C153301 Feasibility Study

• Contract/PO #: C153301
• Contractor Name: Cache La Poudre Reservoir Company
• Contract Type: Loan
• Water District: 3
• County: Larimer
• Bill Number: SB 77-35
• Loan Projects - Doc Type: Feasibility Study

II
II
II
,

II

,

II
,

,>
..'-::"

II
I

Three alt.ernate repairs were classified into two categories; ,_',
one (I), partial cutoff of seepage through pervious subsoil
stratas underlying the existing dams, and t.wo (2), complete
cutoff of seepage through pervious subsoil stratas. Both cate-
gories of repairs dre designed to greatly improve the stability,
of the upstream and downstream slopes although the partial
cutoff design would require a more extensive downstream
drainage system to control the possibility of hydraulic up-
lift pressure on the dam foundation and under seepage that
would reduce the stability of the downstream slope. The three
repairs were designed in order that maximum utilization of
materials at the reservoir site could be made.

....-'

,.'

, -'

!I
!

I
I
:.
I
I
I
I
I
I
I
I
~

The three alternates are as follows:

Alternate No. I (PARTIAL CUTOFF)
Estimated Cost. . . . . . .

. . . . . . ...

$1,125,725

Repairs:

a. Construction of an' impervious earth fill cutoff
along the face of Dam No.2. The new upstream
slope would be 3:1 and the existing riprap would
be reused as much as possible. The,downstream
slope would be reinforced to a 2:1 slope with a
pervious fill to ensure an unsaturated condition.
Construction of a toe drain and encasement of the
drainage ditch will l1elp stabilize the downstream
slope by reducing the saturation level. This item
will be the same for repair Alternates 2A and2E.

b. Construction of an impervious earth fill cutoff
along the face of Dam No.1. This would only be
a partial cutoff and is intended at this time to
extend through the layer of the very soft organic
sandy, silty clay that appears to be directly below
the darn foundation. Construction of,this cutoff
should greatly reduce the seepage that is passing
through this very permeable layer. The new upstream
slope would be 3:1 and special consideration as to
the design of slope protection (riprap) will need to
be made as this darn is aligned for severe wind and
wave abuse. The downstream slope would be reinforced
to a 2-1/2:1 minimum slope as constructed by a pervious
fill. This pervious embankment together with construc-
tion of a toe drain will greatly improve the stability
of the downstream ,slope. By extending the impervious
upstream cutoff, only through the organic sandy, silty,

,.

"-'e