PROJ00137

• Contract/PO #: C153467
• Contractor Name: Greeley, City of
• Contract Type: Loan
• Water District: 0
• County: Weld
• Bill Number: XB 99-999
• Loan Projects - Doc Type: Feasibility Study

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be classified as semi-pervious, with poor drainage capability. Thus, the
rapid drawdown phreatic surface, shown on Figure V.2, is probably more
realistic than conservative.

2.3.3

Material Properties

The major soil properties used in stability analyses are density and shear
strength. Field and laboratory soil tests, as well as past experience with
similar materials, were employed to arrive at the Twin lakes Dam embankment
material properties shown on Figure V.2.

Densities obtained from relatively undisturbed Shelby tube samples of the
embankment, and field tests using a nuclear density gage, indicate that the
embankment materials were poorly compacted. This loose state was modeled in
the stabi 1 i ty analysi s by usi ng soi 1 densiti es whi ch are approximately 90
percent of standard Proctor maximum density for the embankment material;

Determination of the shear strength parameters is the most important phase of
a stability analysis. It is difficult, however, to obtain representative
samples, avoid sample disturbance, simulate external loading and internal pore
pressure conditions, and avoid inherent test error in the testing methods. It
is typically impossible to obtai n samples that truly represent the range of
materials existing in the field. For these reasons, shear strength parameters
are generally determined from samples representing extremes, and parameters
are selected within the range. Hence, a significant amount of judgment and
experience play an extremely important role in the evaluation of laboratory
test resul ts to ensure that the parameters chosen are representati ve of the
materials in place.

Exploratory borings in Twin Lakes Dam revealed a reasonably homogeneous
embankment, consi sting of silty sand with gravel, founded on bedrock. In
drill hole TL-l, however, a 5-foot layer of silty sand with organics was
di scovered at the embankment-bedrock contact. The range of shear strength
parameters obtained for the dam materials was 0' (effective angle of
internal friction) = 350 and c' (effective cohesion) = 1 psi for the silty

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