The Four Seasons Hotel and Tower is one of the largest private developments in downtown San Francisco. The high-rise building is a mixed-use facility, consisting of a hotel, fitness center, retail space, condominiums, apartments and a five-level underground parking garage.

Case Foundation Company was retained to install a reinforced concrete diaphragm wall which would provide initial support for the 60-ft. deep parking garage excavation and then serve as the permanent, load-bearing basement wall.

As part of its contract scope, Case also supervised perimeter pre-excavation of portions of the site to depths of 40 ft. in order to remove old building foundations and tie-back anchors that encroached on the wall alignment. The site was then back filled and compacted to provide a stable working surface at street level.

Site soils consisted of 15 ft. of miscellaneous fill and old building debris overlaying 20 ft. of loose sand. Beneath this was 60 feet of very dense sand, with very stiff “Old Bay Clay” encountered at approximately 95 ft. below street level. Static groundwater was present at 30 ft. The 36-inch thick diaphragm wall followed the building perimeter, penetrating through the overburden soils and keying a minimum of 10 ft. into the underlying clay stratum.

Wall construction was performed by two crawler cranes, operating cable-hung clam buckets. Panels, varying in length from 14 to 26 ft, were excavated and concreted in an alternating sequence to allow the concrete to set-up before adjacent work began. During the excavation process each panel was continuously filled with a pre-mixed bentonite/water slurry which provided ground support for each panel. When excavation reached design depth, a full-length reinforcing steel cage, fabricated on site, was lowered into place through the bentonite slurry. This reinforcing steel was designed to resist the Bay Area’s maximum seismic loading criteria. The panel was then tremie-filled with 5,000 psi concrete, displacing the liquid slurry to a nearby holding tank. A total of 57 panels were constructed to depths varying between 105 and 125 ft. below street level in order to encompass the 1060 lf site perimeter.

In addition to functioning as temporary excavation support and final basement construction, its key in the deep clay also created a permanent groundwater cut-off, eliminating the need for long-term de-watering.

Subsequent temporary support of the wall was provided by tie-back anchors and by internal rakers in areas where external interferences, such as the BART structure along Market St. were present. To date, this is the largest and deepest diaphragm wall ever constructed for a building project in San Francisco. Case completed the installation on schedule and the exposed surface of the wall is considered to be of exceptional quality.