Piling work for a new luxury hotel will deliver the City of London’s first ballroom but the work itself has also created a few firsts for the contractor too.
Recession recovery in the City of London means that sites that have lain dormant and derelict for years are now hives of activity with the rush to ensure these locations are paying their way. However, partial demolition of previous structures leaving former basements and “temporary” propping in place means that there are many unknowns with some of these sites and not every contractor wants this risk burden.
Pan Pacific plans
One such site on Bishopsgate has given piling contractor Keltbray the opportunity to grow its business by taking on that risk while moving up to be principal contractor and deliver bentonite-supported piles for the first time. Completion of the work this summer will clear the way for construction of the Pan Pacific hotel – a new 43 storey, five star development with a public square to its front that will cover an underground ballroom.
Keltbray was awarded the £8M contract for the below ground enabling works in summer 2015 by Singapore-based hotel investor UOL. The contract for the structural construction has yet to be awarded but it is understood that the decision is between Mace and Bouygues.
The challenges presented by the pre-recession demolition of the previous building is clear when Keltbray Piling managing director Stuart Norman outlines that £1M of the contract value covers additional demolition work.
“Keltbray’s demolition division undertook the original work on the site and the former basement walls were propped and backfilled to one floor below ground level,” explains Norman. “The temporary works and sequencing to remove the propping and manage any obstructions within the fill presented a number of risks for the foundation works.”
Norman was confident of Keltbray’s ability to manage these risk as a result of working with the business’ demolition division, as well as temporary design support from sister company Wentworth House.
The 800m long, 300m wide site is bound on all sides at ground level by roads and existing buildings but below ground it is more complicated with Crossrail tunnels passing under the north western corner of the site, the Metropolitan line just to the north, the Houndsditch service tunnel to the south and a Thames Water sewer to the east.
The design of the new hotel effectively splits the site in two with a secant pile wall surrounding the whole development to retain the new basement structures and allow raking props on the existing basements walls to be removed. The front section of the site will form a public space at ground level but below will be three basement levels, including a double height, column free ballroom, which is believed to be the City’s first. The rear section of the site will be the location of the tower itself and has called for bentonite-supported piles – the first ever undertaken by Keltbray Piling.
“Before we started work on the site, we carried out a trial bore using bentonite with specialist advice from Tony Suckling to train our staff in use of the technique and to demonstrate our capability to UOL and its designer WSP,” says Norman. “Both Tony and WSP’s Matt Sharratt were on site during the trial.”
In addition, Keltbray brought in senior construction manager Neil Grace, who has previously undertaken piling under bentonite support, to assist project manager Joe Martin when work reached that stage.
Ground conditions at the site are fairly typical of the area with made ground over Terrace Gravels, followed by London Clay with a 10m thickness of the Lambeth Group starting at 42m below ground level over the Thanet Sands.
The secant wall is formed primarily from 1,200mm and 900mm diameter piles – 258 in total – to a depth of 40m with the larger diameter piles mostly around the plaza area where the retained height will be greater. The original pile design by WSP included larger diameter piles constructed under bentonite support to also support bearing loads from the tower structure but Keltbray used 3D finite analysis to reduce the size of these piles to the standard sizes elsewhere on site.
”The male and female piles had to be carefully sequenced”
Neil Grace, senior construction manager, Keltbray Piling
“The male and female piles had to be carefully sequenced to work between the raking props and install the king posts with a ‘finger’ connection into the existing basement wall to allow the removal of the props and complete the wall,” says Grace.
This sequencing created age differentials between the female piles so there was also a risk of the male piles being deflected during construction.
The mild winter also presented some issues for the female piles. “Pulverised fuel ash (PFA) is normally favoured for the concrete mix for female piles because of the initial strength it offers,” says Grace. “However, due to the warmer winter and switch to more sustainable energy sources there has been a shortage of PFA this year and we had to use ground-granulated blast-furnace slag (GGBS) instead. The GGBS mix created a higher initial strength than the PFA-based concrete which added to the complexity of the sequencing.”
Most of the secant wall has been completed, but a short section along the eastern side of the site has been delayed until after the brick-built Thames Water sewer is relined.
The plunge columns and bearing piles for the ballroom and plaza area had to be completed early on in the programme as the area at ground level had already been earmarked as the best location for the bentonite farm.
In the plaza area, Keltbray has installed 18 permanent plunge columns and three temporary plunge columns as part of the 96 900mm diameter piles constructed to 25m to 30m depth for this element of the scheme. The shorter piles are above the Crossrail tunnels.
“Nine of the permanent plunge columns on the south side feature a 14.5m long square hollow section steel and will support the ground floor slab and the other nine along the northern side of the site contain UC section steel and will form part of the wall,” says Grace. “The three temporary plunge columns will provide support for the third levels of the basements during the excavation phase.”
According to Grace, the square hollow section plunge columns used here are unusual as the design called for them to be free of bolts and fixings for architectural reasons.
Subhead: Bentonite first
The piling for the plaza was completed in January but the sequence was planned to work from north to south across the site so that the slab for the bentonite farm could be constructed while piling work was still underway.
In total there are 68 bearing piles planned for the tower – nine will be 1,500mm in diameter and the remainder will be 1,200mm in diameter – all constructed to a maximum depth of 58m. There are also 13 plunge columns planned for the tower using 16m long UC section steel.
“Additional plunge columns have been added to the schedule to fit with the designs by the contractors for the above ground work,” says Grace.
“Placing the columns in bentonite supported piles will add to the process, so we have opted for concrete with a four hour workability. This also adds flexibility to the delivery timings as traffic around the site can often be congested.”
Constructing each pile takes two days and currently Keltbray is undertaking desanding work on day two but is looking to move the operation to day one in order to improve efficiency.
“At the moment we are installing the temporary casing and then digging down to around 42m depth in the dry before adding the bentonite support to enable us to dig down to 2m above the base and we then test the bentonite to check the sand content,” says Grace.
“On the second day we are digging the final 2m and desanding the bentonite to the specified tolerance, which is 2% but we’re actually achieving 0.25 to 0.5%.
“The bentonite has a higher viscosity than needed so we are looking to reduce this to make solids removal easier. As the desanding is well under the limits set, we are planning to undertake desanding at the end of day one, dig the final 2m on day two and we still expect to be within the specified tolerance.”
When GE visited site, Keltbray was about to start work on the fifth bentonite supported pile but both Norman and Grace appeared confident that the initial trials and expert support will ensure smooth delivery of the remainder of the scheme. “We expect to complete the piling work in early summer,” said Norman. “However, we are also bidding for the capping beam construction and bulk excavation work so the completion of piling may not be the end of Keltbray’s involvement.”
Using this high profile project as the testing ground for its bentonite piling capabilities has already paid off though as Norman says that Keltbray Piling has just secured another similar project in the City of London and is tendering for another.