Innovation in Global Power - Parsons Brinckerhoff

Hydropower – New Technologies, New Considerations
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Deck Slot Cutting and Tainter Gate Remediation
Extend Safe Operations of a Hydroelectric Dam
By Marc Buratto, Boston, Massachusetts, 1-617-960-4973, buratto@pbworld.com; Anthony Plizga, 1-617-960-4972, plizga@pbworld.com; and
Paul Shiers, 1-617-960-4990, shiers@pbworld.com
Cutting expansion slots cut
into a dam’s concrete spillway
deck is one of the
structural repairs that may
be necessary to ensure
that aging hydroelectric
dams operate reliably during
storm or flood events. PB
oversaw implementation of
such a solution for a dam
where structural movement
caused problems with gate
operations. The authors
tell about the slot cutting
and gate remediation, and
measures taken to protect
water quality during the
process.
Figure 1: The Yadkin Project’s
Narrows Dam in North Carolina.
1 A Tainter gate is a type of radial-arm
floodgate used in dams and canal
locks to control water flow.
Narrows Dam is one of four dams that comprise the Yadkin Project, a hydroelectric plant
owned and operated by Yadkin Division of Alcoa Power Generating, Inc. The Narrows facility
includes four units with a total capacity in excess of 110 MW. It generates more than half the
hydropower for the Yadkin Project.
Completed in 1917, Narrows Dam is a concrete gravity structure with a maximum height of
61 m (200 feet). It consists of a main dam section and a bypass spillway structure (Figure 1).
The main spillway deck has an integral concrete slab and steel beam support system spanning
between spillway piers. It is restrained laterally by a non-overflow section at one end and an
intake structure at the opposite end. No expansion joints were included in the original design
of the spillway deck. Twenty-two Tainter gates are installed atop the main spillway to release
surplus water during storm or flood events. Each is 7.6 m (25 feet) wide by 3.7 m (12 feet)
high. 1
The lack of expansion joints caused a portion of the deck slab adjacent to the intake to buckle
under normal thermal conditions in the early 1990s. This buckling caused a redistribution of
built-up forces in the deck to the nearby piers, resulting in the movement of the pier caps
towards the intake. As a result, several Tainter gates could not be fully opened during the
five-year full-open gate testing exercise in 2001, causing concerns about safety. Continuous
dam safety is mandated by the Federal Energy Regulatory Commission (FERC), the licensing
agency, so a remediation program was enacted to restore full opening of all the Tainter gates.
PB was responsible for the engineering and construction oversight of the remediation effort.
Our recommendation called for cutting new slots into the deck and rehabilitating the gates. In
addition to the actual slot cutting, the effort involved:
• Installing instrumentation to monitor deck and pier movements
• Mobilizing and setting up cutting equipment, environmental
compliance equipment and support systems.
• Developing a quality control inspection program to make sure the
field activities were completed correctly during construction.
The Slot Cutting
Three methods of cutting were considered—the diamond wire saw,
concrete saw, and overlapping drill core methods. Most dam sites use
an overlapping drill core method to re-establish expansion joints due
to deck configuration, construction joint layout and other issues, such
as embedded conduits. In this situation, the configuration of the deck
and slots ruled out the use of the concrete saw method, and aesthetic
issues associated with the overlapping drill cores ruled out that
method. The wire saw method allowed the most favorable option for removing the saw
cable if binding occurred during cutting due to slot closure. Removal could be accomplished
by simply making another wire saw cable to cut through the bound cable without significant
demolition activities adjacent to the cuts. A 1.6-cm (0.625-inch) diamond wire saw was used
to cut the slots.
Slot cutting work was confined to the deck of the main spillway, right non-overflow section,
and the deck between the intake and bypass spillway. One slot was cut first at the right
41 PB Network #68 / August 2008