Precast Gets Tolerant
Tight tolerance requirements on
troughs at a Denver water treatment plant were
best handled by precast concrete.
By Bridget McCrea
As one of Colorado’s
primary water utilities, Denver Water can’t
afford to have any of its water treatment plants
out of commission for any length of time. With
2,499 miles of water pipelines, 17 pumping stations
and 34 underground reservoirs located throughout
the city of Denver, the utility is responsible
for the collection, storage, quality control
and distribution of drinking water to nearly
one-quarter of all Coloradoans. More than one
million people and more than 14,000 fire hydrants
in the Denver Metro area rely on its services
on a daily basis.
Denver Water’s treatment
plants employ conventional process designs consisting
of coagulation/sedimentation, filtration and
disinfection processes. The plants must meet
all the standards set by the state of Colorado
and the Federal Safe Drinking Water Act, according
to Martin Garcia, the utility’s project
engineer. But some of the plants are old –
built back when the utility started operations
in the 1920s.
For Denver Water, opening
a new plant and closing an old one requires
meticulous attention to detail and years of
planning. Four years ago, Denver Water knew
its Marston Water Treatment Plant needed a major
overhaul to bring it up to speed. The plant,
with a capacity of 180 million gallons of water
per day, was built back in 1924. After careful
consideration, the utility took the plunge and
decided to spend $36 million to replace it with
a new facility.
“Marston was getting
old and replacement parts were getting hard
to find,” says Garcia, “so we decided
to tear it down and start from scratch.”
Between 1999 and 2001, Denver Water designed
its new plant, then put the engineering and
construction out for bid in 2001. Some of the
key components were 96 precast concrete water
troughs measuring 2-feet by 23-feet 10-inches
by 2 feet 4 inches. Construction started on
the new plant in 2001 and was wrapped up this
September.
Steve Price, senior project
manager for CDM, an engineering firm based in
Cambridge, Mass., says his company won the bid
to design the project. “They had an aging
infrastructure and wanted to expand and be able
to treat more water,” says Price. “It
was time for a replacement.”
Careful
Consideration
Getting Denver Water’s new plant from
concept to completion meant running the existing
plant while building the new plant to avoid
having to shut down Marston’s 180-million-gallon
production stream. Most critical, says Garcia,
were the stringent tolerances that the project
required. The top edge of the troughs couldn’t
have more than 1/16-inch deflection in their
23-feet 10-inch lengths.
Garcia says the tight tolerances
were necessary because of the water that rises
up in the plant’s filter during the backwash
process. “We want it to fill evenly into
the trough so that we don’t get more water
in one section than in another,” he explains.
“If we do get more in one section, it
can upset the media.”
Price says precast concrete
was the right choice based on the stringent
tolerances and the need for structural longevity
– after all, the new plant would replace
one that lasted for 79 years. “We debated
between fiberglass and precast,” he recalls.
“After several discussions with Denver
Water, we decided to use precast, mainly because
of the longevity factor.”
According to Garcia, precast
concrete has been Denver Water’s “material
of choice” for many years. “Fiberglass
was ruled out early because concrete is much
more durable,” he says, adding that the
“ready to go” aspect of precast
concrete made it much more attractive than the
poured-in-place option.
“When they were delivered
to the site, the troughs were prepped and ready
for installation with no extra work required,”
adds Garcia. “That cut down on the time
involved with, say, poured-in-place troughs
that would need forms, time to cure and everything
else. Using another option would have definitely
added more time to the schedule.”
Mike Vaughn, president of
Vaughn Concrete Projects Inc. in Henderson,
Colo., says Denver Water chose precast not only
for its permanence, but for its ease of installation
as well. “If they had installed fiberglass
troughs, I’m sure they would have had
difficulty in supporting the troughs to minimize
distortion, deflection and anchorage problems,”
he says.
According to Vaughn, the
original Marston water treatment plant was constructed
with concrete wash-water troughs that probably
wouldn’t have met Denver Water’s
new tolerance specifications. Nor could they
have expedited the project the way the precast
troughs did. “Doing it the original way
would have easily doubled the installation time,”
says Vaughn.
Pizzagalli Construction Co.
of South Burlington, Vt., and Construction Consultants
Inc. (CCI) of Falls Church, Va., were joint-venture
partners who handled the general contracting
for the Marston project.
Pizzagalli and CCI got involved
with the project in 2001 after winning the bid
to complete the work. Bryan Rufer, Pizzagalli’s
mechanical project manager, says that for the
troughs, precast concrete worked well because
of the project’s tight tolerance requirements.
