Builders of the U.K.’s tallest skyscraper, London’s Shard, will save valuable time by excavating its three-floor basement while slipforming the core. A novel machine is now at work, plunging columns into pile tops some 15 meters below ground to prop up the rising core as soil beneath is removed.
With the core on the critical path, “the month or two” top-down exercise is “giving us breathing space,” says Bob Gordon, chief engineer of the 310-m-tall building’s main contractor Mace Ltd., London. Piling began in March, but he expects the core to go up in November, well before the basement is done.
Key to Mace’s plan is a rig ordered by piling subcontractor Stent Foundations Ltd., Basingstoke. Stent gave equipment developer Alex Cartwright just 12 weeks to turn his paper designs into a $400,000 working system, he says.
They were nail-biting weeks, but the rig works well, says Cartwright. “We can easily meet plan and level tolerances and we routinely exceed verticality requirements by three to five times,” he says.
Old belled piles remain in the ground from the 27-floor building torn down for the Shard, says Rodolfo Giannini, associate director of structural designer WSP Cantor Seinuk, London. “Our piles go between the underreams, but sometimes they clash.”
Stent initially focused on the site’s piled perimeter wall. More recently, it mobilized Cartwright’s rig to install underground columns with sections up to 60 centimeters square.
To form each underground column, the rig is lowered into a roughly 15-m-deep casing reaching to the top of a freshly poured concrete pile of, typically, 1.8 m in diameter. The rig, occupying the casing’s full depth, then steers a steel column into the top 5.5 m of the pile and holds it until the concrete sets before moving to the next position.
“It’s all about speed,” says Cartwright. Once the pile concrete is poured, “it’s vital you don’t waste time aligning the column,” he adds. Placing a column typically takes about 45 minutes, he estimates. Because the columns will end up within the final core walls 15 m down, precision is vital.
The rig hangs from a ground-level frame, which can pivot in two directions like a ship’s compass. At the rig’s bottom, just above the pile, four hydraulic arms open to push against the casing. Guided by self-plumbing lasers, the operator adjusts the arms to ensure verticality.
Once the rig is set, two pairs of rollers at its top and bottom accurately center and hold the descending column. When the rig is withdrawn, the remaining freestanding column is stabilized with cement grout pumped into the void between it and the casing.
“The challenge is the column weight, which is very high, compounded by the requirement for verticality of 1:400, whereas 1:200 has been the norm for a decade or more,” says Cartwright. At ground level, the column...