Concrete frame and groundworks specialist MPB Structures called on Groundforce to provide support for the 85-metre long, 40-metre wide basement excavation for the £96m second phase of the Circle Square development, which main contractor Sisk is building for developer Bruntwood.
The original plan had been to support the excavation with structural steel props in the traditional manner. The change to a two-stage propping solution to advance the construction of the cores meant the flying shores were in for a shorter duration of time.
The basement perimeter retaining wall comprising concrete secant piles and capping beam had already been installed. Groundforce, part of the Vp group, was required to install one level of props, braced against the capping beam to support the top of the piles as MPB excavated.
“This isn’t normally how we do it,” explained Groundforce major projects manager Andy Simms. “Usually the props would be assembled on the ground and secured against the capping beam before the contractor had excavated more than a couple of feet down.
“MPB had already excavated about four metres in the middle of the excavation to allow the piling of the bearing piles to commence and had battered the sides for support,” he added. This meant that the props had to be installed at height.
The props, which were up to 38 metres long, were assembled on the piling mat and then lifted into position by mobile crane. The cost of hiring a 130-tonne telescopic crane was offset by the time saved installing the props, Andy Simms said.
Groundforce provided four days of site assistance to support MPB during the installation. Due to the change in the scheme, no provision had been made in the capping beam for knee braces to connect the capping beam and so Groundforce’s technical team came up with a customised solution. “We designed bespoke shear plates with post-drilled resin anchors which took the load into the capping beam,” Andy Simms said.
Custom-designed connections were required for each of the 16 prop ends. As some of the props were raked in both the horizontal and vertical planes, Groundforce had to modify the swivels by welding wedges to their back plates to achieve the necessary angles.
In total, Groundforce supplied eight props, both 150-tonne capacity MP150s and 250-tonne MP250s with 1.2-metre diameter ‘Supertube’ extensions to achieve the necessary capacity for the long spans.
All of the props were installed in just four days at the end of June. They will remain in place until the basement slab has been cast and the permanent works completed.