Centrifuge model tests were performed to study the stability and uplift mechanisms of the BART Transbay Tube. The tube is a cut-and-cover subway tunnel located in a highly seismic area. The low relative density of the backfill material around the tunnel and the low unit weight of the tunnel might make tunnel suffer uplift movement due to buoyancy forces caused by liquefaction of the backfill material during an earthquake. Three uplift mechanisms were observed in the centrifuge model tests: (1) a cyclic ratcheting mechanism of sand moving under the tunnel associated with cyclic lateral deformations of the tunnel;(2) flow of water under the tunnel; and, (3) heave of the soft trench clay. The FLAC program was used to simulate the centrifuge model tests. A sensitivity study was performed to decide on the final mesh and treatment of interfaces in the numerical model. Results of the sensitivity study, numerical simulations and centrifuge model test results are presented and discussed in this paper