“We could have fabbed them on site,”
he says, “but we chose to have them precast
based on both the fabrication tolerances and
the installation tolerances.”
Jumping
Hurdles
If designing and manufacturing 96 precast concrete
troughs and hauling them out to a job site for
installation sounds like a routine job, think
again. Between the tight tolerances and the
fact that an existing plant had to continue
running adjacent to the new plant, the Marston
Water Treatment Plant was no ordinary job for
the engineers, contractors or the manufacturers
involved.
For starters, working at a
busy job site over a two-year period where a
water treatment plant is still running while
a new one is under construction is hard enough.
Vaughn says his company was “always the
first in the gate every morning when the site
opened.” The punctuality allowed the precaster
to back its truck up to a large crane and unload
the troughs before the site got too congested.
“The site being busy didn’t cause
much of a problem for us,” says Vaughn,
“but I know it did for the contractor.”
From the engineering perspective,
Price says the design challenges included the
ability to maintain the existing plant’s
land operations during construction. “We
worked it out with a consensus-type process
with the existing plant operations,” says
Price.
Adding to the challenges,
says Garcia, is the fact that the two plants
were connected by pipes, making it difficult
to get the new plant up and running while maintaining
the existing plant. To circumvent the challenge,they
shut down both plants at two different intervals
to do all the necessary maintenance on the old
plant. Garcia says that overall, the installation
of the 96 troughs went smoothly and that no
extra work was necessary to get them in place.
One change that took place
during the manufacturing process, says Garcia,
was a request from the precast manufacturer
to build flat-bottomed troughs as opposed to
the original V-shaped bottom troughs, although
the internal part of the trough would retain
its V-shape. Denver Water agreed to let Vaughn
Concrete Products Inc. make the design change,
based on the fact that the troughs would be
easier to form and easier to stack.
“We’ve been testing
and backwashing the troughs and haven’t
seen any problems as a result of that design
change,” says Garcia.
Precise
Measures
Vaughn says his company got involved with the
Marston project in January 2002 and wrapped
up its portion of the project early this year.
The project itself was a tricky one, mainly
because of the specified geometry and the precision
with which the tops of each trough had to be
manufactured.
“There couldn’t
be more than 1/16 of an inch of distortion in
24 feet because the units were acting as a weir
for the water to run over,” Vaughn says.
Vaughn and a representative
from Denver Water sat down together at 10:30
one evening to nail down the details and come
to terms on the price. After the meeting, Vaughn
and his team reviewed the structural design,
taking into consideration both the long-term
and short-term deflections to assure that the
structures wouldn’t sag with time, and
that they wouldn’t exceed the strict tolerances.
“We then went ahead
and built the forms for the troughs,”
says Vaughn, adding that another challenging
aspect of the project was the fact that the
structures were going to be fully engulfed with
water. Because of this, the structures had to
be designed in a way that would cradle or support
one end while the other end was cast into a
wall structure.
“That certainly created
a bit of a challenge during the manufacturing
process,” Vaughn says, “but we worked
around that with a good, structurally sound
design.”
Once the project was underway,
Denver Water made another request of Vaughn
Concrete: a smoother finish on the troughs.
Not part of the project’s original specifications,
Vaughn says his company altered its production
and vibration techniques to ensure that the
resultant finish was satisfactory.
“They wanted a considerably
smoother finish than what was specified,”
he says. “After some experimenting, we
were able to accommodate their request and everything
worked out OK.”
Satisfaction
Guaranteed
After four years of careful planning, design
and installation, the new and improved Marston
Water Treatment Plant began full-scale operations
in September, much to the delight of the utility
itself and the numerous companies that played
key roles in replacing the 79-year-old plant
with a more modern, functional version.
According to Rufer, all 96
troughs are serving their intended purpose,
and all were backwashed multiple times before
use. Garcia says Vaughn Concrete did a “very
good job,” particularly when it came to
meeting the stringent tolerances that the project
required.
Vaughn toured the job site
with the contractor after the installation was
completed. “Everyone seemed very happy
with the outcome,” he says. “Denver
Water’s expectations were definitely met
and everything is working well.”
Project Profile
Project:
Marston Water
Treatment Plant Upgrade
Owner:
Denver Water
Architectural/Engineering
Company: CDM,
Cambridge, Mass.
Contractor/Installer:
Pizzagalli Construction Co., South Burlington,
Vt., and Construction Consultants Inc., Falls
Church, Va.
Precast
Manufacturer: Vaughn
Concrete Products, Henderson, Colo. *
*
Vaughn Concrete Products is a certified plant
under NPCA’s Quality Assurance/Plant Certification
program.
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