September/October 2021 - 17

Dam Conversion
Constructability and dam safety were the main technical
challenges for the project team that included Stantec as the
engineer. The new hydroelectric facilities had to be
constructed within the body of the existing dam while
avoiding any impact to active flood control operations.
Design of the project began in 2010 with a feasibility study,
followed by preliminary design and a geotechnical
investigation program. Detailed design, including of the
temporary works (e.g., cofferdams and retention systems),
began in 2012. Although the construction contract was
awarded in 2014, high water levels caused delays that meant
substantial completion occurred in October 2020.
Cofferdams: Constraints were placed on the design of the
upstream cofferdam due to its proximity to the dam's spillway
and large fluctuations in the reservoir level. A 95 ft (29 m) high
cofferdam extending 40 ft (12 m) above the normal reservoir
level would have been required to ensure year-round
protection of the intake structure and upstream penstock
construction. Such a cofferdam was considered impractical
and unrealistic for construction. Ultimately, the cofferdam for
the intake structure and upstream penstock construction was
designed to provide 8 ft (2 m) of protection above normal
reservoir levels, which allowed extended periods of
construction, but necessitated flooding of the upstream work
when water levels were elevated.
Final Design - Upstream Project Features
Existing
Dam Crest /
HWY T-15
Cutoff Wall
Buried Penstocks
(Upstream, between
intake and
gravity dam)
Existing
Spillway
The powerhouse construction area located at the
downstream toe of the dam was protected by a cellular
cofferdam founded directly on rock. This cofferdam was
designed to provide protection against a 100-year event. Since
the cellular cofferdam was founded on a unit of friable
weathered sandstone, weak and fragmented rock was
removed with an excavator prior to constructing the cells. A
concrete base seal was placed to address the potential for the
rock foundation to erode when subject to under-seepage
across the base of the cells, and to seal potential gaps below
the sheet piles due to the uneven rock surface.
Diaphragm Walls: The upstream diaphragm wall along
the side of the new intake channel permanently supports a
69 ft (21 m) tall cut in the embankment dam. It was designed
as a cantilevered wall without tieback anchors to avoid the
potential for hydrofracturing or otherwise damaging the core
of the embankment
dam during anchor
installation. The wall
was effectively designed
to minimize
deflections and thus
reduce the potential for
related cracking of the
clay embankment. It
was constructed using
abut t ing T-shaped
elements up to 130 ft
(40 m) ta l l . These
heavi ly reinforced
elements included 5 ft
(1.5 m) wide stems that
extended 16 ft (5 m)
into the embankment
from the back of the 5 ft
(1.5 m) thick face of the
diaphragm wall.
T-shaped panel geometry
Diaphragm Wall
Approach Channel
Upstream
Existing Spillway
Approach Wall
Intake Structure
Direction of water flow
The upstream cofferdam consisted of a concrete work
platform constructed on the sloping face of the dam and
founded on steel piles. Lightweight concrete along with preexcavation
into the slope were used to minimize applying
new loads to the dam's existing spillway approach wall. The
temporary excavation support walls for the intake structure
excavation and upstream penstock excavations formed the
seepage cutoff.
Guide wall for upstream diaphragm wall construction
DEEP FOUNDATIONS * SEPT/OCT 2021 * 17
September/October 2021

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