Props away at Mount Pleasant

Main contractor Bouygues UK is constructing a new 214-apartment complex – called Postmark – for Taylor Wimpey at the former sorting office site in central London.

Specialist contractor Keltbray dug out the basement hole as part of the £100m first phase of the development in central London.

Keltbray has the earthworks subcontract, which has involved the piling and excavation of a basement measuring more than 100 metres long by 50 metres wide and at a depth of up to 12 metres. During construction, the excavation was supported by 15 of Groundforce’s MP250 props, braced against a capping beam cast on top of a retaining wall comprising 750mm diameter secant piles installed by Keltbray.

Ground conditions on the brownfield site are variable, but the biggest challenge was the presence of Victorian brick sewers, including one running beneath the road, a few metres behind the retaining wall on Gough Street. This complicated the task of propping the excavation since any ground movement risked damaging the old sewer or even causing its collapse.

It is usual practice when using proprietary hydraulic supports to pre-load the props during installation to counteract the lateral pressure acting on the piled retaining wall. This also helps to accommodate the thermal shrinkage that occurs during low winter temperatures.

“But on this project, too much pre-loading of the props would have risked crushing the old sewer,” explained Andy Simms of Groundforce’s major projects team.

“Settlement of the capping beam and surrounding kerbs was being heavily monitored and Bouygues UK wanted a hydraulic propping system as this would allow additional preload to be added to the struts if excessive movement was observed.”

Load monitoring was also specified on every prop to keep track of any fluctuations in load. The Groundforce system uses load sensors in the connecting pins at the end of the props where they meet the capping beam. Readings from these load sensors allow monitoring in real time and can be calibrated to raise the alarm if loadings fall outside pre-set parameters.

Due to its size, the basement was excavated in phases, with Groundforce props installed as work progressed from one end of the site to the other. The first props were installed in October 2018 and all were in place by the New Year.

Groundforce provided Keltbray with a 3D model of the support solution generated on REVIT software that was then incorporated into the project’s federated model to ensure that the props didn’t clash with any of the permanent slabs or walls.  “We’ve offered 3D modelling for several years now. Having invested heavily in new REVIT families, we now work exclusively with 3D models for Major Projects to ensure our schemes are co-ordinated fully with the permanent works,” Andy Simms said.

MP250 props, each with a working load of 250 tonnes, were used to support the excavation. The two longest props spanned 52 metres and 55 metres. They required custom-made mid-span restraints to limit deflection. These, and eight other long-span props, were equipped with 1.2 metre diameter supertube extensions to increase stiffness and load capacity.

In addition to the five lateral props, 10 corner props were required to brace the excavation. These ranged in length from just five metres to 43 metres.

Excavation of the basement took four months and Groundforce was on site for almost seven months. The last props were craned out in late May after the completion of the basement and lower ground floor slabs, enabling Bouygues to start looking up.

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