Basements: Props for Battersea

Deep excavation supports were put through their paces at a Battersea student

Deep excavation supports were put through their paces at a Battersea student housing project in south London.

For several years, Battersea in south London has been a hive of construction activity – and still the projects keep coming.

Among the latest schemes to get underway is Palmerston Court, an ambitious residential and commercial development by private student accommodation specialist Urbanest. It will comprise three student buildings and a dedicated commercial building, ranging in height from 11 to 19 storeys.

It is on the south side of Battersea Park Road, opposite the redeveloped Battersea Power Station complex.

Palmerston Court will house more than 850 students. The buildings are designed to Passivhaus energy efficiency criteria and are expected to achieve an "Outstanding" rating based on the Building Research Establishment Environmental Assessment Method.

Impressive though this is, the project is at an early stage and attention has been focused more on its structural elements.

As with most high rise developments, the foundations and basement structure of Palmerston Court are a significant undertaking.

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 one contractor Mace involves building a deep basement approximately 36m by 46m and with a base slab two levels – about 8m – below ground.

The ground conditions are predictably poor given the development's close proximity to the River Thames. They comprise made ground and clay with perched water. As a result, a robust support solution has been essential during the basement excavation work.

To address this requirement, Getjar employed Groundforce Shorco, the specialist temporary works and structural support division of equipment hire group Vp plc.

Groundforce Shorco provided a heavy duty, high capacity modular ground support system.

The traditional method of bracing deep excavations is to erect a bespoke structural steel frame which is assembled progressively as the excavation proceeds.

The advantage of the system supplied by Groundforce Shorco is that the components are available off the shelf and can be combined in various configurations. The modular design can also make assembly and dismantling easier.

The support system used for the project comprises a suite of hydraulically extendable 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 strut system allows an adjustment of around 500mm so that each prop assembly can be easily installed and then preloaded hydraulically within the excavation.

Typically, the props brace one side of the excavation against the opposite side and bear against an insitu concrete capping beam or one of Groundforce's waling beams, which are also modular 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 813mm diameter extension tube.

Groundforce Shorco major projects manager (south) Tom Hughes 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 been 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."

The excavation at Palmerston Court was supported by 16 Groundforce props. Seven of these were fitted with the new 813mm extension tubes and five with the 1,220mm Super Tubes. Only four used the standard 600mm extension tubes.

The longest span for any of the props within this excavation was in excess of 33m. For this, the 1,220mm diameter Super Tubes were used.

The excavation is lined with a 173.3m long secant pile retaining wall bored to depths between 10.7m and 35.7m, and the Groundforce props were installed at two levels within the deep excavation. The top level props bore against the insitu concrete capping beam, while Groundforce's largest perimeter waling beam system, the Super Mega Brace, was used for the lower level due to the high loadings inside the excavation.

"The lower level of props has 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. Any adjacent structures or infrastructure has the potential to induce loads within the temporary works. It is important that the temporary works solution has the capacity to withstand any loads that these may impose," explains Hughes.

According to Hughes, Groundforce has worked with Getjar on previous projects, but had not worked with it for some time when the original enquiry for Palmerston Court was received in 2020.

The contractor for the reinforced concrete frame and groundworks has been steadily developing its portfolio to include complementary services, including demolition and piling.

Niel Murdoch, Getjar's project manager on the Palmerston Court project, says: "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 also notes that Groundforce's design was a cost effective and practical solution.

Hughes says: "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."

Mace senior construction manager Sophie Drury says the installed props performed well, no red trigger levels were breached and there was no need to implement emergency measures.

"The environmental benefits of reusing the props and walings rather than them being scrapped at the end of the project was extremely satisfying," Drury notes.

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, the London Borough of Wandsworth, and interested third parties.

A further key challenge on the project was to avoid damaging nearby Thames Water assets which were within 5m of the basement excavation. Mace was able to manage the works successfully, by monitoring the ground above the asset along the capping beam and the top and bottom of the secant piled wall.

The excavation works are now complete and the last prop was removed in early August. The ground floor slab for the development was expected to be completed before the end of August. The project is due for completion in 2024.

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