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Metro method

Secant piling has been used to construct the six, 30m deep underground stations and eight circular emergency shafts on the Copenhagen Mini Metro. The project is being built by the Comet joint venture of Ataldi of Italy, Bachy Soletanche and Tarmac of the UK, Ilbau of Austria, NCC Rasmussen and Schoitz of Denmark, and SAE of France.

Bachy Soletanche helped Comet choose the secant pile method, adopting the hard/ soft technique in the design. Primary, soft, plastic 760mm diameter bored piles are first installed at 1,500mm centres; then the secondary, load bearing, reinforced concrete piles, 1,180mm in diameter which intersect the primaries are drilled through the upper 15m thick layer of till and boulders and down through the harder limestone to the founding depth at 26m.

Once all the piles have been installed, the station's top slab is cast; construction of the remainder of the station then proceeds top down.

Seven Casagrande rigs were mobilised for the work - two C60 HT55's, one C90 HT55, and two C6S bored drilling rigs for the grouting works. Two C60s fitted with grabs are working on the diaphragm wall excavations at the two rectangular escape shaft sites.

Fax: Michael Finch (0181) 963 0346

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