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Taking it to another level: North London basement scheme pushes propping technology

London’s “Billionaires Row” will soon be home to what may well be the UK’s largest residential basement when work is completed on the 110,000m3 excavation.

Developer Harrison Varma promise “effortless living” for future residents of a new luxury north London development. However, the work being put into building the basement by main contractor CJ O’Shea involves every effort to ensure the developer can deliver on its promise.

When complete, Buxmead on The Bishop’s Avenue in Hampstead will boast 20 apartments above ground,
but below ground there will be swimming pools, a cinema, car parking and a spa. To squeeze all that luxury into one plot the basement will cover an area of more than 4,000m2 and extend to three storeys below ground.

If the scale of the basement wasn’t enough of a challenge, the shape is not uniform and has been guided by tree preservation orders (TPOs), plus facades from the original building also have to be retained. The former Balint and Osmond House nursing home that occupied the western side of the site has been demolished but the facade of the Leo Baeck House and coach house, built in 1900, are being kept and will be incorporated into the new development.

At the three-storey deep end of the basement, the loadings were expected to be in the region of 225kN/m

At the three-storey deep end of the basement, the loadings were expected to be in the region of 225kN/m

Although O’Shea moved onto site in late 2012, director of construction Barry Mallon says there is still two months of excavation to be completed before the whole basement area reaches formation level.

“We won the project by competitive tender in 2011 but didn’t move onto site straightaway as the basement element of the project called for some careful planning,” says Mallon. “We worked with our piling contactor Foundation Piling (FPL) and propping supplier Groundforce to develop a solution that allowed the work to be phased.

“Although the site covers around a hectare, only around half of that space is available to use for construction due to the TPOs and most of the available space is taken up by the basement excavation.”

The outer perimeter of the basement is formed by a secant pile wall constructed by FPL using 750mm diameter piles that are generally 30m long, but extend to up to 45m in some areas.

According to Mallon, the ground conditions have been favourable up until the heavy rain this winter when the groundwater caused some issues. The basement excavation lies fully within the Claygate Formation.
Given the unusual shape of the basement as well as the depth, the propping solution was always going
to be key.

“We haven’t worked with Groundforce before but we needed a partner with a track record of bespoke solutions,” says Mallon. The propping solution is indeed bespoke with almost every size of prop and brace offered by
Groundforce - and a few new ones - being used on the project.

“We wanted the least amount of props possible to keep the excavation area open and FPL developed a one
level propping solution for the scheme,” says Mallon. “In reality we have ended up with two levels of
props to minimise ground movements.”

Groundforce senior engineer (major projects) Ajay Nagah says: “There is always a balance between
larger diameter piles and prop numbers. A one prop level solution would be ideal but we also needed to
achieve some large spans here for construction purposes.”

At the three-storey deep end of the basement, the loadings were expected to be in the region of 225kN/m so the excavation is being supported by props with capacities of up to 250t.

Groundforce also developed a larger whaling beam for the project using a twin 533mm by 201mm by 101mm
UC section, which has three times the capacity of its Megabrace product. The whaling beam is mounted 3m
below capping beam level.

“There is lots of bespoke fabrication on this scheme that has been developed through discussions with Barry’s team and the solutions are already being used on other projects,” says Nagah.

One example is the prop hanger that has been used to install additional support at capping beam level as the permanent reinforced concrete is installed to replace the temporary propping.

“Normally props at capping beam level are bolted to a shear key that is welded across two piles, but these are
usually installed before the excavation starts,” says Nagah. “We worked with O’Shea to develop a support that sits
over a single pile. It was easier to install and will avoid working at height when it comes to removal too.”

The need for the additional support came about as a result of Groundforce and O’Shea monitoring the props. “We were aiming to keep the movement to 50mm over the 110m length of the capping beam but we have seen movements of up to 150mm when the weather was hot last summer,” says Mallon. “Groundforce’s monitoring system gave us confidence about whether movements were within expected limits or weather-induced.

“When it came to removing the props as the permanent works progresses, we could see the reaction on the remaining props to understand whether we needed additional propping in place until the next level of permanent works was completed.”

Given that Groundforce’s equipment will have been on site for around 15 months in total, many might think the job could have been more economically delivered using a fixed, welded frame. “The modular props meant that we have been able to install and remove props easily as we have progressed,” says Mallon.

“A fixed frame would have limited access, taken longer to install and prevented a phased approach.” Nagah adds: “The phased nature of the project means that we have only ever had around 50% of the equipment needed on site at any one time. Although there are only a few months of excavation work left now, we still have some further equipment to bring in.”

While Nagah’s reaction to the scale of the scheme at the planning stage may have been one of surprise, it is clear that the work has been a positive learning experience for both O’Shea’s and Groundforce’s design teams.

“Jobs like this are great for learning,” concludes Groundforce major project manager Mark Whitmore. “Most propping schemes are overdesigned so you don’t actually get anywhere near the design loads but this project has pushed our equipment to its limits. The collaboration with O’Shea combined with the monitoring work here has been invaluable for learning.”

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