PROJ00462

• Contract/PO #: FS0036X
• Contractor Name: Idaho Springs, City of
• Contract Type: Loan
• Water District: 0
• County: Clear Creek
• Loan Projects - Doc Type: Feasibility Study

This crack formation is not uncommon in extremely cold climates and
under the right conditions of soil and water. The fine grained soils of the
embankment exert moderately high capillary forces tending to lift water above
a water table and still have a high rate of capillary conductivity. The high
phreatic line within the core material feeds the mechanisms of ice lense
formation which in our opinion causes the crack in the dam.

As the frost line penetrates the downstream slope, small volumes of
water in the soil pores freeze. In freezing; the soil is dried and its capillary
potential decreases. Water from below with a higher potential flows to the
low potential area and encounters the freezing temperatures and changes to
ice crystals. This action continues, changing ice crystals to layers of ice
pushing apart the soil mass at the interface.

In the spring this crack is noticeable because the ice lense melts and
returns to the seepilge flow within the core material. The penetration of the cJ-:.
crack is dependent on the depth of frost penetration into the dam sheIl. It is
IikeIy the depth of this crack is 10 to 12 feet depending on reservoir water
level in the winter. At this depth, the reservoir water temperatures and
frost penetration meet within the dam core.

"'-- \u. \.~:
~~
'l

Seepage

The seepage from the dam is of particular concern in the two areas of
noted cascading water. At the trickling water stations (1 + 92 and 6 + 37)
the underlying rock penetrates upward as shown on Sheet 3. The rock materiaI
is assumed from the drill test data. The drill holes penetrated the central
core to refusal, indicating a rock layer or boulder. However, the depth to
the trickle indicates the foundation rock to have no relation to the zone of
seepage.

The electrical resistivity method was employed to ascertain a change
in material density and possible high water passages in the dam material.
Figures 1 through 11 of the soils report present this data. A good embankment
will have an increasing layer resistivity with increasing depth.

An interpretation of the electrical resistivity data is explained by re-
viewing the curve on Figure 6 of the soils report. The embankment has good
compaction to depth of 16 feet; uniform same density from 16 to about 24 feet;
then a flat straight line from 26 to 30 feet indicates a very hard surface such
as rock; then a decrease in resistivity indicating less density with possible
moisture increase; and at 44 feet depth a rapid decrease in density indicates
a definite water concentration.

III-7