MICROPILE SOLUTIONS FOR CONSTRUCTION OF SAS SPAN OF THE NEW SAN FRANCISCO-OAKLAND BAY BRIDGE-Paper

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Micropiles addressed three construction challenges during construction of the San Francisco–Oakland Bay Bridge’s self-anchored suspension span.<br /><br />First, three temporary support-tower footings stood on steep slopes of Yerba Buena Island. Vertical and batter micropiles provided foundations suited to the restricted, sloping sites, reducing excavation and disturbance near a historic retaining wall. The piles were proof-tested and performed satisfactorily.<br /><br />Second, before installation of the main suspension cable, the 160 m tower had to be pulled 500 mm west using 200 m-long tie-back cables. Ten 18 m micropiles, inclined at 40 degrees and anchored in bedrock, supported the cables. Initial bearing tests revealed weak fill in some locations, so a reinforced-concrete pile cap was constructed with dimensions adjusted to local ground conditions. The micropiles passed testing at up to 2,050 kN and were locked off at 1,200 kN.<br /><br />Third, the west approach bridge deck had to be lowered 75 mm to complete a hinge closure pour. Two rows of 20 m vertical micropiles anchored a cable-and-beam tie-down system. The piles were tested to 1,650 kN using neighboring piles as reaction supports and the same steel beams later used for the deck pull-down.<br /><br />The projects demonstrate how micropiles can provide adaptable foundations for temporary works where conventional piling may be impractical. Testing and design adjustments addressed variable ground conditions, and the micropile systems successfully supported the required construction operations.

Keywords

micropiles

San Francisco–Oakland Bay Bridge

self-anchored suspension span

temporary support towers

Yerba Buena Island

tie-back cables

foundation engineering

bedrock anchorage

pile testing

construction support