Bromley basement construction: Amazing space
The developer’s strapline claims ‘Everything is going to be amazing’ on a new regeneration scheme in Bromley. Claire Smith met the ground engineering team charged with delivering on this promise.
Dealing with 20 neighbours, including a busy rail line, and a public road crossing the construction site does not make for a straightforward project. Add a basement up to 13.5m deep and development of every centimetre of the site, and the challenge gets more complex.
This is exactly what Dawson Wam is facing on construction of the foundations for the mixed-use Bromley South Central regeneration scheme for main contractor McLaren.
“The project was originally planned using a mix of contiguous piled walls and secant piled walls but the design has evolved,” explains Dawson Wam contracts director Colin Russell. “We first started working with main contractor McLaren on their bid for the scheme, and we brought Byrne Looby Partnership on board to help us with our bid for the foundations contract from McLaren.”
The ground conditions at the site, as well as the site constraints and the design itself, are factors that affected the changes to the construction approach.
“Part of the basement requires grade three waterproofing, which is easier to deliver in these ground conditions using a secant pile wall”
Colin Russell, Dawson Wam
“The ground is very dense and goes into the Thanet Sands,” says Russell. “There are lots of properties around the site, plus we also have a road running through the site and it is next to a busy rail line.”
Dawson Wam contract manager Paul Kelly adds: “Our work is very much governed by the public road which runs through the site. There are three phases to move the road for the work and it will eventually pass through a tunnel in the building.”
As well as being tightly constrained, the development uses the full footprint of the site and means the design maximises the space with a deep basement.
“The basement level is up to 13.5m below ground level but typically extends to 11.5m,” says Byrne Looby Parternship (BLP) project engineer Keri Fitzpatrick.
The depth of the basement so close to the rail lines and the number of neighbours meant that BLP had to work closely with Network Rail and undertake finite element analysis to ensure any ground movement created by the work is within tolerances.
The final solution to minimise ground movements but provide the development with the basement and bearing capacity needed was to use a secant solution because of the dense ground and high water table, which is mostly formed by perched water on clay lenses. “Part of the basement requires grade three waterproofing, which is easier to deliver in these ground conditions using a secant pile wall,” says Russell.
The secant wall is hard/hard and is being formed using 35N concrete. “Sequencing is key with a four day maximum turnaround between the male and female piles,” says Kelly.
“The wall is 360m long and formed from 500 piles with the males measuring 920mm in diameter and the females 720mm. Both types extend to up to 18.5m and are being formed using a fully cased CFA design – this was essential for constructing the hard/hard wall and achieving the 1:200 verticality.”
In addition to the secant wall, Dawson is also constructing 350 load bearing piles within the basement area. These 600mm diameter piles are being constructed to 21m without casing and are being formed using 40N concrete to deliver capacities of up to 300t for the main development.
“The secant wall will also carry loads of up to 200t/m and the hard/hard design helps achieve this,” says Russell.
Dawson has been using three piling rigs – a 120t Bauer RTG255, an ABI 2M22 and a Soilmec SF120 – as well as two Caterpillar excavators fitted with boom-mounted sheet piling hammers to deliver the work. Given the size of site and the number of follow-on trades also on site, scheduling the work was not a simple task.
“The sheet piling work has been carried out to facilitate the capping beam construction next to the road and rail line because the final construction level is several metres below the current ground level,” explains Fitzpatrick. “The site slopes so the capping beam is stepped to accommodate this.”
BLP’s work has also included design work for the temporary propping to support the retaining structure as excavation progresses. “There are two levels of propping which has been bespoke designed for the site,” says Fitzpatrick. “We worked with McLaren on the design of the upper level as the party wall issues, especially on the eastern side of the site, mean that the propping is challenging and involved negotiation with tenants.
“The second level of propping includes a 30 degree raker in the car park because of the depth of excavation there.”
To underline the challenges of the design, Russell explains that there are 30 different sections for the design of the basement slab because there are so many variations in final level due to lift pits and different slab designs.
The temporary works has been incorporated into the secant pile wall with a king post cast into every second male pile to allow attachment of the whaler.
“Network Rail set the deflection limits at less than 10mm, so the temporary works is essential to meet these limits,” she adds. “We used Plaxis to carry out finite element analysis on the Network Rail and eastern boundaries. The analysis showed the expected deflection is less than 5mm.”
Inclinometers have been incorporated into the pile cages to allow movement to be monitored during the excavation phase and ensure the tolerances are not exceeded.
Russell adds: “The piling work is within 9m of the rail line and we have been monitoring train movements in a bid to stop work when a train passes, but the site is next to Bromley South station so there is a high level of train traffic.”
“Logistics has been the main issue at the site, but so far everything has gone smoothly,” says Kelly. “There are lots of subcontractors on site and it has been a challenge to coordinate our work around them.”
The other challenge has been the number of different piling platform levels and final formation levels. Nonetheless, Dawson expects its work to be completed by Christmas.
As GE went to press, the site team had completed the piling next to the rail lines and work on the propping phase was underway and Russell was confident that the rest of the work would stay on track.