Extension of car park UG25
Unterer Graben, St. Gallen, Switzerland
Project 2012-2024
Realisation 2019-2025
Opening March 2025
UG25.
Overview
Client
Senn AG, St. Gallen, Switzerland
Structural engineering
INGE UG25, engineering consortium of ZPF Ingenieure, Basel and Bänziger Partner, St. Gallen
Team ZPF
Isak Buljubasic, Heike Egli-Erhart, Rafael Häni, Yannik Jaggi, Antje Käser-Wassmer, Franck Mahler, Jonathan Mazzotta, Kata Aletta Orbán, Jacqueline Pauli, Nico Ros, Sali Sadikaj, Kay Unterer, Sander van Baalen, Robert Vögtlin, Manuel Wehrle, André Weis
To extend the CENTRAL multi-storey car park on the edge of St. Gallen's old town by over 500 parking spaces, the building was given six additional basement levels as well as new entrance and exit ramps, lifts and stairs.
The original building consisted of the four-storey CENTRAL multi-storey car park and a three-storey extension with office and commercial space. The extension of the car park by six basement levels was carried out while the car park was in operation. Additional challenges arose due to the inner-city location, the hillside location and the complex geotechnical and hydrogeological conditions underground.
Excavation support and building stabilisation
The subsoil consists of artificial fillings, old cover layers and a thin moraine, which form the loose rock above the bedrock. The molasse rock is composed of sandstones and marls and sinks towards the NNW.
Before the excavation, bored pile walls with piles up to 30 metres long were constructed on all three sides of the building. The loads of the existing buildings were supported above the newly planned basement floors with pre-stressed floor-to-ceiling concrete discs. These in turn were supported by temporary micropile towers. On the west side, the neighbouring building was underpinned using jetting and secured with anchors.
As the excavation progressed, the previously constructed permanent bored pile wall was stabilised from the ground floor upwards and in the ramp area and the logistics openings using pre-stressed anchors and nails. From the ground floor downwards, stabilisation was provided by two storey ceilings constructed in advance as sprout ceilings. The load transfers between the existing structure and the temporary support structures were realised using hydraulic jacks.
Construction method and realisation
The construction work was carried out using the so-called cover construction method: The excavation was carried out in sections from top to bottom. This always involved excavating two storeys deep and then constructing the aforementioned sprout slabs. Once the excavation base was reached, the foundation slab was constructed and additional intermediate slabs were installed.
The realised concept of bored pile walls, hundreds of anchors and temporary supports proved to be robust and reliable. The entire construction process was monitored around the clock by measuring systems and sensors.