Diversion Tactics
Beneath a busy Phoenix street,
a precast concrete stormwater diversion structure
saves time and money over the long haul.
By Don Procter
Judging a precast concrete
vessel by its price tag can be an unfair and
inaccurate means of assessing its cost effectiveness.
A case in point is a precast stormwater diversion
structure installed under a busy Phoenix street
last year. While the tender price of the precast
vault was higher than that of a cast-in-place
concrete vessel, at the end of the day precast
proved less expensive.
The massive $75,000 diversion
structure, which included two 6-foot by 24-foot
precast boxes, was part of a $26 million wastewater
treatment plant upgrade. The structure was made
of a high-performance concrete by manufacturer
Utility Vault, a division of Oldcastle Precast.
The company, based in the suburban Phoenix community
of Chandler, and project engineer Greeley and
Hansen LLC of Phoenix had to overcome a series
of unusual challenges to ensure the structure
would work.
The idea was to design a diversion
stormwater structure for three stormwater pipelines
that could support the weight of a heavy sanitary
sewer. The problem: The project was being done
30 feet below the middle of the busy Phoenix
intersection at 19th Avenue and Durango Street.
To minimize traffic disruption, the city stipulated
that two of the four lanes had to remain open
to traffic and the job had to be completed within
60 days.
After several months of careful
planning, the building team set to work and
shaved a month off the city’s two-month
requirement, says Rich Meyer, project superintendent
of Archer-Western Contractors Ltd., contractor
for the job. Much of the credit for the speedy
finish goes to the precast concrete structures,
which were placed in one day. A similar cast-in-place
concrete system would have taken three weeks.
The job wasn’t without
its complexities. Buried 30 feet below grade,
the diversion structure had to carry the weight
of the new sewer line and heavy vehicles from
a nearby ready-mix plant, a precast plant and
some specialty industrial equipment manufacturing
facilities. “It is not unusual for us
to see six or seven fully loaded (ready-mix
trucks) come across this intersection,”
notes Jeff Cowee, project manager of Greeley
and Hansen LLC. “It had to be a very stout
structure and it had to be put in place very
quickly. It was a complex job.”
One of the contract stipulations
was that the diversion structure had to rely
strictly on gravity, not a pump station, to
move stormwater, and no standing water was allowed
within the structure. To meet the objective,
Utility Vault manufactured the two 24-foot-long
precast concrete boxes connected by bridges
that were large enough to accommodate the water
coming through the pipe.
One technical innovation was
the engineering and construction of the middle
riser ring section of the vault. The large pipe
penetrations that cut into the riser required
complex three-dimensional finite element analysis
to accurately establish all of the forces on
each section of the structure, explains Michael
Hawes, engineering manager of Utility Vault.
With the aid of Finite Element Analysis Software,
the precaster’s engineering team developed
a cost-effective structure with precise tolerances.
Once completed, the correct amount of rebar
was specified to distribute the moments correctly
and provide adequate shear reinforcement.
But there was another challenge.
“The pipeline we were diverting had to
be straight, but Murphy’s Law was at work
and the line had a little bend. It was enough
of a bend to make our work a bit difficult,”
says Hawes. What saved the company from working
overtime to re-engineer the structure to fit
the bend was the flexible modular design, which
accommodated the in-field inaccuracies. “Basically
we could slide the vault around a bit to fit
the bend.”
Before installing the vault,
the existing stormwater line had to be cut.
Hawes says as a precautionary measure the opening
of the line with the least flow was dammed with
sandbags and two large water pumps were set
up to transport excess stormwater through temporary
pipes that ran up and over the excavation site
and back down into the downstream end of the
stormwater line. The idea was to keep the work
site as dry as possible.
In theory the plan was a good
one. However, no one expected the sudden surge
of water that came through the pipeline which
“blew out” the sandbags and filled
the work area with 2 to 3 feet of water. “Everyone
got wet, but the structure stayed in place,”
Hawes notes, adding that the flood would have
devastated a cast-in-place concrete vessel under
construction. Initially the team expected a
day to clean up the mess, but in less than an
hour work was back on track.
A 140-ton truck-mounted crane
operated by Marco Crane placed each of the massive
precast sections, some of which weighed as much
as 60,000 pounds, in the 30-foot hole. Meyer,
who has never seen a structure that big in the
30 or so years he’s been in the business,
says the parking lot of an adjacent truck stop
was used as a staging yard. Traffic was held
up for 10-minute intervals during the placement
of each section over the day-long operation.
The structure is made of 7,000
psi, self-consolidating concrete (SCC). SCC
was still a relatively new product in the precast
industry when Utility Vault went into production
for the job last year, although now many precast
manufacturers make products using SCC.
Precast concrete won the day
in Phoenix and, in fact, proved that the initial
price tag does not equate to final cost. Dollars
aside, commuters experienced minimal delays
on a busy street, making precast concrete the
clear solution for the project.
Project Profile
Project:
Stormwater Diversion
Structure
Owner:
City of Phoenix
Engineers:
Greeley and Hansen
LLC, Phoenix
Contractor:
Archer-Western
Contractors Ltd., Chicago
Precast Manufacturer:
Utility Vault, a division
of Oldcastle Precast Inc., Chandler, Ariz.
