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Lateral thinking

Concrete Tunnel formwork

A tailored travelling formwork is helping build a watertight underground motorway in central Austria, reports Adrian Greeman.

Green politics increasingly have a major influence on engineering projects in Europe, nowhere more so than in the German speaking countries. In Austria road builders are going to great lengths in highway design to avoid impact on communities, animal life and environment.

For the final 11.1km section of the Innkreis motorway in central Austria 'green' measures are even greater than usual in response to protests during the planning stage. Parts of the route are in cutting or even buried where it passes small towns.

Where it runs on embankment, animal run underpasses and water table culverts are included. Some 90ha of 'biotopes', or ecological protection areas have been set aside.

Such measures have been accepted by client OSAG, Austria's motorway authority, because the link is vital both for the country itself and the European road network. It forms the last part of the north-south A8 highway linking the east-west A1 highway to Vienna with the A9 highway in the south.

However some innovations have resulted, for instance on Austrian contractor Strabag's 2.6km long cut and cover tunnel, where the route passes the town of Steinhaus out of sight and out of mind below fertile fields.

With the ground usually wet the tunnel has to be watertight, explains project manager Walter Hermann. To speed construction and reduce costs, 'white tank' methods are being used to produce impermeable concrete. But to overcome the problem of differential shrinkage and cracking at the longitudinal joints when the concrete is poured at different times, an innovative new system has been devised which allows long sections to be cast in the round.

A more established formwork technique is being used on a shorter 1.6km section of underground motorway around the town of Wels. White tank concrete is ensuring watertightness on a 180m 'bridge tunnel' over the River Traun, with contractor Granit & Haider forming the 12m tunnel wall sections with standard formwork, beginning with casting of a base slab and the central dividing wall, followed later by the outer walls and the roof of the tunnel.

The drawback, says Strabag site engineer Sahir Tahtaci, is that expensive additional steel is needed to control cracking and waterproofing has to be used, which is labour intensive.

Pouring the whole tunnel section, walls floor and roof in one continuous operation avoids these problems, he says. The stringent crack requirements laid down in the Austrian Society for Concrete & Construction Technology's directive on waterproof concrete, can be met more easily.

At Steinhaus, Strabag's operations scheduling department set out to create a formwork that would allow the entire tunnel shape to exist as a continuous void. Working with Austrian engineering consultant Dr Wolfgang Lindbauer and formwork specialist Doka, the three came up with a tunnel casting machine; in effect a modified self-climbing vertical shaft form operating horizontally.

The formwork uses two launching girders supported on six legs, one pair at each end and one in the middle. These girders can be lifted and propelled forward with standard hydraulic pushers within the main formwork to sit in the next tunnel section. Once their feet are set down on two steel I-beam supports, the now static girder can be used to travel the formwork forwards.

The formwork is built up around the girders on a frame using standard Doka elements to support panels making up the inner faces of the tunnel and the underside of the roof. The side panels can be retracted and folded back using hydraulic rams when striking and preparing for the next move.

Crossbeams at the front and rear of the girders support an framework for the 24m outer wall formwork panels, the rear beam sitting on the already completed section. The 7m high panels can be retracted hydraulically during the moving phase and to allow room for installation of pre-formed reinforcement elements.

Stop panels at ground level at each end of a section complete the unit.

'The entire structure not only allows a continuous pour for the whole tunnel cross-section, ' says Dipl Ing Ernst Pürrer managing director of Doka's Austrian division, 'but it allows each tunnel section to be 24m long.' Such a length reduces intermediate joints and accelerates the construction process.

Tahtaci says Strabag is able to achieve a one week cycle for a 24m length of tunnel, half the time of the standard system.

Strabag schedules its pours just before the weekend, which then absorbs much of the three day curing time before the concrete reaches 16N/mm forms can be struck. Steel reinforcement for the floor area on the next section is also positioned in this period, before the formwork is moved forwards.

Reinforcement is designed to give good crack resistance just beneath the concrete surface to ensure impermeability. Cracking restraint also demands a carefully designed concrete mix, 'with a high proportion of PFA', explains Pürrer. The concrete must also comply with stringent temperature limits, reaching no more than 48infinity as it goes off, which in turn demands no more than 22infinity at placing. Aggregate spraying and ice in the mix keep the temperature down.

Concrete is delivered from a ready mix plant 20km away using 18 trucks for a sequence of over 120 loads during a 14 hour period for the pour. Sequencing of the placement is critical because the concrete in the wall sections could easily overflow into the open floor area. The contractor builds up the floor in layers to about halfway and then pours the lowest part of the walls.

As this stiffens after a couple of hours it becomes able to hold concrete above it and the pours can continue while the floor is finished. All the concrete is vibrated with immersed units.

All that then remains is to backfill the sloping sided working trench. The entire project is scheduled for completion in early 2003.

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