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London basement at New Fetter Lane: Coded challenge

Construction of a new London basement proves that the change of approach needed to apply Eurocodes to geotechnical design also presents structural design with some challenges.

As the building moves out of the ground, it will literally be within touching distance of the walkway used to access the site facilities.

As the building moves out of the ground, it will literally be within touching distance of the walkway used to access the site facilities.

When leases are already agreed with future tenants before ground has even been broken for a new development, the pressure is on to deliver the work on time and to plan. When you know that the building you’re working on will be occupied by an internationally renowned law firm, the pressure could be perceived to be greater still.

This was the challenge that main contractor Mace and specialist subcontractors Cantillon and MGF faced when it came to the construction of a new office development in central London. The architecturally striking building at 12 to 14 New Fetter Lane has already been let to Bird & Bird but is not due for completion until 2015.

The new development for Great Portland Estates will have 13 above ground storeys when it is completed but the focus is currently on constructing thetwo-level basement. Work above ground is expected to start soon.

However, getting the project from design to construction called for some additional hoops to be jumped through, as the work had to be designed to Eurocode standards.

Originally the temporary support for the project’s 9m deep basement was designed using a fixed steel frame, but programme constraints meant that modular propping had to be considered to keep construction on track.

“Hiring the props also made more sense for the timescale that the project team were looking at for the temporary works,” says MGF major projects manager Andrew Sharp.

MGF was contacted by Cantillon, which is undertaking the demolition work, temporary works and bulk excavation for Mace. The phased approach that the modular propping system MGF could deliver suited the bulk excavation operations and the extraction would also be much faster too.

The shape of the site is slightly unusual - Sharp describes it as a squared off triangle. The challenge presented by the shape is illustrated by the fact that almost every prop used on the scheme is a different size. The site is 60m across and 50m deep at its widest points.

“On the face of it the project’s geometry is complex, but that didn’t faze us,” says Sharp. “The challenge was in the logistics and the design criteria as it was the first scheme we have designed using Eurocode 3 (EC3).”

The shape of the site is slightly unusual – Sharp describes it as a squared off triangle.

The shape of the site is slightly unusual – Sharp describes it as a squared off triangle.

Although the shape of the excavation is unusual, Sharp says that the loadings on the props are fairly typical but it was the EC3 design that complicated the build up to the move onto site.

“This was our first experience of using solely EC3 and we had a challenge to demonstrate the capacity of the circular sections using the code,” he says. “The props are classed as slender - class 4 - under EC3 due to the wall thickness compared to the diameter and it was this classification that made the approach more complex than under the British Standards.

“It was a case of balancing the yield strength to move into the props into class 3 - semi-compact - with the capacity needed to cope with the large spans here that was the challenge. We used a circular section with an X65 grade so the yield stress was 413N/mm2 that meant the props fell into class 3.

“EC3 will become the norm for such designs, but when you’re applying it for the first time, what would normally be acceptable, may need additional calculations to prove the design.”

Category three checking of the prop design was carried out by Swanton Consulting.

Before moving onto site, MGF delivered a training session and toolbox talk to the site team to ensure it was familiar with the propping design.

“We used imagery that we used at the pitch imagery that we used at the pitch stage that helps bring the project to life and give clarity as to what needs to be done,” says Sharp.

Another major challenge was the lack of storage space at the site - even the site offices are balanced above the basement excavation and as the building moves out of the ground, it will literally be within touching distance of the walkway used to access the site facilities.

The site team are not the only ones with a good view of the work - the site is also surrounded by other office and residential developments and this has led to other restrictions. “Work on site can’t start until 8am and has to be stopped at 6pm,” says Sharp. “There is a need for ‘just in time’ deliveries at this site, which made getting the right prop here at exactly the right time to avoid causing congestion a challenge.”

The shape of the site is slightly unusual – Sharp describes it as a squared off triangle.

The shape of the site is slightly unusual – Sharp describes it as a squared off triangle.

The solution delivered by MGF uses a whaler to whaler connection with a lug and pin arrangement on the front flange that allows it to articulate during positioning, which Sharp says is key on complex projects such as this one where some tolerance is needed. “The props fix onto the whaler using eight bolts and can be completed in 10 minutes,” he says.

The 10 props used at New Fetter Lane were delivered and fixed into place over the course of a week.

They are a mix of MGF’s 400 and 600 series hydraulic props, with the shortest measuring just 5.5m while the longest is 27m. All the props have a safe working load of 2,500kN.

MGF also supplied 161.5m of modular whaling beam for the scheme, which is fixed to PU22 sheet piles that were installed by Berry Range Piling into the underlying London Clay.

The excavation has now reached full depth and Sharp is helping to oversee the extraction of the props as the permanent works progress. He expected MGF’s involvement in the scheme to be completed as this issue of GE went to press.

“The basement slab has been cast and the vertical columns are now being cast within the sheet piles with jacks installed between them to support the sheet piles as the props are removed,” says Sharp. With the basement levels nearing completion, the above ground structure of the design by Bogle Flanagan Lawrence Silver will start to take shape but the eye is still on the clock to ensure the handover is completed on time.

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