The 1951 Festival of Britain presented British advances in science, technology and design. Nearly 70 years later, the site’s development has showcased some of the best in British foundation engineering.
Southbank Place is a £1.5bn mixed-use development being built by a joint venture of Canary Wharf Group and Qatari Diar on the Festival of Britain site, occupied since the early 1960s by the Shell Centre – a 27 storey tower and nine storey horseshoe-shaped block serving as the oil company’s London headquarters.
While the Shell Tower remains, the rest of the 20,000m2 site has been flattened to make way for eight new buildings – two office and six residential blocks, the tallest of which will be 37 storeys high. Alongside offices and 900 homes, there will be restaurants and retail units and a new public open space to complement Jubilee Gardens, the park between the Shell Tower and the river.
Construction management is being carried out by Canary Wharf Contractors, with structural works by Expanded Structures (which is building the concrete frames for the six residential blocks) and Severfield (which is erecting the steel frames for the office blocks). Demolition, which began in 2015, was carried out by specialist contractor McGee, with foundations installed by Expanded Geotechnical.
The team’s first major challenge was how to demolish existing buildings and remove the three levels of the 12m deep basement covering almost the entire footprint of the site, without affecting the Bakerloo Line and Northern Line tunnels passing below. The Bakerloo Line runs across the middle of the site, just 5m beneath the original basement base slab, while the Northern Line tunnels cut across the north eastern corner, where Southbank Place’s tallest building is being built.
Careful investigation and detailed analysis revealed there was no danger of the basement’s post-tensioned concrete walls failing, but demolition had to be sequenced carefully, to ensure the ground was not unloaded too quickly.
This had the potential to cause heave of the Bakerloo Line tunnels and the escalator barrels of the western entrance to London Waterloo underground station on York Road, which forms the eastern site boundary. Demolition had to be carefully phased and extensive temporary works installed, to prevent movement while the above-ground structures were built.
“We made sure everyone knew what was going to happen well in advance,” says Expanded operations leader Matt Smith. “The London Underground Infrastructure Protection Team was actively involved in the piling methodology discussions 18 months before we started foundation work. We held regular meetings leading up to the start of, and during, the project, with Canary Wharf Contractors, McGee, London Underground, Network Rail and Arup – before method statements were officially submitted.”
Smith says the use of 3D digital engineering really helped this collaborative approach.
“No-one had piled this close to the London Underground tunnels before, so being able to provide digital visualisations of the demolition, piling and construction works was really helpful and, because data could be tailored for the audience, it made it far easier for the non-foundation specialists in the team to understand, and interrogate, our plans,” he adds.
“By the time we got to site, everyone – from the client down to piling operatives – was confident about what we were planning and how we were going to do it.”
Seven of the eight new buildings are founded on a total of 388 rotary bored bearing piles, with diameters varying from 700mm to 2,100mm. Ground conditions are fairly typical for a central London site: 5m of made ground, up to 6m of river terrace gravels, 25m of London Clay and 18m of the Lambeth Group, with Thanet Sand beneath.
“Where space permitted, the piles were founded in the London Clay, however, this was not always possible because of the below-ground infrastructure, which meant piles had to be positioned wherever they fitted, increasing some of the loads significantly,” Smith says.
Most of the piles were installed in the dry, using temporary casing, although large diameter piles installed near London Underground tunnels, and those founded in the Thanet Sand, were installed under bentonite, many with permanent casing up to 20m long.
“Arup’s original design called for all of the piles installed under bentonite to include base grouting,” Smith adds. “However, the extensively-instrumented pile testing demonstrated this was not needed in all but one of the piles, making installation much simpler and faster.”
The only significant natural geological hazard was a hard limestone layer encountered about 55m down in the north eastern area of the site. “While the site investigation identified this layer, it was estimated to be only 150mm thick at most,” Smith says. “In fact, it was nearly 500mm thick, which required specialist rock tools to drill through it when installing the deepest piles.”
Alongside tricky logistics, and having to core through a large number of under-reamed piles from the original buildings (the exact locations of which were unknown before work began), three areas of site presented significant challenges for Expanded: Piling near the London Underground tunnels; installing foundations for Four Casson Square (which sits above the western ticket hall for London Waterloo underground station) and construction of a secant pile wall in the north eastern corner of the site under One Casson Square, the scheme’s tallest structure.
The torsion box pile
The biggest challenge at Four Casson Square was how to support the new superstructure, without imposing any load on the underground station entrance, its sensitive escalator barrels and the Bakerloo Line tunnels beneath.
