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Plunge column and basement construction

The plunge columns were fabricated from 45mm thick steel plate to form a box section 550mm square. They were made in 6m lengths and then welded together to from 18m long columns.

The toe of the column was embedded 4.5 m into the pile, as the column was lowered down the pile casing. Shear keys on the face of the embedded column length helped to develop a full bond with the concrete and ensures an even load transfer into the body of the pile, while helical reinforcement around pile head provides bursting resistance.

Once the concrete in the pile had reached a strength of 25N/mm2, the empty bore of pile casing was filled with weak cementitous concrete, while the void in the column box section was either fully reinforced and filled with 40N concrete or left empty where necessary

Next the 325mm thick first basement slab was cast and starter bars positioned to begin the columns and upper floor construction. Once the basement slab was self supporting, excavation commenced down to basement level three, missing out level two, allowing headroom for excavating machinery.

The level three slab was cast 325mm thick, so that it could support the level 2 slab without back propping. The concrete surround to the steel box columns was then cast from basement level three to two, with excavations continuing down to level five once the upper slab had cured, usually within 7 days. Basement construction continued in this manner until the lowest slab was cast.

For the two lift shaft cores the plunge columns were kept slender to fit within the solid concrete walls, and had to be temporarily braced as excavation was taken down to basement level. The secant walls on the perimeter did not require special bracing nor temporary propping, as it this was provided by plate action of the floors supported on the plunge columns as the excavation was opened up.

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