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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The shear strength parameters selected to represent the entire embankment were
0' = 350, with c' = O. Using this lower effective angle of internal
friction assumes that as potential failure surfaces pass down through
horizontal weak seams in the embankment, the average friction angle mobilized
along the failure surface will be reduced.

An additi onal analysi s was performed assumi ng a potenti a1 hori zontal weak
layer exists near the base of the embankment, having shear strength parameters
of 0' = 250 and c' = O. These parameters were selected from the lower
bou.nd of a typi cal range of shear strengths for sandy cl ay. Thi s assumpti on
was made because of the stratified nature of the embankment revealed by the
bori ngs.

2.3.4

Failure Surface

There are two common failure surface configurations used for stability
analysis. A circular arc failure surface is more applicable for analyzing
essenti ally homogeneous or zoned embankments, founded on bedrock or thi ck
deposits of fine-grained materials. A non-circular failure surface; described
by linear segments, is generally more applicable for zoned embankments on
foundati ons containi ng one or several hori zonta1 or nearly hori zontal weak
layers. Non-circular failure surfaces can also be used for relatively
homogeneous dams, which contain continuous well-defined horizontal weak layers.

The fail ure surfaces shown on Fi gure V. 3 represent a wi de range of potenti al
failure modes for Peterson lake Dam. Each surface was located using a
critical slip plane search routine in the computer program IPCSTABl5". For
the upstream rapid drawdown stability, the top of the slip planes were
confined to near the dam crest to represent a critical failure, which could
resul tin loss of the reservoi r. For the downstream steady-state stabil ity;
various critical failure surfaces were located for different heights above the
foundation. The base of the downstream failure surfaces was limited to an
el evati on correspondi ng to roughly the top of the outl et works wall. Usi ng
the out1 et channel bedrock e1 evati on as the control 1 i ng base for the fail ure
surfaces would not result in a representative cross section for analyzing the
entire downstream slope.

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