Barrett Byrd Associates

Tricky ground conditions on reclaimed land beside the Cornish coast have led to use of both precast concrete and steel foundation piles for a new building in Falmouth, reports Mike Walter.

Unknown whereabouts of redundant foundations buried beside Falmouth Marina has led the ground engineering specialist Roger Bullivant Ltd (RBL) to specify a combination of piling systems on the site of a new mixed use development. The new residential and commerical building at Discovery Quay will stand four storeys high and will overlook the town’s picturesque harbour alongside the National Maritime Museum of Cornwall.

Technicians from RBL had been told that steel tie bars which retained a former dock wall lay buried beneath the ground, but the exact position of each tie bar was not known. Land on which the development is taking shape is reclaimed and the site had previously been infilled.

Precast concrete piles were the preferred choice for providing a foundation for the new building but, as a means of contingency, it was decided that lengths of hollow steel tubular piles should also be brought to site. The precaution paid off, with over 40 pile positions out of 242 on site requiring the use of steel after precast concrete piles could not be completed.

Despite their durability, precast concrete piles can sometimes be diverted off course or refuse to be driven to full design depth if they hit a large obstruction under the ground. Steel tubes can, however, overcome most obstructions with relative ease.

“We knew that old foundations would have proved impenetrable for precast concrete piles so we brought a stock of steel tubes to site should any piles not reach full depth,” said RBL’s area manager Peter Merry. “Both the precast concrete and steel piles are designed to safely accommodate a load of 500kN and were driven using the same rig through soft clays and socketed into a bedrock of shale. It is fair to say that we had a fairly flexible foundation design to work towards.”

Each precast concrete pile measured 250mm² and was driven using the rig’s 5.5t Quiet Hammer until a set number of blows was achieved for a given penetration. Piles were driven to between 10 and 16m depending on the depth of the shale, which became deeper the closer pile positions were to the sea. Sections of precast concrete pile measured between 3 and 6m and were connected together using a Collet locking mechanism as the pile descended into the ground.

Each of the steel replacement piles measured 178mm in diameter and arrived on site on the back of a low loader in sections of between 4 and 6m. Once a steel pile was fully installed, it was filled with concrete and a reinforcing bar was inserted to provide added strength. RBL’s piling contract took two weeks and works came to a close on 28 February.

“We were approached to carry out the piling on this job because the engineer knew that we would be able to switch from using one type of pile to another fairly easily using the one rig,” added Merry.

The new mixed use development taking shape on site will incorporate penthouse appartments on the top floor, a restaurant on the middle floor and retail units on the ground floor. The steel frame structure will be finished with oak cladding to match the appearance of the neighbouring maritime museum.

Falmouth features many new housing and cultural developments and has some of the fastest rising property prices in the south west of England.

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