inward at an angle of 30 degrees. Because each bulkhead
is customized, this unusual geometry was not a problem;
Ayres created a design that allowed one caisson in the
bulkhead to flip up at an angle instead of staying vertical.
Another six months is typically necessary for the
construction of the bulkhead, and then it’s ready for
installation.
HOW DOES IT WORK?
Those who have seen the floating bulkhead installed often
compare it to a garage door closing. Each bulkhead is made
up of separate caissons with ballast tanks. To submerge it,
the ballast tanks are sequentially filled with water until the
whole bulkhead is floating in a vertical position. Once it
is in this position, the bulkhead is either pushed or pulled
into place at the gate opening for sealing. The bulkhead
is then positioned and stabilized by powered hoists or
other manual rigging devices. Typically, every bulkhead
ultimately seals on each vertical side and along the bottom
at the gate opening. The manner of sealing and bearing
for the floating bulkhead can vary with each facility;
some bear and seal against existing dam structures, while
others require that side- or bottom-sealing surfaces be
added as an integral part of the dewatering system. The
seal material varies as well and is selected based on the
condition of existing or installed sealing surfaces on the
facility. The Conowingo facility, for example, requires that
Crofton fabricate a custom sealing surface on the floating
bulkhead for each installation.
Variations in the formwork used to construct the dam,
coupled with the age of the concrete, require that the
floating bulkheads be installed without seals at the
beginning of every installation. Divers, using the bare steel
sealing surfaces of the bulkhead as a template, measure
the required thickness of the seals every foot. Following
these measurements, the bulkhead is refloated, the seals
are custom-fabricated from wood and soft foam rubber,
and the bulkhead is reinstalled. The crest gate is then
lifted, and the developing differential pressure compresses
the soft rubber seals into the “nooks and crannies” of the
concrete surfaces of the gate opening, creating an effective
seal.
It doesn’t take long.
“Typically, the process at the Conowingo facility takes one
week to complete. That includes removing the floating
bulkhead from a rehabilitated gate opening, rehabilitating
the floating bulkhead seals, and reinstalling it at a new
Floating Bulkheads Across
the Country Designed by
Ayres Associates
4│TRENDS
crest gate opening,” Feairheller said. “Every facility is
different, and installation times can vary from a couple of
days to a couple of weeks depending on the configuration
of the upstream face of the gate opening.”
COST-EFFECTIVE SOLUTION
“At first when people are looking at the cost of a floating
bulkhead, they say, ‘That much for design and fabrication?
Can’t we do something else?’ ” Rudolph said. “But then
they look at the labor and material costs for a cofferdam
or other methods for their situation or realize they have
a walkway bridge that won’t support heavy machinery,
or find out there are environmental concerns for the
construction of a cofferdam.”
Besides the fast installation, another cost-saving benefit
is the bulkheads’ reusability. Because the seals can be
changed for each use, they can be customized to fit
another dam if it’s similar enough in size and geometry.
People have leased them out to other dams, Rudolph
said, and Xcel Energy used its bulkhead at multiple sites
in Wisconsin. He also recalls one owner in California that
used the same bulkhead on 24 different structures that
used eight different seal configurations in 1998 and 1999.
“The seals were variable, and during the design phase we
actually made a cardboard model and would put it against
all the structures to see if it would work,” he said.
Another collaborative effort between Ayres and Crofton in
Puerto Rico resulted in the adaptation of one of the largest
floating bulkheads for use on multiple water resources
structures in the region, simply by modifying the bulkhead
seals.
Other clients have used the floating bulkheads when they
are required to perform a full-gate opening test by the
Federal Energy Regulatory Commission. If the gates are
open full blown, Rudolph said, it not only wastes water
that could be used to generate power, it could also damage
infrastructure downstream.
“Floating bulkheads are nice because they span the dam,
they’re mobile, relatively easy to install, and reusable,”
Rudolph said. “Five years down the road you might have
something come up, and you have it there for you.”
A rendering shows the underwater
view of a bulkhead being lowered to
the dam face. To see a video rendering
of an installation, visit
http://crofton-survey.com/vid1.html.
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