“Due to the restricted depth to the Bakerloo Line tunnels, the original piled basement wall had a rectangular structure to add lateral restraint,” Smith explains. “This ‘torsion box’ required one of the largest piles on the contract to be placed through it, without loading any part of the structure.”
Extensive temporary works allowed the torsion box pile to be installed through 2,180mm diameter temporary casing inserted through three basement levels.
“This pile was the only one on the project to incorporate base grouting and was permanently cased to the invert of the Bakerloo Line tunnels, which sit about 5m below the basement’s base slab,” Smith says.
“Installation was very difficult: a small section of the original basement structure was left by McGee to form a piling platform for our large Bauer BG40 piling rig. Temporary steel columns and beams were installed to shore up the basement slabs and steel plates laid on the ground slab to provide a stable platform for piling.”
The secant pile wall
A secant pile wall had to be built to extend the basement in the north eastern corner of the site under One Casson Square, allowing the original basement wall to be demolished.
“The Northern Line tunnels run across this part of the site, with the toes of some of the male wall piles just 1.4m from the tunnel crowns in places,” Smith says.
“We also had to avoid the historical temporary works that were used to support sheet pile walls during the original basement wall construction. Additionally, the Hungerford Viaduct, that carries mainline trains from London Waterloo, is just metres away, with some of the pile positions coinciding with its foundations. Finally, Network Rail has a sensitive signalling control room nearby.”
Some of the piles were cored through the Hungerford Viaduct footings. The wall was propped internally during demolition of the original one, to prevent any movement of the viaduct, which was monitored throughout piling.
“Any piling within 25m of London Underground’s assets must have its prior approval,” Smith says. “The usual exclusion zones of 3m to the sides and 6m above the tunnels did not work for this project. We therefore had to obtain special dispensation to drill the secant wall piles to within 1.4m of the tunnel crown and the large diameter rotary bearing piles to within 1m either side.
“We had to install 18 large diameter piles in these exclusion zones across the site. This demanded a special approach, with temporary casing installed to seal in the top of the London Clay, with very tight tolerances of 25mm in plan and 1 in 200 vertically.
“Permanent casing was installed to the tunnel invert by incrementally screwing the casing down and drilling out the plug; augering was not allowed to progress in front of the casing. Piling then continued under bentonite to the final depth.”
The casing installation methodology was trialled during pile testing, with lateral movement of the ground monitored using shear sensor arrays, which predicted the likely forces that would be transferred to tunnels.
Tunnels were monitored for movement throughout piling. When the permanent casing reached the depth of the tunnels’ centre line, work was paused to ensure movements were within the allowable range. Casing was then progressed over the final few metres.
“Originally, London Underground only gave approval for us to install piles during engineering hours,” Smith says. “However, after the first pile was installed without any problems, we were given the go-ahead to pile while services were running.”
Collaboration was key
Since Expanded finished the foundations, construction has continued at a rapid pace: the first two residential blocks are nearing completion and the structures of the two office buildings, One Southbank Place and Two Southbank Place are also finished.
“One of the biggest challenges on this project was ensuring that everyone – the project team, London Underground and Network Rail – worked together to deliver the scheme with minimal disruption to both construction and the transport network,” explains Expanded geotechnical business leader John Chick.
“We only had one route in and out of site, plus there was very little space to store equipment and materials, and the interfaces between the various teams were very close. Planning and coordination was crucial – we were the filling of the demolition and construction ‘sandwich’ – requiring everyone to take an open-minded, can-do approach.”
Three, 900mm diameter test piles were installed to confirm Arup’s pile design, two founded in the London Clay, to depths of 32m, and the third founded in the Thanet Sand at 54m.
One of the London Clay piles and the longer pile were installed under bentonite. All were instrumented with strain gauges (conventional and fibre optic) to measure horizontal movement and extensometers to measure vertical movements.
“Arup’s original design included base grouting in any piles being formed under bentonite,” explains Smith. “However, results from the tests showed this could be omitted from all but the Torsion Box pile. This resulted in simpler pile cages much faster piling.”
Anchor piles installed to carry out the tests were also used to confirm Expanded’s installation methodology in the London Underground exclusion zones. One had permanent casing installed, which was grouted into place between tunnel crown level to the ground surface.
Working test piles (three 750mm diameter and one 1,500mm diameter) were installed to represent the 388 piles being installed across the site.