Background

The San Francisco Bay Area Rapid Transit District (BART) is conducting a comprehensive Earthquake Safety Program of the entire BART system, which includes the 3.6-mile long Transbay Tube running under the Bay from Oakland to San Francisco. The tube was originally installed by lowering Tube segments onto a foundation course gravel layer in an excavated trench along the Bay bottom.

Various fill materials were placed around and over the Tube. Seismic retrofit studies for the majority of the Transbay Tube alignment have focused on:

· The magnitude of seismic loads on the tube, and

· The condition and geometry of the tube backfill materials

Fugro's Role

As a component of this evaluation, Fugro under contract to Bechtel Infrastructure Corporation, conducted a site characterization study along the Transbay Tube alignment. The study included a marine geophysical survey and integration of the geophysical data with available geotechnical data. The goals of the site characterization were to:

· Map the regional stratigraphy down to bedrock in the project area

· Characterize local conditions around the tube subsurface stratigraphy in the vicinity of the Tube

The regional and local stratigraphy provide input to the engineering analyses and evaluations of the Transbay Tube under design seismic loads. The local Tube evaluations are also intended to help delineate conditions for the design and execution of seismic retrofit strategies such as ground improvement.

The data analyses and interpretations included:

· Processing of the geophysical data;

· Compilation of the available geotechnical data (i.e., lithologic descriptions, laboratory data, velocity data) into formats and displays that allow for engineering evaluations;

· Mapping of bayfloor features by integrating previously collected hydrographic survey data with the geophysical data collected as a part of this study;

· Integration of the BART geophysical and geotechnical data sets to map regional and local stratigraphy. This effort also included integration of BART data with data collected by Fugro for the Bay Bridge Replacement and SFO Runway Reconfiguration projects;

· Preparation of charts and cross sections that illustrate the findings of this study;

· Submission of interim interpretive products to allow for parallel engineering evaluations by others; and· Preparation of a summary report summarizing the geophysical field program and presenting the site characterization products.