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Columns beat peat on Austrian viaduct An innovative driven pile system is being used to combat difficult peaty ground on a motorway project in central Austria.

THE SOUTHERN section of Austria's A9 north-south autobahn from the A1 junction near Linz towards Graz was built in 1984, but near Kirchdorf, where the road crosses the River Steyr valley, it reduces to a two-way restricted section and traverses the mountains in tunnels.

Now, more than 40km of the A9 is being widened to remove the bottleneck. The work involves the construction of a new 1km long viaduct across the Alpine valley. The ASch167M (pounds8.4M) contract is being carried out by Tiefbau- technik, a subsidiary of German Strabag. Client is Ostereichische Autobahnen und Schnellstrassen Aktiengesellschaft.

The main problem for the project team is that the valley floor is made up of a 25m thick peat layer overlying glacial moraine deposits. The peat comprises two main layers of equal thickness separated by a 4m layer of sand.

A novel foundation approach was found with a system invented by Austrian foundation con- tractor Grund und Pfahlbau.

Support for the 1km long viaduct and its 230m long sliproad, is provided by 45 columns sitting on driven 'ductile' concrete piles that pass through the peat into firmer glacial moraine deposits that provide the necessary bearing capacity.

The core of each pile is formed by a 170mm diameter, 10mm thick hollow steel tube driven into the ground. However, the peat is so soft that the tubes almost sink under their own weight, with driving only needed through the sand layer. Each tube is fitted with a special flat shoe, which allows the pumped concrete to be forced out and around the tube itself, forming a coating of concrete as the tube goes down. By the time founding depth is reached, a concrete pile has been created with a steel tube reinforcement through its centre.

The viaduct is made up of six sections between 2m and 11m high. For each, there are two types of twin column support. The first type are connected to the deck by bearings and each sit on a pile cap with six piles, with the pile caps connected by a ground beam to provide rigidity. And for each section are two sets of twin columns fixed directly to the deck, each column sitting on 10 piles.

The team is confident con- struction will be finished on time in December 2000, with a couple of weeks in hand after a good winter last year.

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