PROJ00519

• Contract/PO #: C153597
• Contractor Name: Windsor Reservoir and Canal Company
• Contract Type: Loan
• Water District: 3
• County: Larimer
• Bill Number: HB 91-1006
• Loan Projects - Doc Type: Feasibility Study

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

Woodward. Clyde Consultants

A cement-bentonite slurry wall was considered as one method of reducing seepage
through the embankment in the area of the outlet works. The slurry wall would be
constructed using current construction techniques. The wall would extend from the
ground surface down into bedrock, a depth of approximately 70 feet. The cement-
bentonite slurry wall would extend about 100 feet either side of the existing outlet works
and would encompass the outlet pipe.

The second alternative considered was a three part remediation. This alternative would
require the construction of a clay blanket on the upstream face of the existing
embankment. The blanket would be approximately 5 feet in thickness normal to the
slope, approximately 300 feet wide, and would extend from the crest of the dam to the
toe of the embankment. A slurry wall would be constructed near the upstream toe of
the embankment. The wall would be the same width as the clay blanket and would tie
into the blanket. The slurry wall would extend from the ground surface into the
bedrock. This alternative would also require the construction of a buttress on the
downstream face of the embankment. The buttress would be constructed by excavating
the downstream slope to an approximately 2:1 (horizontal to vertical) slope and placing
a sand blanket with a clay cover. The final downstream slope would be approximately
2.7:1.

The third alternative considered includes reconstruction of the existing embankment
above the outlet works. This construction would require excavation of the embankment
from above the outlet pipe, at an approximately 2:1 slope, to the crest with a bottom
width of about 30 to 40 feet. A cutoff collar would installed around the outlet pipe.
The embankment would then be replaced and compacted to the proper moisture and
density requirements. A chimney/blanket drain would be constructed within the
reconstructed embankment above the outlet works. A soil-bentonite slurry wall would
be constructed near the upstream toe, approximately 40 feet either side of the inlet pipe,
and extend down to bedrock. This wall would serve to reduce seepage along the
embankment/foundation interface.

The third alternative, reconstruction of the embankment, was selected to remediate the
seepage conditions at Douglas Dam. This option is the most rigorous but it provides
several benefits the other options do not. It allows for inspection of the outlet works,

22539G/DOUGLAS.DAM 12-17-90IREPORTS3

24