The ETH Zurich is to receive a new research and laboratory building for medical technology. Construction will take place in a sidehill cut at Gloriastrasse in Zurich. First, however, a supporting structure is required that will support the 100-metre-wide slope. The engineers of Basler & Hofmann found an elegant solution for doing just that – the like of which has never been seen before.
Creaking loudly, a crane lifts a giant platform at one end. On the platform stands a reinforcement cage of enormous proportions: the colossal structure measures 25 by 10 metres. Despite its size – or rather precisely because of it – the cage is extremely delicate. If the crane were simply to grab it at one end, the steel cage would fold like a sheet of paper. Only once the platform is positioned at a 70-degree angle can a crawler crane take on the precious cargo and lift it above the ground.
Dozens of builders and experts observe the tricky manoeuvre on this icy-cold December morning. Among the assembled crowd is Cornelia Malecki, a geotechnical engineer at Basler & Hofmann. She has a considerable cold, having to blow her nose between repeated bouts of coughing. However, it would never have crossed her mind to stay at home and miss out on the premier at the construction site of the new ETH Zurich’s research building at Gloriastrasse. After all, the results of the development work performed over the past five years by the team assembled here will be turned into reality today: the first of a total of 14 hefty diaphragm wall panels for an innovative slope stabilisation system is to be placed in the ground.
Weighing 50 tonnes, the reinforcement cage now dangles on the arm of the crawling crane. It is to be lowered into the narrow slot that has already been cut out at the other end of the building site. The crane makes a loud din as it starts to move. A group of site managers and builders march alongside, observing everything like a hawk. Nobody has any prior experience of working with a reinforcement cage of this size. A tense atmosphere thus fills the air. A site manager waves the following crowd, made up of engineers, planners and photographers and already observing from a safe distance of a few metres, even further back. “It is rare to find so many onlookers on a building site”, exclaims Cornelia Malecki against the noise.
Conventional won’t do
The great level of interest is not without reason: “Never before have diaphragm walls of this kind been used to anchor a slope stabilisation structure”, says Carlo Rabaiotti, team leader at Basler & Hofmann. The 100-metre-wide and 40-metre-high slope is still secured by a conventional solution. Carlo Rabaiotti points to the imposing piled curtain wall along the northern side of the building site: “The wall is currently fixed to the earth and rock masses behind it by around 500 anchors.” However, as the geotechnical engineer explains, this would only serve as a temporary solution. The anchors extend far below neighbouring plots of land and are too much of a limitation on construction opportunities in these areas. The team of engineers from Basler & Hofmann has therefore worked meticulously to develop a solution without anchors.
Diaphragm walls with a new task
The concept utilises diaphragm walls in a completely new way. What sets the concept apart is the incorporation of six prestressing tendons, each as thick as an arm, in every one of the 14 diaphragm walls. Now that the reinforcement cage hangs upright on the crane, they clearly stand out from the cage’s regimented structure. In future, they will run through a retaining wall positioned above made from reinforced concrete and link it to the diaphragm walls. The construction will be further reinforced by perpendicular retaining walls (bulkheads), with their individual bases being positioned on the diaphragm walls. “This will allow the entire supporting structure to be stabilised horizontally and vertically without anchors”, explains Cornelia Malecki. “Using this construction method, we can design the concrete wall in a slender and elegant manner without compromising its strength.”
In the meantime, the crane has now positioned the first reinforcement cage directly above the 25-metre-deep slot in the ground. A site manager signals to the crane operator, who then slowly lowers the steel construction into the slot. For structural reasons, the cages have to be set in concrete in one piece. These are put together on site by the builders on the specially developed platform – they would have been far too large to transport.
The crane operator lowers the steel cage into the slot centimetre by centimetre. As he does so, two specialists attach optical fibre cables to the steel bars. In future, these will lead from the reinforcement cage to a measuring device that will register with micrometre precision how the steel bars expand and compress in the finished structure. This will allow the performance of the new system to be monitored at all times. Team leader Carlo Rabaiotti is not worried: “Our design is very robust.”
The reinforcement cage has now completely disappeared in the ground and the builders begin to fill the slot with concrete. The crowd slowly scatter, and Cornelia Malecki and Carlo Rabaiotti also head back into the warm. The show is over for now Next week, the work will start on the second diaphragm wall element.