The wait has been worthwhile. After having originated as a project idea back in 1999, on 17 December 2021, the new underground car park at the Kunstmuseum Basel will go into operation. The car park offers 350 parking spaces in the immediate vicinity of Basel’s city centre. To minimise disruption to traffic during construction, it was built using a particularly space-saving method: the top-down method.
It’s a clear December day in late 2020. As always, the areas around the Bankverein, the gateway to downtown Basel, and St. Alban-Graben, home to the two major banks UBS and Credit Suisse as well as the Kunstmuseum and the Antikenmuseum, are busy. Since the beginning of 2018, however, there has been even more going on here than usual: a new underground car park is being built under St. Alban-Graben. The tube-shaped car park will be located between the Bankverein and the Kunstmuseum. It will have 350 parking spaces on four underground storeys with a total length of 169 metres and a width of up to 17 metres. The project idea dates back more than 15 years, and various political hurdles had to be overcome before the car park could be built. In April 2018, an investor was found in the form of a real-estate fund managed by Credit Suisse Asset Management. In 2019, after years of planning, the construction work finally began, and in March, the first bored pile for the car park was driven into the ground.
It is extremely cramped on the construction site. However, anyone expecting to see a huge excavation pit is disappointed. Even a tram line has remained in operation above it. “St. Alban-Graben is an important artery in Basel’s public transport system. To ensure that it remains passable throughout the construction project, the car park is being built using a special method known as the top-down method that is as minimally invasive as possible,” explains Mario Brand, Subproject Manager Support Structure, who is visiting the construction site on this December day with Talei Madden, Project Engineer for Special Civil Engineering at Basler & Hofmann. Basler & Hofmann was already represented in the project development company founded in 2006. Within the commissioned planning consortium consisting of Basler & Hofmann, Rapp Infra and Stefan Meyer Architekten, Basler & Hofmann’s experts are responsible for the special civil engineering, the supporting structure, the vibration monitoring and the ventilation and electrical installations in the new car park.
About a year before the car park is opened, we climb through one of three excavation openings under St. Alban-Graben together with Mario Brand and Talei Madden. All of a sudden we find ourselves in the middle of a huge underground construction site. While there was hardly any construction noise above ground, down here it is difficult to understand each other at times. “We are now standing where in the future the ceiling of the second basement floor will be. Above us is the cover on which the tram runs,” explains Talei Madden. For the construction of the car park, bored pile walls and intermediate supports were first of all created on the ground surface. A concrete cover was then built on top in two stages. Thanks to this staged process, one tram track always remained in operation on one side of St. Alban Graben. The excavation and construction work is now taking place under the protection of the cover. “We’re kind of building from the top down. For us structural engineers, this meant a complete rethink. In structural engineering, loads on the upper storeys get smaller and smaller. Here, however, the loads are increasing as construction progresses. Accordingly, the planning of the supporting structure had to be completely finished before execution began,” adds Mario Brand.
The vertical dependencies and accuracy requirements were challenging for the planners and contractor team. “The drilled solid steel columns are 11.5 to 13 metres deep and extend through all four storeys. If the entire volume of 36,000 cubic metres were excavated at once, the supports would collapse in the middle like spaghetti,” explains Mario Brand. Excavation is therefore carried out in two partial steps – first of all, down to the ceiling of the third basement storey. To prevent the supports from buckling, buckling brackets are installed at the level of the second basement ceiling. Once the future ceiling of the third storey has been concreted as a floor slab, the ceiling of the second basement storey can be constructed using conventional construction methods. The third and fourth basement storeys will be excavated in the same way. The team of planners faced a special challenge with the Heuwaagestollen – a tunnel that runs just five metres beneath the entire floor slab. It was not to be allowed to absorb any load. “In places, the column grid of the cark park overlaps with the location of the tunnel. The affected columns have been designed as non-load-bearing for the construction period. During the construction phase, auxiliary construction supports are carrying the load to the side of the tunnel. A transfer beam will then be put in place on the fourth basement storey. Once this is erected, a controlled load transfer will take place to the main supports and the auxiliary supports cut out,” explains Mario Brand.
Due to the tight space conditions on St. Alban-Graben, the construction machines sometimes virtually scraped the foundations of the Antikenmuseum and the Kunstmuseum. This meant that continuous vibration monitoring was required, because valuable cultural and art objects are exhibited and stored in both museums. Basler & Hofmann’s experts consequently installed two measuring units, each with around ten measuring points, and in this way were able to continuously monitor the vibrations via a web platform. “Both the installation and monitoring were far from straightforward. First, we had to install the geophones partly in the visitor area of the museums. There they were to be as inconspicuous as possible. We also had to comply with very low limit values. That meant just a child jumping around could be enough to trigger an alarm,” explains Thomas Rupp, Project Manager for Vibration Protection. “During very tricky stages, we sometimes had to also be directly connected to the excavator operators in order to be able to intervene quickly in the event of critical vibrations,” says Thomas Rupp. In May 2020, the Antikenmuseum experienced strong vibrations, which is why various exhibits then had to be packed in a vibration-proof manner.
In the meantime, it is December 2021. Instead of descending through excavation openings, the car park is now entered through the official entrances. On 17 December, just before Christmas, the first visitors to the museum and city will drive their cars through the parking barrier. Even though stronger tremors were experienced from time to time during the construction work, no significant damage was caused. “In any case, I for one now know where I’ll be parking in Basel in the future,” says Mario Brand with a smile on his face.