PROJ00318

• Contract/PO #: C153618
• Contractor Name: Greeley, City of
• Contract Type: Loan
• Water District: 3
• County: Larimer
• Bill Number: SB 87-15
• Loan Projects - Doc Type: Feasibility Study

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embankment materi a1 s were poorly compacted. Thi s loose state was model ed in
the stabil i ty analysi s by usi ng soll densi ti es whi ch are approximately 90
percent of standard Proctor maximum d~nsity for the embankment material.

Determination of the shear strength parameters is the most important phase of
a stabil ity ana1ysi s. It is di ffi cu1 t, however, to obtai n representati ve
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 obtain s~mples that truly represent the range of
materials existing in the field. For these reasons, shear strength parameters
are generally determined from sampll!s representing extremes, and parameters
are selected within the range. Hence, a significant amount of judgment and
experience plays 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 bori ngs in Peterson lake Dam reveal ed a homogeneous sil ty sand in
the upper 10 to 12 feet, and then alternating 3-to 6-inch lenses of silty sand
with gravel, clean gravelly sand, Clayey sand, sandy clay, and low density
peat down to the bedrock foundation. Typical shear strengths of all the
layers containing predominantly sand and/or gravel fall within a relatively
narrow range. From a stability standpoint, these layers can be modeled using
a single shear strength. The OCCasional lenses of sandy clay and peat
represent potential weak seams within the embankment.

Cansol i dated-undra i ned tri axi a1 shear tests were performed on samp1 es from
Peterson Dam containing predominantly sand and gravel. The effective angle of
internal friction (~I) for these samples ranged from 39 to 44 degrees with 0
to 2 psi effective cohesion (c'). These values are at the upper bound of the
typical range of shear strength for thi s type of materi a1. It is al so
alarming that these high friction angles were obtained for such poorly
compacted soils.

Because of the granular nature of the primary materials comprlslng the
embankment, soil cohesion should not be depended on for shearing resistance;

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