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A case for change

Foundations - Piling

A major foundations rethink was needed when ground conditions at a water treatment site were not as expected. Andrew Mylius reports from Scaffati, near Naples.

Costs of piling for a Lira 98bn ($45M) water treatment works at Scaffati, near Naples, are set to double after original designs were found to be unsuited to ground conditions.

Specialist piling subcontractor Trevi was forced to halt work and redesign foundations after it became clear the Franki piles specified by site engineer Politecnica Societa Italiana (PSI) would not deliver required bearing capacity.

Trevi regional manager Gino Zanchini explains: Eight tanks are to be built at the Scaffati water works for different stages of purification. Site information supplied to main contractor Prefettura di Napoli and PSI showed ground made up of about 3m of organic material, clays and sand over a further 3m to 4m of loosely compacted sand and, below that, dense sand. On that basis PSI specified a foundation system of 30m to 35m long, 508mm diameter reinforced Franki piles - constructed by driving a steel casing to refusal, and then, as concrete is poured, extracting the casing. Piles were to have a bearing capacity of between 40t and 150t, with 134 piles per tank at 1.5m centres, united by a grid of beams.

On extraction of the casing concrete seeps into the ground - up to 30% may be 'lost' in this way. The resultingly irregular profile increases friction and enhances bearing capacity.

But at Scaffati Trevi was losing between 200% and 300% of concrete, says Zanchini. And tests carried out on one in every 50 piles showed they were regularly well below strength. After piling had been completed for two tanks in what was known to be more competent ground, it became clear that a review of the method was called for.

Trevi embarked on its own site investigation to discover that, though the strata of organic/clayey, soft sand and compacted sand were more or less as described, there was an intermediate 3m to 4m deep layer between the organic/clay and sand layers made up of fluid silts. This layer was capable of absorbing almost indefinite amounts of concrete, chronically undermining pile strength.

Accordingly, Trevi has adopted two alternative designs, both driven, cast insitu piles. Pile length has been increased to between 40m and 60m and a sacrificial corrugated steel casing is now used. Trevi is using a 346mm diameter casing in the good ground. In the weakest areas a twin section pile is used - the lower part of the pile is 346mm diameter and the upper section 457mm. Trevi has designed a piling hammer that will strike the bases of both casings simultaneously to ensure the sections do not become detached.

Corrugated casing costs 40% more than conventional, says Zanchini, and sacrificing it increases piling costs by 30% to 50%. Problems have put the contract three months behind schedule, he says.

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