Groundforce Shorco’s MP250 hydraulic props have enabled specialist contractor Keltbray to excavate a huge basement as part of the £100 million first phase of a major residential development in central London.

Main contractor Bouygues UK is constructing a new 214-apartment complex – called Postmark – for client Taylor Wimpey on the site of the former Royal Mail sorting office at Mount Pleasant, Farringdon.

Piling and excavation of basement

Keltbray has the earthworks subcontract which involves the piling and excavation of a basement measuring more than 100m in length by 50m wide and at a depth of up to 12m.

The excavation is 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 is 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 in order 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,” says Andy Simms, of Groundforce’s major projects team.

Hydraulic propping system

“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,” says Simms.

Groundforce’s powerful MP250 props – each with a working load of 250 tonnes – were used to support the excavation. The two longest props spanned 52m and 55m and required custom-made mid-span restraints to limit deflection.

Increase stiffness and load capacity

These, and eight other long-span props were equipped with Groundforce’s 1.2m diameter supertube extensions to increase stiffness and load capacity.

In addition to the five props spanning from side to side, 10 corner props were required to brace the complex excavation. These ranged in length from just 5m to 43m.

Excavation of the basement took four months and Groundforce was on site for almost seven months, the last props being craned out in late May after the completion of the basement and lower ground floor slabs.

Bouygues will now progress with construction of this phase which comprises 151 apartments, private lounge and rooftop terrace.