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

6.0 Dam Material Properties
6.1 General
The dam and its foundation soils consist primarily of three types of materials:
1. Dam Fill
2. Native Material
3. Sandstone Bedrock
Plates 2, 3, and 4 show the site plan, profile, and cross-section of the dam along with the subsurface
stratigraphy. The dam fill materials consist of interbedded medium dense to dense
clayey sand and stiff to very stiff lean clay and sandy clay. As mentioned; this dam was built in
1894, making it 114 years old. Because the dam was constructed by the Oakland Water
Company, the District does not have plans or specifications used for the construction of this dam.
The dam fill materials were likely obtained from the native on-site materials.
Table 3 provides the average moisture contents, densities, void ratio, and saturation values for
the fill, native material, and sandstone. Based on these average material parameters, the degree
of compaction of the Dam fill materials is estimated to be 85 percent. Although considered low
for current state of practice (90 percent or higher), the relative compaction of the dam fills was
typical for construction performed in the late 1890's. The actual soil properties were used in the
analyses so that a determination of adequacy could be made. Shear strength results are presented
in 6.4 of this report.
6.2 Classification and Index Tests
Grain size distribution and Atterberg limits determinations were performed on numerous samples
of the dam fill, native material, and sandstone bedrock in both previous and present studies. The
results indicate that the dam fill materials predominantly consist of medium dense to dense
clayey gravelly sand and stiff to very stiff sandy silty clay. The native material deposits
represent both alluvial deposits as well as residual soils that have been weathered in place.
Distinguishing the two is practically not feasible at the site, and therefore, the alluvial soils and
residual soils have been combined and are referred to as native materials. The term native
material is also consistent with the term used by the consultants AKA for projects they have done
in the vicinity. These native materials have a moisture content that is slightly higher than the fill
materials and an in-situ unit weight that is lower than the Sandstone Bedrock. Index properties
of the three materials are described below.
Dam Fill Material
The in-situ moisture contents of the dam fill varied from 9.2% to 27.6%, with an average of
19.7% and a standard deviation of 4.5%. In-place dry unit weights of the dam fill ranged from
96.6 to 117.5 pounds per cubic foot (pcf), with an average of 106.9 pcf and a standard deviation
of 5.6 pcf. The average void ratio and degree of saturation are 0.6 and 90.2 %, respectively.
Seismic Stability Evaluation Report
Dingee Reservoir Dam
12
August 2008