Centrifuge Modeling of Seismically Induced Uplift for the BART Transbay Tube

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. DOI: 10.1061/(ASCE)GT.1943-5606.0000489. (C) 2011 American Society of Civil Engineers.