Seismic rehabilitation of two immersed tube tunnels

Two immersed tube tunnels in the seismically active San Francisco Bay Area were built over 30 years ago with little consideration for seismic effects. The tubes were not damaged during the 1989 Loma Prieta Earthquake. However, liquefaction of loose sand backfill over the tubes occurred at relatively low seismic accelerations. A seismic rehabilitation design for the two tubes required the improvement of the backfill to prevent extensive liquefaction and potential flotation of the tubes. Conventional ground improvement concepts to prevent liquefaction were considered impractical and inadequate. An isolation wall concept consisting of either rows of stone columns or jet grouted soil was adopted. Theoretical considerations, as well as limited data from the literature on centrifuge tests and structure performance during the 1995 Hyogoken-Nanbu (Kobe) earthquake, show the effectiveness of the isolation wall concept.