REP14926

• Permit No: M2001035
• IBM Index Class Name: Report
• Doc Date: 7/8/2001
• Doc Name: ENGINEERING GEOLOGIC SITE CHARACTERIZATION STUDY TROUT CREEK DAM SITE & RESERVOIR AREA CHAFFEE CNTY
• Media Type: D
• Archive: No

-~ ~S
R. J. Irish
Consulting Engineering
Geologist, Inc.
on the downstream side of the ridges. Floods flowed both east end west over the western ridge et
the damsite. This is evidenced by potholes that face westward on the western side of the crest of
that ridge, and by even larger and more numerous potholes that face eastward on the eastern side
of the crest.
Overtopping of the western ridge by Trout Creek is evidenced also by a southwest-trending
alluvial fan that has its apex near the top of the ridge about 250 feet upstream from the planned
damsite.
Trout Creek, probably late in the Ice-Age, degraded its channel through the gorge section to a
depth of about 25 to 30 feet below the present floor elevation of the stream, which now flows
across alluvial soils (Fig 3}. That channeling probably was facilitated by a set of northwest to
southeast striking, near-vertical joints and possibly by a narrow shear zone that trend through
that topographic section. Moreover, the rock there appears to be particularly pegmatitic {i. e.,
partly very coarse grained},thus was more susceptible to erosion by scouring than the finer
grained varieties of the grenodiorite that neighbor it.
The buried ravine section of the gorge at the planned damsite appears io be asymmetric to the
northeast, with a steeply sloped, possibly terraced northeastern side, and a less steeply sloped
southwestern side (Fig. 3}. A channel a few feel wide and deep may incise the floor of the buried
ravine. This is common for ravines formed in this type of geologic environment.
The alluvial soils refilling the buried ravine section of the gorge at the damsite appear to be
mainly fine to coarse grained, silly sands chat incorporate scattered gravels, cobbles, and
boulders, mainly of granitic rock debris. The fines cronstitute an estimated 15`g of the alluvial
soil, thus soil permeability probably is on the order of 1 x 10 -3 to -4cm./sec. A water level
measurements in Borings DH-M-1 and -2 indicated that these soils are ground water saturated to
about the creek level.
The fresh grenodiorite is a strong rock that requires a strong hemmer blow to break. We
estimate that the unconfined compressive strength of the finer grained variety is on the order of
20,000 to 25,000 psi, whereas the Uc strength of the coarser grained variety is estimated to be
on the order of 15,000 psi. The Uc strength of the pegmatite, a very coarse grained rock wish
relatively weak grain-to-grain bond, is estimated to 6e on the order of 6000 to 1 D,000 psi.
These estimates ere not confirmed by the results for four grenodiorite samples laboratory-tested
for Uc strength {Appendix I },but too few samples were tested to ensure a representative
sampling, and the test results appear to be influenced by microfracturing. Some of the surfaces of
the broken rock evidenced iron oxide steins on crystal boundaries. Even with those structural
weakness, however, even the weakest of this rock has an intergranular strength well in excess of
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