The Marks

Before the residential towers in the "Neu Marx" urban development area in Vienna's third district could grow into the sky, a stable foundation was required. Just the right challenge for the experts from PORR Bau GmbH's Specialised Civil Engineering department. From 2019 to 2020, they took on the execution planning and construction of the foundation pit system, including diaphragm walls, anchors, dewatering, bored piles and earthworks. This was realised instead of the originally planned sheet pile wall excavation. A special feature here was the merger of four different clients to construct a joint excavation pit.

A single-shell construction method was planned for the majority of the excavation pit (towers T1 and T2). The diaphragm wall, which is up to 22m deep, acts as the final inner wall of the garages and cellars and is also subjected to vertical loads from the rising supporting structures of the buildings, which are up to 140m high. By guaranteeing tightness class A2 in accordance with the "Diaphragm wall guideline", the required quality standards are achieved over the entire 8,600m² diaphragm wall area. In the T3 tower area, a double-shell design was used in the form of 40cm thick diaphragm walls with an inner shell in front.

Perfect preparation for a smooth construction process. Extensive route clearance was required prior to the construction of the guide wall. A large number of massive reinforced concrete obstacles were removed at great expense in terms of time, equipment and materials. Accompanying excavation monitoring confirmed that the area was free of explosive ordnance. The subsequent diaphragm wall construction could then be realised without the use of chisels and with minimal vibration.

Water law requirements fulfilled. A special solution was provided for the excavation pit shoring in order not to have a permanent negative impact on the groundwater communication of the surrounding properties. Small diaphragm wall windows made of special concrete located below the deepest excavation level fulfil the tightness requirements. If required, this special concrete can be jetted using a jet grouting process with the use of water once buoyancy safety has been achieved.

This creates a water-permeable recess in the diaphragm wall.

Parallel work for greater efficiency. At the same time as the diaphragm wall construction, 18 extraction wells, six monitoring wells and eleven infiltration wells were constructed from the top edge of the ground and an advance excavation level of around -4.5m. Infiltration of the drainage and residual water was carried out on the site itself via infiltration wells. Anchoring the diaphragm wall with 239 bar and strand anchors posed a particular challenge for the construction process and the specialised trades working in parallel. In areas of the crane foundations located on the outside, the construction pit shoring was reinforced by placing a second layer of anchors. This meant that cost-efficient special solutions could be offered, planned and implemented for the client at short notice.

Load test successfully completed. The deep foundations of the three high-rise buildings and the base building were laid on 634 auger cast concrete piles with a diameter of 90cm. The basis for the pile design was the realisation of five pile test loads (up to 9MN individual load) including the production of the required test piles. A tangential bored pile wall was constructed to ensure the planned level jumps of the floor slab. This meant that space-consuming embankments in the base area of the excavation pit could be avoided.

The economically and technically best solution. The decision in favour of a diaphragm wall instead of the originally planned sheet pile wall was exactly right for several reasons: whether it was the obstacles encountered, from brickwork to solid reinforced concrete obstacles, which would have meant the end for a sheet pile wall, the increase in the impermeability of the excavation pit or the vertical loads that could be transferred via the outer shell produced in this way. The overall package of the turnkey construction pit decoupled the interface between civil engineering and structural engineering. This synergy is reflected in the execution between the two major contracts for building construction and civil engineering. It enabled the clients to plan in detail and optimise the awarding of the master builder work. The successful work created a strong foundation for three new towers on the Vienna skyline.