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

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
~

I
~

I

The results of the geotechnical ihvestlgatlon showed the upper 10 to 12 feet
of the, dam to be composed of light brown, slightly silty sand and gravel. The
remainder of the dam Is made of alternating three-to-six inch layers of clean;
multi-colored coarse sand, medium tand, fine sand and gravel with dark gray to
black layers of clayey sand, rich with brown spongy organic material including
wood and plant roots.

The flattening of the downstream slope In 1972 was accomplished with material
of uniformly lower permeabll Ity than some layers I n the dam. In addlti on, it
was placed directly on top of a foot or two of Impervious top soil,
representing the old downstream face of the dam. Therefore, rather than
fonnlng a free-draining buttress to the downstream toe, It acts as a barrier
to seepage and forces fl ow to surface on the downstream face about one-tlli rd
of the way up the slope.

The Impervious blanket placed on the upstream face of the dam during the 1972
repal rs was found to be only a few feet thick and composed of very unstabl e
organic soil. There Is no bedding under the rlprap.

A borehole drilled adjacent to the outlet conduit drop confirmed the
suspicions from previous work that the backfill surrounding the conduit Is a
nest of boulders surrounded by lOOSely consolidated sand.

The dam is located In a "V"-shaped valley Incised Into precambrian bedrock and
Is founded on sound to slightly weathered gneiss. The borings showed the rock
mass to be lightly jointed and reaSonably impervious.

Abundant volumes of glacial till, consisting of clayey sand with gravel; were
located within the reservoir. This material would serve as either Impervious
embankment material or could be processed to produce filter and drain
material, or aggregate for RCC or soil cement. In addition, outcrops of sound
bedrock occur within the reserv~lr and could be quarried for riprap or
concrete aggregate. A deposit of potential impervious material exists at the
extreme south end of the reservoir, but the soil fs nearly a pure silt, and
its use in the embankment would not be recommended.

v