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https://nap.nationalacademies.org/catalog/27432/critical-issues-in-transportation-for-2024-and-beyond

INNOVATIVE DESIGNS OF SEISMIC RETROFITTING THE POSEY AND WEBSTER STREET TUBES, OAKLAND/ALAMEDA, CALIFORNIA

This paper presents state-of-the-art and practice for an innovative seismic retrofit of the Posey and Webster Street Tubes in the San Francisco Bay Area. The tubes are immersed tunnels, connecting Oakland with the Island of Alameda, California. The tubes are located between two major faults, Hayward Fault and San Andreas Fault in the State of California, both capable of generating horizontal ground acceleration of about 0.5 g at the site. The Loma Prieta earthquake in 1989 with an estimated horizontal ground acceleration of about 0.2 g at the site was the most recent major seismic event that occurred near Posey-Webster Street Tunnels. The California Department of Transportation (Caltrans) officially recognized these two tubes as candidates for retrofit in the comprehensive seismic retrofit program for highway structures throughout California. This paper discusses the innovative design of seismically retrofitting the two tubes. Liquefaction of the hydraulically placed backfills could cause buoyancy of the tubes with the resulting displacements causing catastrophic failure of the tubes' precast joints. Two different design approaches are used to mitigate liquefaction potentials. For the Posey Tube, the design involves installing jet grout columns to create cut-off walls adjacent to the tube to prevent potential buoyancy of the tube during and after a magnitude 7.25 earthquake. This design is innovative as it isolates the liquefied soil from flotation by the jet grout walls, and is considered to be first of its kind to be employed in an immersed tunnel. For the Webster Street Tube, two methods of installing stone columns are used to densify sandy soils surrounding the tube; one is called the Pipe-pile method and the other the bottom feed vibroflotation method. The Pipe-pile method is believed to be the first of its kind to be applied to densify soils adjacent to an immersed tube in the world. The bottom feed vibroflotation method, though used to stabilize loose soils, has never been employed to densify soils surrounding an immersed tube in the U.S. To verify the design concept and the constructibility of the three methods, a demonstration program was instituted and implemented by Caltrans in the summer of 2000. Results of the demonstration program have been incorporated into the Phase I construction where the joints between precast segments and the joints between the tubes and portal buildings are being released to allow displacements and limit the forces. This paper also presents some of the findings and recommendations of the demonstration program as well as the joint repair work performed at the site.

Language

  • English

Media Info

  • Features: Figures; References;
  • Pagination: p. 271-284

Subject/Index Terms

Filing Info

  • Accession Number: 00933601
  • Record Type: Publication
  • Files: TRIS
  • Created Date: Nov 4 2002 12:00AM