Complex conversion and new build in the heart of Basel
July 17, 2020
ERNE is carrying out a complex construction project at the intersection of Steinengraben and Leonhardstrasse in Basel. The corner building of the row of houses was gutted and converted, and adjacent buildings were demolished and rebuilt. Apartments are being built on the upper floors and offices on the lower floors. The project includes demolition, pollutant clean-up, special civil engineering, excavation, conversion and new construction. Precise construction site logistics were required due to the limited space and the location on busy roads.
Demolition of two storeys and dismantling with the demolition excavator
Before a building complex from the 19th century was converted and rebuilt, it was comprehensively dismantled. Our specialists removed contaminated materials such as asbestos and slag floors and ensured they were disposed of professionally. The building was gutted and non-load-bearing walls and ceilings were removed.
The dismantling team then demolished five buildings adjoining the corner building on both sides, with the upper two floors at 27 Leonhardstrasse being removed by hand to protect the adjacent building fabric. The materials were removed "just in time" due to the limited space available.
The skeleton of a corner building was retained. The ceiling was demolished with a 4.5-ton excavator, which was lifted onto the roof with a Pnau crane.
Conversion of the corner building at Steinengraben 28
Once the ceiling of the building had been removed, the outer walls were shored up. Work could then begin on a complete core renovation. Walls were built using in-situ concrete and the façade was renewed. Earthquake-proof walls were built and climbing zones created, i.e. space for the installation shafts for the building services such as ventilation, water pipes and electrical cables. ERNE drilling and cutting specialists were used to mill the heating recesses in the basement. The structure was then reinforced with glued lamellas and is ready for the interior work as soon as the ceiling has been completed.
Excavation and special civil engineering for the new buildings 30-36
Excavation pits for the foundations and underground garages had to be dug for the new buildings. As historical remains were suspected in the area, archaeologists accompanied the excavation work. They discovered the remains of an old city wall. This was left standing and later integrated into the new building.
Securing the excavation pit proved to be a complex task. In the area of the road, nail walls made of fast-hardening shotcrete with anchors provide stability. The anchors reach eleven meters deep into the ground. Underpinning and micropiles provide additional support for the buildings.
Large downstand beam and exposed concrete stairs for the building at Leonhardstrasse 27
An unusually large main beam was installed in the new building at Leohardstrasse 27, which runs in the future car parking garage on the mezzanine floor and will bear the main load of the building. It is 26 m long, 1 m high and 80 cm wide and extremely heavy with an iron content of 580 kilograms per cubic meter of concrete!
The walls for the new building, which adjoin the corner building no. 28 and the building on the other side, were built using prefabricated components. The elements were constructed using Peters panels and high-performance columns and filled with concrete. In contrast to conventional formwork systems, these remain in the wall. They are extremely pressure-resistant and also serve as fire protection insulation.
The stairs and stair cores were constructed in fair-faced concrete type 4.1.4. This quality class places high demands on the appearance of the concrete. During preparation, care had to be taken to ensure that the formwork was fitted very precisely. When pouring the concrete, particular care had to be taken to avoid unsightly blowholes and to achieve an aesthetically pleasing, even surface.
