PROJ00180

• Contract/PO #: C153639L
• Contractor Name: New Cache La Poudre Irrigating Company
• Contract Type: Loan
• Water District: 3
• County: Larimer
• Bill Number: HB 93-1273
• Loan Projects - Doc Type: Feasibility Study

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Stability. The stability of the existing structure was estimated based on rough physical
dimensions of the structure and estimates of material strengths. No drawings of the as-built
condition of the structure are available, and the conditions of the foundation to rock contact
are not known and can only be estimated.

Tests were not conducted on the concrete, and the compressive strength was estimated at 2000
psi. The shear strength of the shale bedrock, for purposes of sliding, was estimated at 380 for
friction angle and cohesion of 1000 pounds per square foot. This is based on our past
experience and testing of the Pierre shale on various projects.

Three conditions were analyzed for stability of the diversion structure similar to those
typically checked on a concrete dam. The three conditions are USUAL, UNUSUAL, and
EXTREME. The USUAL loads include water to the top of the structure; UNUSUAL loads
include the structure overtopped during a flood; and, EXTREME loads are earthquakes with
the water level to the top of the structure. An additional normal load case was checked using
an ice load of 10,000 pounds per linear foot of structure with the water two feet (2') above
the base to simulate the normal winter condition. It would not be normal to have a full pool
behind the structure during winter with ice on the river as the sand gate is always left open in
the winter.

Table V-I below summarizes the conditions and the factor of safety calculated for each
condition.

Table V-I
Stability Analysis for Existing Structure

USUAL-W/O ICE
USUAL- W/ ICE

UNUSUAL

EXTREME

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1.5/4.0

1.65
1.04

0.98

1.41

9.7
2.0

4.6

6.5

1.3/2.7
1.1/1.3

As shown in the table above, the USUAL and EXTREME cases have adequate factors of
safety; however, the UNUSUAL condition and the NORMAL condition with ice both have
somewhat low factors of safety. The UNUSUAL case assumes the gates are not open and that
water is flowing five feet (5') over the structure. Opening the gates would obviously reduce
the pressure significantly and increase the overturning factor of safety . We would expect the
gates would normally be open during a flood event.

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