Seismic Upgrade on the Californian I-280 Retrofit Project
At 5:04 p.m. on October 17, 1989 the Loma Prieta Earthquake, with a surface-wave magnitude of 7.1, occurred with its epicenter located 95km south-southeast of San Francisco, California. The main highway network in the San Francisco Bay area sustained serious damage at several locations. One of them is the Interstate 280 Viaduct (I-280), which traverses the southern part of San Francisco from west to east and proceeds in a northern direction along the eastern shoreline. Damage was observed at two locations: several blocks south of Army Street and near China Basin.
The seismic retrofit scheme at China Basin included numerous foundation retrofits. Design engineers determined that the bridge structure needed to be anchored down with a lot more than its original footing capacity. The original retrofit plans called for increasing the width, length and depth of numerous existing pile cap footings and supporting them on Ø 406 and 610mm reinforced concrete drilled shafts. Pile design capacities were 890 and 1,335kN respectively in tension and compression. Depth to the weathered bedrock varied from 10 to 20m.
Varying soil and rock conditions in addition to high groundwater level caused many difficulties during shaft installation. After numerous attempts to install the drilled shafts, the owner, Caltrans, and the general contractor, Atkinson Construction, determined that a pile re-design was necessary to successfully complete the foundation retrofits. DBM Contractors Inc. was asked to assist Caltrans engineers in developing a technically and economically feasible pile system to replace the drilled shafts.
DBM’s engineers selected a composite tiedown pile consisting of a Ø 406mm steel pipe driven to bedrock and internal double corrosion protection Ø 63.5mm GEWI® THREADBAR® (St 550/700, gr 75) drilled and grouted below the pile tip.
The DYWIDAG tiedown pile was selected over multistrand tiedown piles and micropiles due to its superior axial and lateral stiffness performance. The project consists of over 600 DYWIDAG tiedown piles with a design capacity of 1,335kN in compression and tension. The average length of the threadbars is 24m including a bond length of 10.7m. A total of 360t of Ø 63.5mm GEWI® THREADBAR® has been supplied for this project.