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

4.0 Geologic and Ground Motions Studies
4.1 Geologic Study
A site-specific geologic evaluation study was prepared for the District by Mr. James M. Joyce,
PG, CEG, of Joyce Associates, who performed the work as a subcontractor for the prime review
consultant for this project, Alan Kropp & Associates. This geologic study is included in
Appendix D of this report. The purpose of the geologic study is to provide pertinent regional and
local geologic information for use in the seismic stability evaluation of the embankment.
The geologic report states that the seismic activity within the northern Coast Ranges is generally
associated with active faults of the San Andreas system, including major active faults both east
and west of the site. The principal active fault in the region is the Hayward Fault, the main trace
of which is approximately 1,100 feet east of the fill embankment. In the year 2002, the U.S.
Geological Survey estimated there was a 62 percent or higher chance of a large earthquake
occurring in the San Francisco Bay Region by the year 2032.
The geologic study found that within the region, many valleys have been partially filled with
unconsolidated sedimentary deposits of Quaternary age. These deposits, which include alluvium
and colluvium, underlie the gently sloping valley bottoms and hillside swales, and consist of
clay, silt, and gravel. The regional evaluation found that the site is principally underlain by
sandstone bedrock throughout the reservoir area. The western side of the reservoir consists of a
fill embankment consisting mainly of silty clay. The fill overlies a layer of native soil consisting
principally of sandy to silty clay.
The geologic evaluation found no evidence of landsliding around the reservoir, and found no
evidence of creep or movement within the fill embankment along the west side of the reservoir.
Additionally, the study found no indications that active faults cross or project towards the site
and conclude that the risk of fault-related ground rupture in the reservoir area is very low.
4.2 Ground Motion Study
The site-specific ground motion study was developed by Dr. Walt Silva of Pacific Engineering,
who was a subcontractor to AKA. The study is included in Appendix E of this report. The
purpose of this study was to develop the design response spectrum, and three spectra matched
time histories for the Dingee site. The design response spectrum was based on the 84 th
percentile, fault average component of ground motion level. Three time histories were
developed and were matched to the target spectrum. The three time histories were developed
from the 1) Kobe, 2) Kocaeli, and 3) Landers earthquakes. The time histories were used to
estimate the earthquake-induced slope displacements which are presented in Section 7.4.2.
Seismic Stability Evaluation Report
Dingee Reservoir Dam
10
August 2008