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CITY SLICKER

PILING & FOUNDATIONS

Preparation and teamwork are helping the smooth running of the piling operation on a big office development in the City of London.

Another million square feet (800,000m 2) of office space is taking shape in the City of London, in a Land Securities development consisting of five buildings ranging in height from 18 storeys to two.

The New Street site used to house a printing works which was badly bombed during the Second World War and redeveloped as offices in the late 1960s/early 1970s.

Although these buildings are now being demolished, structural engineer Pell Frischmann plans to reuse some of their piled foundations.

The £2.3M piling contract sees subcontractor Stent working as part of a wider team on the design and build of the office complex, which includes a communal basement and outdoor areas for the five buildings.

Ground conditions consist of between 2.9m and 4.7m of made ground overlying up to 2m of sands and gravels and 31-34m of London Clay underlain by Lambeth Group deposits.

Following Stent's geotechnical analysis to assess the soil and ground conditions, preliminary test piles were constructed in two separate visits before the main works began.

These required the installation of strain gauges, which were adopted to accurately determine the capacity of each test pile below the intended cutoff level for the main works piles.

Adoption of this method removed the need for installing a costly isolating liner.

Stent project manager Stephen Chambers says: 'The test piles had to be constructed close to existing buildings and prior to the demolition works, so the timing was critical to allow the piles to be safely constructed away from the demolition areas.' Stent also took the opportunity to construct some of the new building piles at this early stage. 'Because these piles are positioned outside the basement footprint, they would have proved more dif'cult to construct during the main works, ' Chambers explains.

After the preliminary tests, the piles were constructed to a global factor of safety of two, with working tests being carried out as the works progress.

Using a combination of rigs, Stent is now installing 720, 600-2100mm diameter rotary bored piles to depths of 40m.

Concrete deliveries are averaging 120m 3 a day, peaking at 180m 3; and it is expected the total volume will reach 9,000m 3 when the contract 'nishes in December.

Stent is working for main contractor Sir Robert McAlpine. 'The enabling works contractors are releasing the piling areas gradually as they complete. We are working to a fairly tight programme so to adhere to the schedule we need to make sure all the trades involved are communicating effectively, ' Chambers says.

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