PROJ00077

• Contract/PO #: FS0008X
• Contractor Name: Longmont, City of
• Contract Type: Loan
• Water District: 0
• County: Boulder
• Loan Projects - Doc Type: Feasibility Study

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to geologic factors.
The abutment and foundation rock at Bluebird provide a good arch d~~
.
site. The strength and deformability of the rock mass are relatively high. The
orientations of the discontinuities in the rock mass are favorable with respect
to sliding. Susceptibility of the foundation and abutment to "plucking" during
an overtopping flood event appears low. The rock mass is semipervious.
Five rock samples were tested in unconfined compression and values ranged
from 11,130 psi to 20,280 psi with an average of 15,310 psi. This can be
characterized as medium to high strength rock. Two samples were tested for
modulus of elasticity with results of 8.5 and 8.6 million psi (tangent modulus
at 50% ultimate strength). Tensile strengths of the same two samples were 1,330
and 1,990 psi. The rock can be characterized as dense with an average density
of 178 pcf.
The modulus of deformation of the rock mass was estimated at one million
psi. This was based on a correlation (Deere, 1967) for intact laboratory modu-
lus vs. in-situ modulus. A correction factor of 0.12 was used based on low RQD
(Rock Quality Designation) values. In addition, values reported in the litera-
ture for modulus measured by in-situ testing for similar rock types were sur-
veyed and compared (Rocha 1964, Deere 1967).
RQD for the core ranged from 0 to 90 and averaged 50 which can be
described qualitatively as fair. The RQD values are unduly biased, however,
because hole B-2 intersected one near-vertical joint for much of its length.
The orientation of jointing within the abutments and foundation is
favorable with respect to sliding. Based on the measured joint orientations and
on an average friction angle of 300 along these joints, sliding is not kinemati-
cally probable.

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