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Roof reload: London’s Olympic stadium

Construction of a new roof as part of the transformation work London’s Olympic stadium has called for some major ground engineering work.

Anyone who believes that use of the word mini in context of piling means something small scale should definitely take a look at some work currently underway in east London. Major minipiling work may sound like an oxymoron but that is exactly what Balfour Beatty Ground Engineering (BBGE) is undertaking as part of the scheme to transform the Olympic Stadium.

BBGE is working to install groups of up to 20 minipiles at each V-column location

BBGE is working to install groups of up to 20 minipiles at each V-column location

The current works will more than double the capacity of pile caps below the stadium’s iconic V-columns to support a larger roof structure being built as part of a £153M contract by BBGE’s parent company. When the stadium fully reopens in 2016 every seat will be covered by a new cable net supported roof that will be cantilevered from the compressive truss that encircles the stadium.

“The stadium is being converted to a multi-use facility for athletics, pitch sports and events,” says Balfour Beatty deputy project director Kevin Pluckrose. “A major part of the work is to build a new roof that will cover all of the seats, extension and enlargement of the west stand and improved spectator facilities, as well as other improvements.

“The roof is a large engineering challenge as it is twice the size and weight of the original, which is why extensive strengthening of existing foundations is needed.”

The new roof structure will create additional loads of up to 5,000kN onto the pile caps below the V-columns, according to BBGE contract manager Matthew Truelove. The existing pile caps are designed for loads in the range of 3,000kN to 4,000kN, so the loading for the new roof means capacity in some locations needs to be more than doubled, which is why the minipiling solution being delivered by BBGE is anything but mini.

The majority of existing pile caps are supported by up to five 450mm diameter cast insitu piles that are 12m long, although some of the piles have been formed using CFA techniques. BBGE is working to install groups of up to 20 minipiles at each location and, while there are challenges on site to meet the design, actually designing the piles was also a complex task.

“We are governed by the shear capacity as much as the weight imposed by the new roof,” explains BBGE head of minipiling Ken Gailey. “The settlement and lateral deflection criteria of 5mm were also a challenge.”

Design of the foundation solution was undertaken in conjunction with Buro Happold’s Alice Spence, who recently won a high commendation in the GE Awards Young Geotechnical Engineer of the Year category. According to Gailey, eight of the piles from of each group are designed to cope with the additional compressive forces, while the rest are designed to support the extra lateral and shear loads. “The new roof creates around 2,500kN of additional
compression on each pile cap,” he says.

Pile cap design

Pile cap design

The foundation has been analysed using various software packages, including Plaxis to prove the pile cap design meets the settlement criteria and Oasys and Wallop to investigate the loading on the piles themselves. The resulting design uses 323mm diameter bottom driven minipiles to 12m below ground with heavy reinforcing and circular hole sections (CHS) to cope with the high bending moments. “The cage extends to the full depth of the pile with the CHS in the top 1.8m and extending a further 1.2m into the pile cap,” explains Truelove.

The extent of reinforcement in the piles means that BBGE will use a grout mix to form the piles rather than concrete at most of the locations. The piles will extend through up to 10m of made ground to found

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