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Piling & Foundations: Rebuild, reload

Transforming London’s Olympic stadium has called for some major ground engineering work. Claire Smith reports.

Balfour Beatty Ground Engineering (BBGE) is undertaking a major minipiling operation as part of the scheme to transform the Olympic Stadium in east London.

The 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,
Balfour Beatty.

“The stadium is being converted to a multi-use facility for athletics, pitch sports and events,” says Balfour Beatty deputy project director Kevin Pluckrose.

The work includes building a new roof that will cover all of the seats; extending and enlarging the west stand; and improving spectator facilities.

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 encircling the stadium.

“The roof is twice the size and weight of the original, which is why extensive strengthening of existing foundations is needed,” explains Pluckrose.

The new roof structure will place additional loads of up to 5,000kN on the pile caps below the V-columns, which are designed for loads in the range of 3,000kN to 4,000kN.

Most 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. “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.”

Foundation design was undertaken in conjunction with Buro Happold.

“The new roof creates around 2,500kN of additional compression on each pile cap”

Ken Gailey, Balfour Beatty Ground Engineering

Eight of the piles from 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,” says Gailey.

The design uses 323mm diameter bottom driven minipiles to 12m below ground with heavy reinforcing and circular hollow sections (CHS) to cope with the high bending moments.

The cage extends to the full depth of the piles, with the CHS in the top 1.8m and extending a further 1.2m into the pile cap.

The extent of reinforcement in the piles means that BBGE is using a grout mix to form the piles at most of the locations, rather than concrete.

The piles are driven through up to 10m of made ground to found into the terrace gravels or the top of the Lambeth Group.

Access to the pile cap locations is a challenge. “From the north west to south east side of the stadium there is a podium level outside the stadium with a suspended floor slab, and the V-columns in this area are supported on concrete columns, so the pile cap work has to be carried out from the undercroft below the floor slab,” explains Balfour Beatty package manager Chris Hearne.

Minipiling was selected because the rigs had more mast height flexibility.

“Rotary bored piles were considered but the access issues meant this was not possible,” says Hearne.

Once all the piles are completed Balfour Beatty will install 240, 25mm diameter, 500mm long dowel bars into the existing pile caps to tie the new cast insitu pile caps into the existing structure. “Installing the dowels and pile cap will definitely be a challenging task,” says Hearne.

Minipiling started in March and Gailey expects most of the work to be completed by mid-July, although there is likely to be a further phase of piling for other new facilities being developed at the site.

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