Jet Grouting at Posey Tube, Oakland, California

This paper presents an innovative seismic retrofit of the historic Posey Tube in the San Francisco Bay area of California. The tube, which forms an underwater link between the cities of Oakland and Alameda, is located between two major active faults, the Hayward Fault and the San Andreas Fault. The Posey Tube, completed in 1928, was then the first pre-cast underwater tunnel to be built in the world. Approximately 40 years later, another tube, the Webster Street Tube, was built parallel to and 152 meters (500 feet) west of the Posey Tube. Construction of the two tubes required excavation of a trench along an approximately 976-meter (3,200-foot) stretch of alignment over land and water, and backfilling of the trench with dumped fill. During a major earthquake, the tube 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 could become buoyant in the heavily liquefied material, causing substantial displacements and damage. The seismic retrofit in the Posey Tube consisted of installing jet grout columns along both sides of the Posey Tube, and while that in the Webster Street Tube was composed of pipe pile stone columns which is discussed in a separate paper. The objective of the seismic retrofit in the Posey Tube was to prevent flotation of the tube due to liquefaction of the sandy soils underneath the tube. This was achieved by constructing a row of 1.83-m (6-ft) diameter overlapping jet grout columns on both sides of the tube, each to a depth of about 13.7 meters (45 feet) from approximately 1.5 meters (5 feet) below the crown of the tube. These two rows of columns serve as barriers to contain the liquefied, loose, saturated sand in place during a major earthquake. This innovative design concept is considered to be the first of its kind to be applied to an immersed tube of this magnitude in the world. This paper discusses: (1) challenges confronted by the unique site and soil conditions with the use of triple fluid system of jet grouting, (2) the use of jetting specific energies to determine the overall structural integrity of grout cores, and (3) results of field and laboratory testing that included pre- and post-construction borings, permeability testing, and unit weight/unconfined compressive strength testing. These are followed by a discussion of lessons learnt from the jet grouting operation along the Posey Tube.