One of the most exciting current construction sites for civil engineering company Stump-Franki sits tightly enclosed by Alexanderstrasse, the Gruner Tunnel, and the Alexa Mall: The construction pit for the ABC-Tower. With a base area of just 60 x 25 m, the planned ABC-Tower will be a rather narrow high-rise. The overall construction pit depth of 21 meters offers space for four basement levels, which will accommodate tenant areas and technology. On the fourth basement level, a passage will be built to the existing adjacent underground parking garage of the Alexa Mall.
Geological, geographical, and temporal challenges
A high ground water level, sandy soil, and the narrow construction pit, enclosed by the heavily trafficked Alexanderstrasse, as well as Grunerstrasse with the Gruner Tunnel, made the special underground engineering measures particularly challenging. By the time the construction pit was excavated, just under 1,800 truckloads totaling 27,000 m³ of excavated earth had been removed from the site. This was a logistical challenge in this narrow construction pit in the city center, every detail of which had to be considered in the implementation planning.
A foundation slab almost 4 m thick combined with 44 large-diameter piles (diameter 1.20 m) and the in-situ concrete diaphragm wall, which is 40 m deep in places, provides stability for the future high-rise. This is a singularly cost-effective and safe solution given the site conditions. The combined piled-raft foundation is particularly suitable for higher structures that will be erected on soft foundation soil, i.e. soil that is sensitive to settlement. The watertight trough is designed as an in-situ diaphragm wall with depths to 40 m and a braced, jet-grout floor that is 2 to 4 m thick. For the bracing, 230 micropiles with drilling depths to 45 m were manufactured. These micropiles are also used to brace the foundation slab after wet pit pumping is switched off.
Planning from PORR
pde Integrale Planung GmbH, a PORR Group company, was responsible for general planning tasks, sustainability, and construction preparation. The company is also commissioned to provide the implementation planning for the ABC-Tower.
