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

7.0 Stability Evaluations
7.1 General Approach to Evaluation
Conventional limit equilibrium slope stability and earthquake-induced deformation methods
were used to assess the performance of the dam. Static and pseudo-dynamic stability evaluations
were carried out for both the upstream and downstream slopes of the dam. Effective shear
strength parameters were used in the static stability analysis. Undrained shear strength
parameters, developed as described in Section 6.4.2, were used in the pseudo-dynamic stability
analysis.
Slope stability analyses were performed using the computer program SLOPE/W, Version 5
(GEO-SLOPE International, 2002). The program uses two-dimensional limit equilibrium
technique and the method of slices, and allows the user to select various methods including
simplified Bishop, simplified Janbu, Spencer, Morgenstern-Price, etc., to calculate the minimum
factors of safety for both circular and block failure surfaces. The results presented for the
upstream and downstream cases represent the surface type yielding the least factor of safety.
Slope stability analyses presented in this report were performed using the Spencer Method. In
the Spencer Method, compatibility of both moment and force equilibrium is enforced, which
provides a higher level of reliability than methods that compute moment or force equilibrium
separately.
7.2 Dam Profile and Cross-Section
A representative dam profile and cross-section was chosen to best represent the surface and
subsurface conditions over the entire site. Plates 3, 4, and 5, respectively, show the plan, profile
and cross-section for the dam. The cross-section analyzed and shown in Plate 5 is considered to
represent the most critical and maximum height of the embankment.
Based on our review of the historic reservoir surface elevations and piezometric levels, two
different water surfaces and corresponding phreatic surfaces were evaluated: Elevation 772 feet
and 768 feet. High water storage elevation of 768 is more representative of the storage levels for
the past 8 years, or since about year 2000. Prior to year 2000, high water storage levels were
around 772 feet. The high water storage level of 768 was selected since this level represents the
upper bound elevation of the reservoir, in the more recent past, present, and most likely and
anticipated future use. The phreatic surface was developed by reviewing the piezometric levels
shown in Plate 5. The piezometric levels fluctuate directly with the reservoir storage levels. The
data shows that piezometric levels were higher prior to year 2000, when the upper bound of
reservoir storage level was around 772 feet, and then decreased after year 2000 as the reservoir
storage levels decreased to upper bound of 768 feet.
The limit equilibrium stability evaluations presented in Section 7.2 were performed using these
two possible water storage and corresponding phreatic surfaces. The plates only contain the
graphical results corresponding to reservoir storage elevation of 768 feet, and not for 772 feet.
Seismic Stability Evaluation Report
Dingee Reservoir Dam
16
August 2008