The project involves the construction of four underground sports halls beneath the sports field, directly adjacent to the existing school. The ground conditions are challenging, with metres of fill characterising the excavation area within a former, backfilled brick pit. Three groundwater levels, two of which are confined within the Miocene silts, clays and sands of the Vienna Tegel, require groundwater lowering and dewatering using wells and gauges during the construction phase. An analysis of the existing building revealed that the existing foundations required reinforcement. As these are over 100 years old, they no longer met current technical standards. As a safety measure, the foundations were underpinned with large-scale jet-grouted bodies. Additionally, prestressed anchors are used to secure the jet-grouted body. To secure the excavation pit, which is approximately 10 metres deep, a low-deformation capping method was chosen, implemented with dense diaphragm walls on three sides and, on the side adjacent to the existing building, with a DSV sealing screen in addition to the DSV underpinning. The reinforced concrete cover is supported by bored piles (auxiliary piles), which will, however, be removed once the excavation is complete. Due to the inner-city location, space for the site facilities is severely limited; the reinforced concrete cover is therefore used for this purpose and also for the temporary storage of excavated material. The temporary bored piles are therefore also essential for bearing these additional loads. The massive reinforced concrete floor slab, which also forms the horizontal waterproofing against the subsoil, is founded on the extensive diaphragm walls and the bored piles. Once the groundwater control measures have been discontinued, the deep foundation elements also serve to secure the building against uplift in its final state.

The construction and specialist civil engineering works are being carried out by the Granit & Keller Grundbau consortium and its subcontractor PORR. We would like to extend our sincere thanks to Mr Michael Schmidle (BIG), Ms Gerda Polig (architect), Ms Alexandra Gamperl and Mr Patrick Hollensteiner (both site supervisors), Mr Christoph Hartleb (Granit) and Mr Thomas Dolesch (Keller Grundbau) for the comprehensive project presentation and guided tour of the construction site.