Austrian based formwork specialist Doka has entered the competitive market of tunnel shuttering with claims of an innovative travelling rig able to form a 24m length of complete twin tube cut and cover road tunnel in a single pour.
And the technique’s first trial use, creating a 2.3km long autobahn tunnel in north west Austria, has been triggered in part by the presence of hundreds of frogs.
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The £750,000 ($1M) self-propelled formwork frame is claimed to be slashing construction costs and time by a third, casting in one pour a monolithic tunnel section twice the length possible with previous shuttering systems.
Fewer construction joints, less waterproofing and use of preformed reinforcement cages means the largely automated rig can be operated to a weekly concreting cycle.
The total 21m-wide, two compartment tunnel, being built by German contractor Strabag, forms part of an 11km missing link in the otherwise long completed A8 autobahn. This runs 350km south east, right through Austria, to link Germany with Yugoslavia.
The delay completing this final section near Wels has been caused both by the civil unrest in Yugoslavia and the high environmental value of the land the closing link crosses.
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By GlobalDataTraffic flows to Yugoslavian war zones have long been declining, leaving the 40km diversion around Wels temporarily acceptable. And the delay allowed a solution to be sought to the challenges of an originally open road, routed in cutting through environmentally sensitive countryside rich in ecological value.
At least 18 fish ponds, complete with their occupants, have been relocated along with plants and woodland. But the main obstacle was the several hundred frogs waiting every day to cross the autobahn cut en route for mating territory.
The contractor’s winning suggestion was to run the road through a box-shaped cut and cover tunnel – to limit noise, intrusion and, above all, to provide a bridge for crossing frogs. This solved all challenges at a stroke.
And when Doka came up with the technology to double the previously possible length of tunnel section concreted in one go – saving the time ecologists had to man frog fences to prevent an invasion of the construction site – client operating concessionaire OSAG said ‘yes’ enthusiastically.
Cut and cover tunnels for environmental or avalanche protection are becoming increasingly popular in the Swiss-Austrian regions. But previous formwork technology limited cast sections to 12m.
Doka has not only doubled that previous maximum at a stroke but, by providing two formwork rigs, both working from the centre of the tunnel outwards, the technique allows a total 48m of complete tunnel to be cast every week.
Doka has traditionally avoided the bored tunnel market, finding it too cut throat.
“It is based only on low cost, rather than the quality and standardisation targets we prefer,” says Ernst Pürrer, managing director of the company’s civil engineering division. “But this growing cut and cover market allows us to concentrate on higher specification, less running maintenance needs and extended shutter use.”
The technique, claimed to be a European first, converts the principles of Doka’s well proven vertical climbing formwork, with its suspended shutters, into a horizontal format. The 600t rig allows the total tunnel profile of 700mm thick floor slab, outer and central walls, plus roof, to be cast in one operation.
It consists of two central, side by side, telescopic lattice trussed girders, 54m long and from which all the formwork hangs. This wheeled, square section launch girder is jacked forward into position so that 24m of its length extends out over already placed tunnel floor slab reinforcement.
The cycle begins with the two outermost, 7m high wall shutters being rolled out along the top of both launch girders. They are hung from the ends of fore and aft cross beams spanning across the top of the two girders. Lower ends or each shutter run on tracks laid just outside the line of the tunnel.
The internal, three sided shutter box follows for each tube, again rolled out along its girder to form inner faces for outer and central walls, plus a soffit shutter for the roof. With tunnel rebar largely prefabricated into cages, all is now ready for a total 24m long section of both tubes to be cast in a continuous 14 hour, 960m3 pour.
The floor is formed first in several layers with a short, 1m high section of all three walls poured at the same time. This allows concrete in wall stubs to start setting, while the tunnel base is completed, eliminating the possibility of uplift in the floor when walls are poured full height.
A 35N concrete mix, with 30% cement replacement by pulverised fuel ash is specified to keep the heat of hydration down to a very low 38°C. This minimises, and helps control, shrinkage cracking in the monolithic section.
The tunnel cutting boasts a near ground level water table and the specification called for an “impermeable” tunnel section. Monolithic construction allows fewer construction joints to waterproof and none at all longitudinally.
Shutters can be struck after 36 hours with the inside formwork folding inwards to release the frame. This box section is then jacked downward onto its travelling beam before the whole weekly cycle is repeated by walking forward the front 24m section of girder.
“The entire operation is much quicker and more economic than previous techniques,” Pürrer claims. “Overall I reckon it cuts time and costs by 30%.”
An advantage of cut and cover road tunnelling over bored tubes is that the former can have less rigid dimensions so profiles can often be adjusted to suit formwork available.
The birch plywood shutter facing is designed for at least 50 uses on the Wels contract. And, unusually, Strabag – a leading roads contractor – has bought, rather than hired, the rigs for a total $1.8M, hoping no doubt for further applications in later projects after the Wels tunnel is complete next summer.
