Centrifuge Modeling of Seismically Induced Uplift for the BART Transbay Tube
Publication: Journal of Geotechnical and Geoenvironmental Engineering
Volume 137, Issue 8
Abstract
The BART Transbay Tube (TBT) is an immersed cut-and-cover subway tunnel that runs from Oakland to San Francisco, California. The loose sand and gravel backfills placed around the tunnel are considered to be liquefiable, and the clays under the backfill are soft in some zones along the alignment. These conditions could potentially result in uplift of the tunnel during strong earthquake shaking. This paper describes centrifuge model tests performed to verify numerical methods used to assess the stability and to evaluate the potential uplift mechanisms of the TBT. The observed mechanisms of uplift were a ratcheting mechanism (sand migrating under the tunnel with each cycle of relative movement), a pore water migration mechanism (water flowing under the tunnel), and a bottom heave mechanism, involving soft soils below the base of the trench. A fourth potential mechanism, viscous flow of liquefied soil, was not observed. The volume of the tunnel relative to the volume of the trench and the densities and permeabilities of the nonhomogeneous backfill were important parameters affecting the uplift of the tunnel. From the experiments reported here and analyses reported by the designers, it was concluded that the magnitude of uplift is limited and, hence, that an expensive ground improvement project to densify the backfill was unwarranted.
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Acknowledgments
This project was funded by a subcontract to UC Davis from Fugro West, Inc. Fugro West was a subcontractor of the Bechtel Infrastructure Corporation, with funds originating from the San Francisco Bay Area Rapid Transit District.
Anthony Hitchings, Tom Horton, and Kathy Mayo of BART and Mark Salmon and Ken Mark from the Bechtel team must be credited for having the courage and vision to go beyond conventional engineering procedures to use the most advanced tools available to address this difficult problem. BART’s Peer Review Panel and Design Review Board provided valuable guidance and oversight to the project. In particular, Jonathan Bray, Raymond Seed, and I.M. Idriss provided important input to the centrifuge model tests as part of this study.
Dan Wilson and Ross Boulanger of UC Davis participated in the design and testing. Stephen Coulter of Fugro West, Inc., participated in every phase of the model construction and testing. Yan Lucille of Fugro West, Inc. participated in the early phase of model construction and testing. Lijun Deng and M. Ilankatharan, UC Davis graduate students, assisted with instrumentation, sample preparation, testing, and data processing. Lars Pedersen, Chad Justice, Ray Gerhard, Mark Hannum, Peter Rojas, Nick Sinikas, and Joel Mireles, staff of the UCD Center for Geotechnical Modeling, provided necessary assistance and valuable expertise. Although this work and equipment maintenance costs were entirely funded by industry, the support to develop the centrifuge equipment by the National Science Foundation through the Network for Earthquake Engineering Simulation (current award number NSFCMS-0402490) over several years was a prerequisite for this work.
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Information & Authors
Information
Published In
Journal of Geotechnical and Geoenvironmental Engineering
Volume 137 • Issue 8 • August 2011
Pages: 754 - 765
Copyright
© 2011 American Society of Civil Engineers.
History
Received: Feb 5, 2010
Accepted: Nov 24, 2010
Published online: Nov 26, 2010
Published in print: Aug 1, 2011
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