Centrifuge Modeling of the Piled Raft Foundation of a High Rise Building

The Millennium Tower in San Francisco, which was the tallest building in western United States, recorded excessive settlements shortly after construction. There were two possible reasons for the abnormal behavior, inadequate foundation design to support excessively heavy building load, and changes in subsurface conditions due to dewatering at the nearby construction site. This research aims to identify the most reasonable causes of the settlement of Millennium Tower. In this study, a centrifuge model was constructed to simulate the settlement of Millennium Tower and to predict the settlement trend in the future. A loading device using airbag was used to apply incremental load in-flight. Dewatering was also simulated during consolidation. The centrifuge modeling was performed based on the field subsurface data obtained from geotechnical reports. Ground materials were prepared by mixing different kinds of soil, and the foundation system was modeled in detail based on the information of foundation design. The model was tested under a centrifugal acceleration of 120-g. An airbag loading system was used to simulate the multi-stage construction sequence. After simulating the construction sequence, long-term consolidation settlement over a period of 20 years was conducted. The groundwater level was lowered to study the change in settlement caused by dewatering at the nearby construction site. Eight laser transducers measured the settlements of building and each ground layer, and the pore water pressure transducers measured the pore water pressure in the clay layers at four different depths. The average settlement obtained from the centrifuge model test showed good agreement with the field measurements. The centrifuge model testing showed that the temporary change in groundwater level did not affect the long-term settlement.