September/October 2021 - 20

wall on the top of an approximately 18 to 24 in (45 to 60 cm)
layer of soil. It appeared that the grout increased the density of
the soil, with the overlying limestone acting as a confining
roof above the zone.
During rock excavation, angled drain holes were drilled
from the excavation face down into the voided horizon to
relieve water pressures behind the grout prior to excavating
through the zone. Many of the drain holes produced little to no
flow. However, some had outflows up to 20 gpm (75 L/min),
which flowed throughout excavation. The grouting program
and drains installed during excavation were effective in
controlling and monitoring the seepage flows.
Controlled Blasting: Drill and blast rock excavation for the
powerhouse and tailrace channel was performed within an
area protected by the downstream cofferdam after
overburden excavation and consolidation grouting were
3
completed. Approximately 26,000 yd (19,900 m ) of rock was
excavated to a depth of approximately 40 ft (12 m) below the
foundations of the existing spillway stilling basin wall,
cellular cofferdam and anchored secant pile wall. Given the
close proximity of the rock excavation to these structures,
blasts were designed to achieve peak particle velocities less
than 2 in (5 cm) per second and measured at multiple points
around the excavation. After a successful test blasting
program, blasting was successfully performed within 5 ft (1.5
m) of these structures by utilizing blast control measures
such as added line drilling, boosters, reduced burden and
spacing and lower pounds per delay.
3
The excavated rock walls were generally observed to be
undisturbed by the blasting methods. Where shotcrete was
placed to protect the weathered upper sandstone unit, it was
similarly undisturbed by blasting. After excavation, the
patterned rock bolts originally specified for excavation
support were reduced to spot bolting for much of the
Downstream view of completed powerhouse, to right of spillway
excavation to take advantage of the quality of the vertical
faces and favorable joint spacing and orientation.
Dam Safety Monitoring: A surveillance and monitoring
program was used to evaluate the performance of excavation
support systems, cofferdams and the existing dam during
construction. By combining different instrumentation -
including survey points, inclinometers, piezometers, pressure
cells and strand load sensors - readings could be evaluated in
multiple ways and compared with anticipated behavior.
Conclusion
The Red Rock Hydroelectric Project on Iowa's Des Moines
River presented several unique challenges in the design and
construction of a new hydroelectric facility within and
through an active flood control dam. The potential dam safety
ramifications necessitated a series of sophisticated design
analyses, a range of specialty geotechnical construction
methods and a robust dam safety surveillance and monitoring
program - along with teamwork and coordination during
construction - to enable this unique renewable energy project.
With the completion of the hydroelectric project in 2020,
the dam now provides up to 55 MW (55,000 kW) of clean relia,
ble
power to the surrounding communities and will generate
approximately 178,000 MWH (178,000,000 kWH) annually.
Thomas G. Andrews, P.E., is a vice president for the Stantec Power & Dams
group out of its Chicago office. He has over 22 years of diverse project
experience in civil and geotechnical engineering for hydroelectric power and
dams. He was the project manager for the Red Rock Hydroelectric Project.
Rachael V. Bisnett, P.E., is a senior associate and civil engineer for the
Stantec Power & Dams group out of its Charlotte, North Carolina, office. A
geotechnical engineer with 11 years of domestic and international
experience, Bisnett has focused her work on the analysis, design and
construction of dam modifications. She was the project technical lead for the
Red Rock Hydroelectric Project.
September/October 2021

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