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Setting a sequence

Expanded had to develop an installation process that would meet the tight criteria but also to allow relatively straightforward placement so that it could keep to the planned rate of three piles a day. This has been achieved by means of a specially designed tube which guides the steel columns into place, the tube being developed and extensively tested at the company's workshops in Grimsby.

The 356mm diameter steel circular hollow section columns used at Bristol are 12m in length. They are fitted with a variety of welded structural bearing plates and gussets to support the three carpark floor slabs, which make them difficult to handle and guide.

At the top of each column (8m AOD which coincides with the piling platform level), and at 3m below this, square bearing plates provide connections for the top and middle floors. A circular plate 3m below this (2mAOD) provides support for the bottom floor slab, and below this rows of shear studs are fixed to the column to help bond the column into the concrete pile.

First stage of installation is to drill in a 6m length of 1,350mm diameter outer casing, which is firmly secured at the ground surface. This is then drilled out using an auger and 1,200mm diameter casing up to 17m long is hydraulically vibrated up to 1m into the bedrock. This casing is then drilled out and the bore progressed to achieve the required rock socket.

With the pile bore complete, a reinforcing cage fabricated on site from 15m lengths of 16T32 rebar is placed in the hole, and concrete is tremied in up to the required depth, just above the base of the outer casing, and the 1,200mm casing vibrated out.

Expanded's procedure then involves plunging the columns through a 1,050mm diameter, 6m long guide tube, suspended within 1,350mm diameter outer casing.

The guide tube has full length internal locating rails down which run the gussets and spacers providing points of contact at four levels on the columns (at the three structural bearing plates and upon a pair of diametrically opposite shear studs). Column verticality is achieved because at least two of four guide mechanisms are within the guide tube at any one time during installation.

To ensure that the guide tube itself is vertical and in the correct position, the tube has four adjusting scissor jacks at its bottom end. These are operated from ground level and used to adjust and lock the position of the guide tube before the columns are plunged.

It is then a race against time to get the plunge columns in. Once in place, the column is left overnight and in the morning, the outer casing and the guide tube removed and loose granular fill placed around the steel column. This is to prevent damage to the exposed steel during excavation of the carpark.

This all seems, and is, very complicated and so Expanded decided to carry out trials in advance of the main contract to ensure that the procedure would work. Using an extra column manufactured specifically for the task, the company bored two typical piles on site giving then the opportunity to refine the retardation properties of the concrete and the column installation technique.

The months of planning and perfecting the technique seem to have paid off. Expanded started on site in February and has keep to its original plan of three piles a day enabling successful completion of the foundation work last month.

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