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

We have tabulated the results for both reservoir surface levels in this section. For estimating the
long-term and current and future seismic stability, we have elected to use storage elevation of
768 feet.
7.3 Long-Term Static Stability
For long-term static stability, a drained analysis with steady state seepage was analyzed for both
upstream and downstream slopes of the dam. For each slope condition, both deep and shallow
failure conditions were evaluated. The analyses were performed using unit weights and effective
strength parameters of the materials shown in Table 4A and 4B, an assumed phreatic line based
on maximum recorded data (since year 2000) of the existing piezometers, and an average-high
reservoir water level (at Elevation +768 feet).
The results of the long-term static stability analysis are shown in Plate 18 for the upstream and
Plate 19 for the downstream slopes. The lowest factors of safety against long-term static stability
for the upstream and downstream slopes are presented in Table 5 below.
Table 5: Long-Term Static Slope Stability Analysis Results – Factors of Safety
Section Upstream Downstream
A-A’ (Water EL = 772 ft) 2.74 1.45
A-A’ (Water EL =768 ft)
1.57
2.31
Block Surface
(See Plate 18)
(See Plate 19)
A-A’ (Water EL =768 ft) NA
1.67
Circular Surface
(See Plate 20)
7.4 Short-Term Seismic Stability
A pseudo-dynamic approach was utilized for evaluation of slope stability under seismic
conditions. In this approach, earthquake forces are represented by an equivalent dynamic
horizontal force which consists of the estimated effective horizontal ground acceleration
multiplied by the mass of the potential slide material. For earthquake loading conditions, the
analyses were performed using static total strength parameters as the undrained shear strength for
the materials as shown in Table 4B. No phreatic line was used in these undrained loading
analyses.
Based on the in-situ blow-counts and strength test results, the dam fill materials, native materials,
and the sandstone bedrock are not anticipated to undergo strength loss during cyclic or seismic
loading. As mentioned previously, typical strength increases for rapid loading can be 20 percent
or greater; however, a conservative approach of no strength increase was used for this project.
In the pseudo-dynamic stability analysis, the result is the yield acceleration, K y , an effective
horizontal ground acceleration at which a potential sliding surface would develop a factor of
safety equal to unity. The results of the pseudo-dynamic stability analysis are shown on Plates
Seismic Stability Evaluation Report
Dingee Reservoir Dam
17
August 2008