O-Cells in Stuttgart, Germany
SWR, Stuttgart’s premier radio and television station have found themselves needing to expand by extending their existing premises in a heavily built up area of Stuttgart.
Project: SWR Media Building
Location: Stuttgart, Germany
Client: SWR
Consulting Engineers: Henke and Partner GmbH
Foundation Contractor: Franki Grundbau GmbH
Variable underlying strata and the presence of highly prized mineral water source lead to the planning of an O-cell testing program. A primary concern of this project was not to breach or contaminate a mineral water aquifer. Pile length should be long enough to hold the load but short enough not to risk infiltration of the water source. Utilizing a multilevel O-cell test allowed the characteristics of two separate soil strata consisting of clays and sands with the toe founded in a limestone rock socket to be ascertained.
Preparation of the reinforcement cages with O-cell assemblies was undertaken off-site in a steel fabrication workshop. The completed assemblies were then shipped to site for installation into the auger bored piles constructed by Franki Grundbau GmbH.
The second pile was situated along a narrow one way street with a complex array of overhead cables. The test was installed on the pathway beside the road. One half of the narrow street was closed to traffic for safe site access.
Test pile one consisted of an 873 mm auger bored pile with multilevel O-cell assemblies located at depths of 2 m and 8 m above the pile toe. Each assembly was made up of 2 x 330 mm diameter O-cells. The pile was bored to a depth of 25 m with the final concrete level approximately 6.5 m below ground level. The second pile was a single level bi-directional test, constructed 23.25 m deep with the O-cell arrangement placed at 5.34 m above the toe. The top of concrete was 5.0 m below ground level.
Isolation of pile sections on test pile one was only possible by multilevel bi-directional means. The inclusion of strain gauges within the pile section gave mobilised unit friction values previously unknown in these materials. If the second pile had been tested by top-down means, it would have required complete closure of the road adjacent to the test location. Bi-directional testing has allowed the pile design to be optimized to ensure the piles do not penetrate the valuable mineral water reserves located directly below the site. The multilevel test mobilised an excess of 30MN.
