Micropile Solutions for Construction of SAS Span of the New San Francisco-Oakland Bay Bridge
December 10, 2020
The new east span of the San Francisco-Oakland Bay Bridge features a single tower self-anchored suspension (SAS) bridge that became the largest of its kind in the world when completed in 2013. During construction of the SAS bridge, micropiles were effectively used to solve three significant construction issues:
(1) Construction of the SAS bridge required temporary towers and trusses to support the box girders until erection of the suspension cables is complete. Three of these towers were located on steep slopes of Yerba Buena Island bedrock outcrop. For these towers, vertical and sub-horizontal micropiles were used as a viable alternative to conventional piles, considering constructability, cost-effectiveness, schedule and environmental impact. All the micropiles were tested and performed well during construction of the bridge.
(2) The installation method used for the main cable of the suspension bridge required the 160 m high main tower to be pulled 500 mm to the west, using 200 m long pull-back cables anchored to bedrock at Yerba Buena Island prior to the erection of the main cable. 18 m long micropiles were used as foundation for the tie-back cables inclined at 40 degrees. A reinforced concrete pile cap varying in thickness and width was used for testing the micropiles and for locking them off before the tiebacks were attached.
(3) The bridge decks from the west approach connecting to the west pier of the SAS bridge needed to be pulled down 75 mm in order to make the hinge closure pour during construction. A tie-down system consisting of two pairs of steel beams with ten pull-down cables on each beam was used. Two rows of 20 m long vertical micropiles were used as foundation for the tie-down cables. The micropiles were tested using the adjacent micropiles as reaction piles, without a pile cap, and using the same steel beams. This paper describes the design and construction of the micropiles and the connection details between micropiles and the structure, focusing on some of the challenges faced during the design, installation and testing of the micropiles.
Thavaraj, T., A. Sy. and D. Dowdell. 2017. “Micropile Solutions for Construction of SAS Span of the New San Francisco-Oakland Bay Bridge,” in Proceedings of the 13th International Workshop on Micropiles, March 29-April 1, 2017. Vancouver, BC: ISM, ADSC and DFI.