Sunset Reservoir is located in a densely populated residential neighborhood of San Francisco, with homes and businesses located immediately downstream of the dam. The reservoir has a storage capacity of approximately 177 million gallons or 543 acre-feet and supplies roughly 60 percent of the City’s water. The San Francisco Public Utilities Commission (SFPUC) owns and operates the reservoir.
The seismic stability of the dam, a critical City facility, was the focus of the project. The SFPUC’s criterion is for the dam and reservoir to remain functional after a major earthquake on the San Andreas Fault, which is located 5 kilometers from the site. The 74-foot high earth embankment was built in 1938 on top of saturated native sands and silts. Geotechnical investigations revealed that these soils could be susceptible to significant strength loss during a major earthquake.
This project marks the first time the California Department of Water Resources, Division of Safety of Dams (DSOD) has approved Deep Soil Mixing (DSM) to remediate a potentially weak foundation and thereby improve the seismic stability of an earth embankment dam. The design for the seismic remediation was completed in November 2004. Construction began in June 2005 and was completed June 2006 with minimal disruption to these residents because of the low noise and no vibration soil improvement system.
The DSM layout consists of multiple 47 ft. square grids (blocks) of DSM columns that are placed in three rows (treatment zones) aligned parallel to the longitudinal axis of the embankment. The consultants recommended the discrete block layout, with gaps between the individual blocks to provide a conduit for regional groundwater.
DSM technology was new to DSOD, but they recognized it as the solution for a site that would have otherwise been difficult to remediate. The DSM technology proved successful in addressing the difficult site constraints while achieving the required improvement to meet the seismic performance objectives that were established for the project.
Coring and testing indicated the minimum 28 day unconfined compressive strength of the DSM within the treatment zones was 179 psi (minimum 120 psi required). The average strength for any compete core, in the treatment zone, was found to vary between 322 and 1,177 psi (minimum 300 psi required). Core recovery average 96 to 100 percent within the treatment zone (minimum 85 percent required) and the percent of unmixed soil ranged from 0 to 15 percent (maximum 15 percent allowed).