PROJ00072

• Contract/PO #: C153592
• Contractor Name: West Divide Water Conservancy District
• Contract Type: Loan
• Water District: 0
• County: Garfield
• Bill Number: SB 89-85
• 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

-

Test pit 1 (TP 1) was excavated with the backhoe into the existing dam

embankment to a depth of six feet for ~n in-place density test. The wet density

was found to be 108 pounds per cubic foot. The material was sandy clay

containing basalt gravel and cobbles. Test pit 2 (TP 2) was located at the

upstream edge of the embankment in probably original ground of the landslide

~ material. The soil was lean clay with basalt cobbles and boulders to a depth of

10 feet. This test pit remained dry for several hours during the exploration

even though the edge of the existing lake was about 15 feet away. This

condition suggests the clayey material is quite impervious.

5.4 Geologic Evaluation:

The only geological concern regarding the reservoir's enlargement at the

Alsbury site is the landslide forming the left abutment. The area proposed for

this abutment appears stable at the present time and raising the existing

embankment by about 12 feet and creating a larger reservoir are not expected to

result in stability problems for the following reasons:

1.

The proposed dam construction would involve such a minor extent of the

landslide toe that such construction and reservoir operation would not

be expected to cause renewed movement of the original massive slide

(the slide is about one mile wide and 2-1/2 miles long). However,

small slumps and slides on steep areas of the larger mass have

occurred in recent times, probably in high precipitation years, and

will undoubtedly continue.

2.

The proposed dam would abut a gently sloping portion of the large

landslide. This area appears to have been stable for many years, even

though it has been in contact with both the hypothetical original

landslide-created lake and the smaller, man-made lake built in 1961.

3.

The proposed increase in water surface elevation by 19 feet (from

existing lake elev. 9,042 to new maximum water surface elev. 9,061)

should not effect the local groundwater regime enough to cause

stability problems in the slide mass because the groundwater is

undoubtedly tributary to the creek; that is, the water table presently

rises in elevation away from the creek back into the abutments.

-14-