The West Mission Bay Drive bridge was constructed in 1949 to span nearly 1,300 feet across the San Diego River. The existing 11-span bridge was constructed using steel girders with a cast-in-place concrete deck. The bridge is supported by reinforced concrete pier walls and abutments founded on groups of 128 to 160 vertical and battered timber piles per support. The California Department of Transportation (Caltrans) has classified the bridge as structurally deficient and functionally obsolete. The principal deficiency is failure of the foundation system under seismic loading. However, many of the steel and concrete structural elements that support the 70-year old structure are visibly in a state of disrepair.
Group Delta Consultants evaluated the geotechnical conditions at the site between 2009 and 2019 to develop detailed foundation design and construction recommendations for the two new 7-span replacement bridges. The subsurface exploration program included 16 rotary wash borings, one sonic core, and two cone penetration test (CPT) soundings that were used to characterize the geotechnical behavior of the soils that will support the new pile foundations. Most of the explorations were drilled through the existing bridge deck at night under traffic control, or from a temporary wooden construction trestle. The explorations were extended below the pile tip elevations, and to depths of over 240 feet below grade in some areas. An extensive laboratory testing program was conducted using hundreds of soil samples collected from the borings. The data obtained from the field and laboratory investigation was used to design each individual pile per the AASHTO LRFD Design Code (6th Edition).
The abutments will be supported by groups of 70.9-inch diameter Cast-in-Drilled-Hole (CIDH) piles. Each new pier be supported by two 118.1-inch diameter CIDH piles. To reduce the potential for construction anomalies, the CIDH piles will include permanent steel casings. The casings will be installed to the specified pile tips using an oscillator of rotator, and the piles will be excavated from within the casings. Once the pile tips are cleaned and inspected, the casings will be withdrawn to the permanent tip elevations. To verify the pile design and construction concept, a full-scale pile load test was conducted using the O-Cell method on February 14th, 2019. The test pile was 118.1-inches in diameter and 175-feet deep and was installed using the same means and methods planned for the production piles. The test pile results verified that production piles that are designed and constructed in the same manner as the test pile will experience less than 1-inch of total settlement under the Service, Strength and Extreme design loads, consistent with the design intent.
Location: San Diego, California
Client: T.Y. Lin International