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ThyssenKrupp Develops Revolutionary Prop System for Shoring Excavation Pit and Foundation Piles

For the large Postareal Salzburg construction site ThyssenKrupp Xervon Austria’s stationary shoring experts came up with a solution comprising not vertical but horizontal shoring involving a hydraulically adjustable one-off system comprising heavy tubular props designed to secure the terrain next to the excavation pit and its foundation piles.

Since 2005, a completely new district has been under construction on the almost 27,000 square meter area in front of Salzburg main station. Included are a hotel, shopping center, underground parking lot (for 290 vehicles), a residential complex (100 homes) and a 12-storey office building. The project is now in its final phase consisting of the construction of the shopping center and the neighboring parking lot. This latter extends up to 50 meters under the earth's surface. The individual phases of the necessary construction work need to be planned in great detail. The adjacent building surrounding the excavation pit (railway station, trackage, etc.) are mounted on foundation piles. If during excavation work, the side pressure acting on these piles gives way, stability is endangered. So, until the complete structure exerts the necessary counterpressure, some temporary support or shoring is required.

Together with the executing consortium (DOLL / Seekirchen und Fa. SPILUTTINI / St. Johann i. Pongau), the shoring experts at Vienna-based ThyssenKrupp Xervon Austria GmbH, Maria Lanzendorf, have come up with a highly creative solution: a horizontal hydraulic support comprising heavy-duty tubular props. Their special feature: at certain points pressure cells are mounted for meticulous monitoring of this shoring function. Whenever the reading changes - depending on the excavation depth, the supporting pressure can be readjusted with the aid of hydraulic set-collar presses so that the piles are always under the necessary pressure.

The tubular props are from Voestalpine and measure 50 to 110 cm in diameter. The smaller ones have to cope with loads of between 100 and 200 t, the large ones with loads of up to 400 t, with an unsupported length of between 12 and 20 meters. To achieve this high load-bearing capacity, the individual tubular elements are flanged to each other. Adjusting plates from 5 to 10 cm in thickness ensure the necessary close fit across the entire length for each prop. The final sections are then held by the hydraulic presses.

The large-scale Salzburg construction site has altogether 20 supporting elements waiting to be used which, end to end, measure a total length of around 360 meters. Depending on how the work progresses on site, the giant props are applied whenever soil is excavated between the sheet-pile wall (behind this are the foundation piles) and an about-to-be constructed part of the buildings and structures. At the sheet-pile wall, the props are applied to transverse roll steel beams. The cross beams distribute the load in a transverse direction of the foundation piles. Under the most favorable conditions, the end of the props meet with a vertical rolled steel beam resting on an already completed part of the building.

Not everywhere, however, is a situation that straightforward. There are some tricky areas involving shoring angles of under 45 degrees. In these circumstances, the forces are rerouted with special heavy-load prop structures and via auxiliary foundations. “It’s all a question of know-how and meticulous detail planning,” says Anton Stricker, head of the Stationary Shoring team in Austria. “We’ve been in this business for a long time and, at the end of the day, each contract is a one-off job that calls for experience, expertise and ingenuity.”

Despite all this, the Salzburg job certainly ranks among a rarity that, however, is handled with the same routine as all the other contracts. Since May 2008, the ThyssenKrupp experts have been working on the site and installing stage by stage the necessary props. In this way, all the parts of the underground parking lot to being built and the propping process is likely to continue until the end of the year by which time the parking lot will be in place and the floor slabs will take on the supporting function

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