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Taking the plunge Plunging columns to tight tolerances for an underground car park currently being built in central Bristol has led to an intricate construction process.

Grimsby-based piling contractor Expanded Piling has just completed foundation work for an underground car park at a site close to Bristol city centre. When complete, the two floor deep structure in the Canons Marsh district will be topped by a new landscaped square which will give little evidence of the complex foundation work undertaken below it.

Walls for the 6m deep box have been formed by permanent steel sheet piles with fully welded clutches to ensure water tightness. Main contractor Tarmac Civil Engineering will excavate the car park in a classic top down sequence with propping from floor slabs at three levels. Vertical support to the floor slabs is provided by tubular steel columns each supported by a single large diameter pile.

Consultant Ove Arup & Partners has designed the 138, 1.2m diameter piles to act in tension, as well as compression - with loads varying from 490t in compression to 180t in tension on the same pile. The tension capacity is required to resist uplift from groundwater which, although lowered by dewatering during construction, will recover once the dewatering system is turned off.

Arup's pile design is based on up to 6m penetration into Triassic sandstone of the Mercia mudstone group, which is present at between 11m and 13.5m below ground level, and overlain by fill, alluvium and gravels.

The contract required trial piles to be installed and load tested in tension to confirm the rock adhesion.

For this purpose, Expanded installed three, 600mm diameter piles, with only the rock socket being concreted. The temporary casing was withdrawn to just above rockhead to stop the overlying alluvium collapsing onto both the reinforcement and the displacement monitoring equip- ment.

During testing, a tension load of 500t was applied by pulling on six, 36mm diameter Dywidag thread- bars, with the trial being carried out in a similar way to testing developed last year by Expanded for a contract at Finham near Coventry (GE November 1997).

The trial pile test results showed that rock strength varied across site - as had been anticipated. Having established the basis for design Expanded proposed a sequence of construction for the working piles.

Each pile had to be concreted to a level 6.5m below ground level into which the tubular steel supporting columns would be plunged while the concrete was still wet.

Very tight tolerances were specified for column placement. A verticality of 1 in 200 was required, while the top of each column had to be within +5mm and -10mm of level, sitting within 10mm of the surveyed position and 1 of plan.

Even the piles into which the columns were plunged had to be drilled to far tighter plan and verticality tolerances than usual, to avoid the risk of the lower end of the plunge column clashing with the pile reinforcing cage.

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