ABC Tower, Berlin

Alexander Berlin’s Capital Tower, known as the ABC Tower, is one of four new residential high-rises to go up as part of a comprehensive renovation project around the Alexanderplatz, in the centre of Berlin. The ABC Tower will be the tallest residential building in Berlin, and one of the tallest in all of Germany. The 150m high-rise comprises 35 above-ground and 4 basement floors. PORR Spezialtiefbau was contracted to create the construction pit for the project and complete all of the associated sealing and foundation works.

Hemmed in by Alexanderstrasse, the Gruner Tunnel and the Alexa shopping centre, the ABC Tower construction pit is one of the most exciting projects to be tackled by specialist civil engineering company PORR Spezialtiefbau. A relatively slender tower has been planned for the 60 x 25m site. The construction pit descends 21m into the ground, to allow for 4 basement floors that will house the building technology and provide additional space for tenants’ use. The fourth basement floor provides access to the adjacent underground parking garage of the Alexa mall.

Challenges presented by geology, geography and time

The high groundwater level prevalent in Berlin, sandy soil and a necessarily narrow construction pit, restricted by the busy Alexanderstrasse, Gruner Strasse and the Gruner Tunnel, presented major challenges for the specialist civil engineering team. Just 1,800 lorry loads were required to remove 27,000m³

of excavated material for the construction pit. In this narrow, inner-city construction pit, logistical challenges had to be factored into every detail of the execution planning for the project.

The future high-rise’s stability is ensured by a floor slab up to 4m thick in combination with 44 large-diameter bored piles, each measuring 1.2m across, and an in-situ concrete diaphragm wall up to 40m in depth. Given the conditions on site, this represents an outstandingly economical and secure solution. Combined plate-pile foundations are particularly suitable for taller structures located on soft or settlement-prone soil types.

The waterproof trough was constructed as an in-situ concrete diaphragm wall extending up to 40m into the soil, in combination with a deep-anchored, medium-height jet-grouted floor between 2m and 4m thick. The anchoring comprised 230 micropiles bored to depths of up to 45m. These micropiles will also serve to anchor the base plate after the groundwater control system is switched off. The construction pit walls are supported by a double-layer steel reinforcing structure with pipe diameters up to 1,200mm.

General planning by PORR

PORR subsidiary pde Integrale Planung GmbH was awarded the contract for general planning, sustainability and construction preparation as well as the execution planning for the tower.