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

21 and 22 for the upstream and downstream slopes, respectively. The yield acceleration under
seismic conditions is 0.45g for upstream slope and 0.15g for the downstream slopes. Tables 6
and 7 provide a summary of the upstream and down stream factor of safety and its variation with
applied horizontal seismic coefficient.
Table 6: Seismic Slope Stability Analysis Results
(Water El = 768 ft) Factor of Safety
Downstream Upstream
Seismic Coefficient
0.10 1.12 2.22
0.15 1.00 1.83
0.20 0.92 1.75
Table 7: Seismic Slope Stability Analysis Results
(Water El = 772 ft) Factor of Safety
Downstream Upstream
Seismic Coefficient
0.10 1.04 2.35
0.15 0.92 2.00
0.20 0.83 1.70
It should be noted that on the two plates, the failure plane has an average slope of about 0.5.
This slope will be discussed in Section 7.4.2, where vertical deformation estimates are made for
the slope deformation estimates. Typically this would imply the vertical component of
deformation is about ½ of the estimated horizontal component.
7.4.1 Yield Acceleration Evaluation
The yield acceleration, K y , for a slope is defined as the value of estimated effective horizontal
ground acceleration at which a potential sliding surface would develop a factor of safety equal to
unity. This value is the minimum pseudo-static acceleration required to produce instability and
any earthquake-induced deformation of the slope.
The yield accelerations for the various scenarios were calculated from pseudo-static slope
stability analyses. Initially, the potential sliding surface searches were performed without any
earthquake forces to find the sliding mass with the minimum factor of safety. Then earthquake
forces, represented by equivalent static horizontal force, were applied incrementally to the
sliding mass until a minimum factor of safety of unity was obtained. The critical slip section
associated with the minimum factor of safety of unity and the corresponding yield acceleration
for each section are shown on Plates 22 and 21 for downstream and upstream slopes,
respectively. Table 8 summarizes the results of the yield acceleration for the critical section for
downstream and upstream slopes.
Seismic Stability Evaluation Report
Dingee Reservoir Dam
18
August 2008