POWER September 2021 - 23

TOP PLANT
there were no flat or gently sloping areas
available from which heavy equipment
could operate, nor vehicular access to the
area of the future intake structure, which
complicated the construction of the earth
retaining features. Temporary work platforms
and access roads had to be constructed
on the upstream face of the dam
to facilitate the work, " Andrews said.
The construction of the upstream diaphragm
wall, which is one of the largest
walls of its type in the world, presented
unique challenges. Given the confined
working area, careful oversight was needed
to stage the work properly. For example,
transporting the 130-foot-long, 90-ton
reinforcing cages to the work zone, executing
critical lifts to take the cages from
horizontal orientations to vertical, and
then positioning the cages for installation
in the diaphragm wall required significant
coordination. Quality control of the specialty
concrete mixes, welding, and other
project components was also critical for
successful completion of the project.
Design and Implementation
The RRHP's upstream diaphragm wall
along the new intake channel supports a
69-foot-tall cut in the embankment dam
without anchors and was designed to
minimize deformations that could otherwise
potentially induce cracking of the
dam's clay embankment. The wall was
constructed using abutting T-shaped elements
up to 130 feet tall. These heavily
reinforced elements included five-footwide
stems that extended 16 feet into
the embankment from the back of the
five-foot-thick
face of the diaphragm
wall, which made the T-shaped wall elements
a total of 21 feet across.
The construction sequence for each Tshaped
element required three overlapping
passes-or " bites " -of a hydromill
trench cutter to achieve the design geometry.
Each bite consisted of a vertical
pass of the 10.5-foot-long by five-footwide
hydromill cutter head from the
ground surface to the bottom of the rock
socket. The rock embedment depth of
the upstream diaphragm wall was established
to minimize deformations and
extended up to 35 feet. A self-consolidating
tremie concrete mix with an initial
set time of 16 hours and negligible bleed
was developed to facilitate the extended
duration required to place concrete for
each T-shaped element.
" Each of two intake openings are fitted
with a wheel gate capable of closing
against flow for protection of the unit, "
said Andrews. " The feature is vital in the
September 2021 | POWER
event of a turbine runaway speed condition
due to failure of the turbine wicket
gates to close and also in the event of a
penstock rupture. "
The gates are vertical-lift, fixed-wheel
gates and each closes an opening approximately
18 feet wide by 21 feet high.
The wheel gates are also capable of closing
the intake under balanced pressure
when necessary for turbine maintenance
and were used to close the intake water
passages during construction of the penstock
dam penetrations.
Two circular reinforced concrete penstocks
with 21-foot interior diameters convey
water from the intake structure to the
powerhouse. The two penstocks are sized
to provide a velocity of 7.7 feet/second at a
full flow of 10,235 cubic feet/second.
At the intake structure and powerhouse
connections, the penstocks transition to a
rectangular section. The two penstocks
penetrate through the concrete gravity
dam monoliths adjoining the spillway. The
penstocks are steel-lined downstream
and designed to withstand operating
pressure-including transient pressures
from a load rejection of the units-and to
resist buckling when unwatered.
Penstock construction required excavations
up to 70 feet deep within the existing
dam and its foundation. In addition to the
importance of minimizing deformations to
maintain the integrity of the existing dam,
the design of the excavation support systems
had to account for the differential
loading from the embankment dam slopes.
The extensive excavation support included
braced excavations supported by
steel combi-walls (sheet piles interlocked
with steel beams). The penstocks penetrated
through two adjoining concrete
gravity dam monoliths immediately adjacent
to the spillway. Wire-saw cutting was
used for each of the 24-foot-wide octagonal
penetrations through the concrete
dam monoliths, which were designed to
be self-supporting during construction.
Successful Completion
Design work began in 2011, but the construction
contract and onsite work did not
start until 2014. During the RRHP, more
than 2.3 million onsite worker hours were
logged, with about 175 workers involved
at the height of construction. Skilled laborers
and operators were hired out of the
local union hall to complete all necessary
tasks, and 39 separate contractors were
utilized during the project. The overall job
incident rate was 1.9, which compares
favorably to the national average (2.8). At
least some of that safety success can be
www.powermag.com
POWER POINTS
Winning Attributes
✔ The successful addition of
hydroelectric generation to a
previously existing non-powered dam
provides a model for similar projects.
✔ Construction adjacent to an
operating spillway required a
focused surveillance and monitoring
program to maintain safety and
reduce impacts to the dam's
integrity.
✔ Construction of the upstream
diaphragm wall-one of the largest
walls of its type in the world-
required innovative engineering and
extensive coordination.
attributed to pre-activity meetings that
were regularly held by the engineer with
contractor teams prior to significant and/
or particularly challenging tasks.
Engineers were surprised by one discovery
early in the construction phase.
" While grouting around the land-side
perimeter during routine pre-excavation
work, the construction team encountered
a voided zone in the rock created by previously
undetected gypsum, " said Andrews.
" The voided zone had originally been partially
infilled with soil and rock fragments,
but it was still characterized by artesian
flows. These flows came from grout holes
and reached volumes of 20 gallons per
minute and higher. " The solution involved
a combination of extensive cement and
chemical grouting to treat the voided rock.
The RRHP also experienced extensive
flooding of the upstream work during
construction, which delayed completion.
Periods of high water were expected,
but it was not practical to construct a cofferdam
to protect against the full range
of possible fluctuations. The reservoir
level in 2018 and 2019 exceeded expectations
and remained elevated for several
months. Stakeholders worked together
to mitigate and manage the issues, avoiding
unnecessary cost and time extensions,
and also minimizing the collateral
effects to other portions of the work.
Wet commissioning of turbine generator
units was completed in July 2020,
and substantial completion of the project
was acknowledged on Sept. 2 that year.
The facility was declared commercially
complete on June 10, 2021. ■
-Aaron Larson is POWER's
executive editor.
23
http://www.powermag.com

POWER September 2021

Table of Contents for the Digital Edition of POWER September 2021

Contents
POWER September 2021 - Intro
POWER September 2021 - Cover1
POWER September 2021 - Cover2
POWER September 2021 - Contents
POWER September 2021 - 2
POWER September 2021 - 3
POWER September 2021 - 4
POWER September 2021 - 5
POWER September 2021 - 6
POWER September 2021 - 7
POWER September 2021 - 8
POWER September 2021 - 9
POWER September 2021 - 10
POWER September 2021 - 11
POWER September 2021 - 12
POWER September 2021 - 13
POWER September 2021 - 14
POWER September 2021 - 15
POWER September 2021 - 16
POWER September 2021 - 17
POWER September 2021 - 18
POWER September 2021 - 19
POWER September 2021 - 20
POWER September 2021 - 21
POWER September 2021 - 22
POWER September 2021 - 23
POWER September 2021 - 24
POWER September 2021 - 25
POWER September 2021 - 26
POWER September 2021 - 27
POWER September 2021 - 28
POWER September 2021 - 29
POWER September 2021 - 30
POWER September 2021 - 31
POWER September 2021 - 32
POWER September 2021 - 33
POWER September 2021 - 34
POWER September 2021 - 35
POWER September 2021 - 36
POWER September 2021 - 37
POWER September 2021 - 38
POWER September 2021 - 39
POWER September 2021 - 40
POWER September 2021 - Cover3
POWER September 2021 - Cover4
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