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Cutting edge solution

Geotechnics - Tubular steel and concrete piles were needed for a Falmouth brownfield development, reports Mike Walter.

When piling specialist Roger Bullivant started installing foundations for a new development on the quayside at Falmouth, it knew there were old steel anchor bars in the ground.

But it did not know where.

Driven precast concrete piles were to be used for the bulk of the job, but Bullivant knew that if it hit obstructions it would need an alternative piling method.

Bullivant therefore decided to bring lengths of hollow steel tubular pile to site for use where inserting the precast piles would have been hampered by the anchor bars.

The precaution paid off, as more than 40 of 242 piles required the use of steel after the installation of precast concrete piles could not be completed.

Bullivant is just completing foundations for the four storey housing and offi e project on Falmouth's Discovery Quay, next to the Maritime Museum and the town's picturesque marina.

The site had been reclaimed, with an old quay wall buried behind new fill. Steel tie bars, anchoring back the old wall, lay within the ground, but the exact position of each tie bar was unknown.

Precast concrete piles may be diverted off course or refuse if they hit a large obstruction .

But the more expensive tubular steel piles can overcome most obstructions with relative ease.

'The precast concrete and steel piles are designed to accommodate a load of 500kN and have been driven using the same rig through soft clays and socketed into the shale bedrock which falls away in line with the original shoreline profile. It is fair to say that we had a fairly flexible foundation design to work towards, ' says Bullivant area manager Peter Merry.

Each precast concrete pile measures 250mm 2 and has been driven using a 5.5t Quiet Hammer to a depth of between 10m and 16m.

The concrete piles came in 3m and 6m lengths, and were connected using a Collet locking mechanism as the pile descended into the ground.

Steel replacement piles measured 178mm in diameter.

When fully installed, the steel piles were filled with concrete and a reinforcing bar was inserted for added strength.

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