Dingee Reservoir Final Seismic Report - East Bay Municipal Utility ...

8.0 Conclusions and Summary of Dam Performance
The dam fill materials consist of medium dense to dense clayey gravelly sand and stiff to very
stiff sandy silty clay with gravel. Based on our material characterization and liquefaction
potential evaluations, the dam fill materials and native materials within the Dam are not likely to
liquefy and will not undergo significant strength loss due to a seismic event.
The survey monument data at the site indicate some movement of survey monuments D-2, D-3,
and D-4. Some of the piezometer water level readings have increased over time. So far, the
changes in the survey monuments and piezometer levels are within tolerable levels. These
instruments are being monitored and evaluated as part of the District’s routine dam inspection
program.
No known active fault trace is present at the site, thus earthquake-induced ground rupture is not
anticipated. Landslide maps do not indicate any areas of instability at the site, nor were any
observed. Minimum factors of safety for long-term steady state conditions range from 2.3 and
above for the upstream slope to 1.45 to 1.57 for the downstream slope.
The results of the seismic stability analyses show that the dam will perform satisfactorily when
subjected to ground shaking from the maximum credible earthquake of magnitude 7.25 on the
Hayward Fault. The results also indicate about 2-feet of slope deformation and less than 1 foot
of crest settlement. The reservoir is operated about (El. =768’) 5.5 feet below the dam crest
elevation (El. 773.5 feet). Uncontrolled release of water is not expected from a major seismic
event. Crest settlements of less than 2 feet could occur for water storage elevations of 772 feet.
The reservoir water storage is typically held to Elevation 768 feet due to operational needs.
The results of rapid drawdown stability analyses also indicate satisfactory performance of the
upstream slope of the Dam in case of rapid reservoir drainage. Minimum factors of safety for the
short-term rapid drawdown condition range from 3.4 and 2.8 when the reservoir water level is
dropped from Elevation 772 to 768 feet to a factor of safety of 1.9 when the reservoir is fully
drained to elevation 755.9 feet.
Seismic Stability Evaluation Report
Dingee Reservoir Dam
21
August 2008