Full-round concrete placement for Austrian tunnel
Environmental protection issues shaped the plans for a twin-tube vehicular tunnel 1.45 miles long outside the village of Steinhaus in Upper Austria. A cut-and-cover operation requiring the excavation of 650,000 cubic yards of soil, the tunnel itself called for 130,000 cubic yards of concrete to be placed. Because the tunnel stands in groundwater, it was built as what the Austrians call a white tank, a water-- impermeable concrete body with no added impervious external skin. Demanding Austrian standards for these impermeable concrete structures include a high degree of crack limitation and control of concrete temperatures; the mix temperature must be below 65 deg F at placement and cannot rise above 105 deg F during setting.
Monolithic tunnel cross section
At Steinhaus, the contractor built successive 80-foot-long sections of the twin tube using the full-round method, with the base slab, walls, and cover slab all completed for an entire section at a time in a single pour. Monolithic construction contributes significantly to meeting the Austrian white tank requirements because the structure has no longitudinal construction joints where the nonuniform creep and shrinkage of components of different ages could lead to cracking. Eliminating the longitudinal joints also does away with the laborintensive job of waterproofing them. At the same time, it presents a challenging placement schedule.
The tunnel formwork machine When the tunnel contractor, Stra bag Bau AG, proposed doubling the originally planned section length from 40 feet to 80 feet and pouring each section monolithically, the form supplier (Doka) responded by designing and delivering two large-area, highly mechanized, self-propelling form units. Strabag planned to use each forming machine 50 times, working from mid-length of the tunnel toward each end, forming an area of about 15,000 square feet with each successive pour.
Using the full-round construction technique, there is no supporting track on which the forms can advance. Doka adapted the tried-and-tested principles of the climbing drive used for its self-- lifting formwork (see CONCRETE CONSTRUCTION, August 1999), transposing it out of the vertical into the horizontal. A launching girder that advanced using a hydraulically driven "walking mechanism" was equipped with three sets of feet, one set at each end and one set in the middle. After the girder was advanced, it set its forward feet down on the ground so that it was ready for the outside and inside form panels to be moved along it. Bulkhead forms had a device for holding the joint waterstop material and fitting the joint doweling.
A front cross girder, resting on the two launching girders, supported the leading end of the outer form panels, which were 40 feet long and 23 feet high. The other end of the panels was supported on another cross girder, which could be repositioned on rails mounted to the previously cast section of the tunnel. The outer forms could be backed off as much as 3 feet from the concrete surface, enabling workers to safely place wall reinforcement from scaffold platforms on the formwork. Lattice girders stiffened the outside formwork in both vertical and horizontal directions so that it could sustain wind loads while being repositioned. Wall panels for the inside formwork measured 18x40 feet, and the cover slab elements were 31x40 feet. The wall panels folded inward with the aid of hydraulically operated toggle lever devices, and then the entire inside formwork unit was lowered using the walking girder mechanism. All of these largearea form panels were built of 8-inch and 12-inch timber formwork beams supporting 7/8-inch Finnish birch plywood sheets, all manufactured by Doka. The plywood panels were expected to last throughout the 50 cycles of use for each of the two formwork assemblies with only incidental repair. Weekly...