Groundforce Shorco has supplied a modular hydraulic support system to shore up the basement excavation on a prestigious city centre development in Manchester.
Concrete frame and groundworks specialist MPB Structures called on Groundforce to provide support for the 85m long, 40m 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 2 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 were 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,” explains 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 4m in the middle of the excavation to allow the piling of the bearing piles to commence and had battered the sides for support,” he adds. This meant that the props had to be installed at height.
The props (which were up to 38m in length) were assembled on the piling mat and then them lifted into position by a mobile crane. Groundforce provided 4 days of site assistance from their dedicated project co-ordination team 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 bespoke solution. “We designed bespoke shear plates with post-drilled resin anchors which took the load into the capping beam,” explains Andy.
Despite the complex geometry of the capping beam, which incorporated multiple steps, penetrations and cast-in items, Groundforce very quickly provided the custom-designed connections for each of the 16 prop ends.
Similarly, some of the props were raked in both the horizontal and vertical planes. This cannot normally be accommodated as the connecting swivels act only in one direction. Groundforce therefore 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 the larger (250-tonne) MP250s with 1.2m diameter “Supertube” extensions to achieve the necessary capacity for the long spans.
“The need to hire a 130-tonne crane was costly, however this was offset by the time it saved installing the props. We were able to offer the quickest and most efficient solution,” says Andy.
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 are completed.