Pipe Pile Stone Columns at Webster Street Tube, Oakland, California

This paper presents state-of-the art and practice for an innovative seismic retrofit of the historic Webster Street Tube in the San Francisco Bay area of California. The Webster Street Tube, along with the adjoining Posey tube, forms an underwater link between the Cities of Oakland and Alameda. The Webster Street Tube is located between two major faults, the Hayward Fault and the San Andreas Fault. The Posey Tube, completed in 1928, was the first pre-cast underwater tunnel to be built. Approximately 40 years later, the Webster Street Tube was built parallel to and 500 feet west of the Posey Tube. Construction of the both tubes required excavation and backfilling of a trench along an approximately 3,200-foot stretch of alignment over land and water. During a major earthquake, sandy backfill materials and the sand bedding immediately below the tubes would be susceptible to liquefaction, resulting in the loss of soil strength. Consequently, the tubes would become buoyant in the heavily liquefied material, causing substantial displacements and damage. The seismic retrofit consisted of installing stone columns along both sides of the Webster Street Tube. The pipe pile stone column method is the first of its kind to be applied to an immersed tube of this magnitude in the world. The objective of the seismic retrofit was to prevent flotation of the tube due to liquefaction of the sandy soils underneath the tube as well as to densify the surrounding saturated sandy backfill material. This paper discusses (1) the use of pipe pile stone columns to density the loose to very loose saturated sandy backfill surrounding the tube, and confine the loose to very loose sandy soil immediately under the tube — an innovative design to isolate and control the liquefied sand below the tube in place; (2) the overall practicality and constructability of using pipe pile stone columns to density loose saturated sands to silty sands; (3) the impact of the pipe pile stone columns in densifying the in situ soils by means of displacement and vibratory action of the pipe piles; and (4) results of field testing via pre- and post-construction borings and Cone Penetration Tests (CPTs). These are followed by a discussion of lessons learned from conducting the construction of the pipe pile stone columns over water and on land.