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JUMBO PLUNGE

The need for fast paced piling on the Park Plaza Westminster Bridge hotel has led Bachy Soletanche to take the plunge.

Piling has just started on the new Park Plaza Westminster Bridge hotel in London.

The developer is a joint venture of Frogmore Property, Galliard Homes and Park Plaza, with investors able to buy into the scheme by purchasing one of the hotel rooms. This funding mechanism demands a fast return, so the project has a fast-track programme. The hotel must be open for business by 2010.

One of the first contractors on site was Bachy Soletanche, which has just completed a six month, £5M foundations package. Its project engineer Jonathan Spyvee explains what was involved: "To enable this project to run on a tight programme, the project manager, GCPM, has adopted the top-down method which requires the early installation of the ground floor slab."

With the slab in place, work can continue above and below, knocking two months off the schedule. But before the ground floor slab could be constructed, Bachy Soletanche had to complete a range of ground works and piling. This included installing 140 bearing piles and constructing a 260m long secant piled wall around the site perimeter.

Most of the bearing piles incorporate steel plunge columns, which will eventually support the ground floor slab. They extend from the cut-off level of the concrete piles at lower basement level to ground slab level.

At the start of the installation process there is no difference between the technique used for installing the piles with plunge columns and those without: the core of the pile is bored out, reinforcement is placed inside and then the concrete is poured into the pile.

Immediately after the concrete has been poured, however, an adjustable steel frame is lowered into the pile shaft, supported from above by a temporary steel casing. The steel column is then fed through the frame and held firmly while position and verticality are checked.

Finally, the column is embedded into the top of the concrete, leaving up to 2m of the column exposed above ground level. The columns range in size from 200mm x 200mm up to 400mm x 400mm, with varying lengths of up to 20m.

Spyvee continues: "It is extremely important to install the steel column accurately down the centre of the pile. Tolerances are in the region of 10mm to ensure compatibility with the follow-on works."

In addition to the standard plunge columns, Bachy Soletanche’s contract included installing four "jumbo" plunge columns, each of which required specially prepared plunging frames, meticulous planning and two days to construct. The first day was spent augering the 2.1m diameter pile to a depth of 35m, and the second day was used to place the massive 21m long columns into the pile.

The four jumbo piles are non-standard sizes - two are 300mm x 1,100mm and the other two are 500mm x 800mm. They had to be specially commissioned and, at 37t, are among the heaviest plunge columns Bachy Soletanche has ever installed.

The secant piled wall forms the retaining wall for the excavation of a deep basement for the hotel.

On the east side of the site, Bachy Soletanche used the classic Kelly/segmental casing method, with a large diameter heavy duty rotary rig. On the opposite side it went for cased CFA secant piling for the 750mm diameter piles.

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