The east span of the 3.5 km long SF-Oakland Bay Bridge is supported by
different types of foundations along its span. Starting from the west, its first 460
m section comprising twin concrete box-girders is supported by sands and rock
on Yerba Buena Island. A 620 m long steel suspension span then bridges a 25
m deep channel that is formed in sediments overlying rock. Twin concrete box
girder structures, each 2.4 km long, complete the stretch over the remainder of
San Francisco Bay to the engineered earth fill landing at its east end. For most of
its length, the Skyway bridges water 3 to 10 m deep over soft surficial Young Bay
Mud. The foundation types include spread footings with hold-downs, steel and
concrete piles that are set in drilled rock sockets up to 3 m in diameter and 2.5 m
diameter steel tubular piles driven to penetrations of up to 100 m. The design
and construction-related considerations are presented. The different foundation
types, influences of the site geology and earthquake demands on the
substructures are discussed. The seismic motions of the bridge superstructures
and substructures, as well as the depth-varying soil motions, influenced the
foundation design. Together with performance-based design criteria, including
post-earthquake footing displacement limits, these seismic demands influenced
various aspects of the foundation design, including the use of battered and
vertical piles.
Author: Gerry Houlahan, Moffatt & Nichol
Publisher: DFI