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The city of Tucson Arizona was recently disrupted by the
largest sewer bypass operation in the nation's history. Over 20 miles of
high-density polyethylene (HDPE) pipe has been fused together in just four and
a half weeks to bypass a ruptured sewer line. The 42-inch concrete mainline,
which was built about 40 years ago, burst in several places and the resulting
deluge caused three massive sinkholes in the downtown area. The repair alone is
estimated at over $15 million, not including property damage.
The major concern for the Arizona Department of
Environmental Quality is the 5'7 million gallons of untreated sewage that
was released per day into the Santa Cruz River during the first seven days of
the spill. The river is temporarily dry at this time. The need to minimize the
environmental impact of the spill, and to complete the bypass before water
returns to the river, was the driving force that spawned this massive
The break was a result of naturally occurring chemical
reactions combined with erosion and forces resulting from natural fluid flow.
First, hydrogen sulfide in the waste material interacts with
bacteria in the steamy conditions inside the pipe to form a sulfuric acid that
literally eats away at concrete. Secondly, the sheer force of the waste flowing
through the pipe has a strong eroding effect on concrete and this pipe went
from being several inches thick to paper-thin before it finally broke. Finally
the first break also occurred where the pipe makes a 90-degree turn where
forces and pressure are magnified.
The bypass consists of four parallel 20-in. and 24-in. HDPE
lines approximately five miles each. The pipeline will handle the flow of
sewage for four to six months while the mainline is being repaired.
"Due to the aging infrastructure in the U.S., there is
a lot of opportunity for bypass jobs because mainlines are beginning to fail at
an increased rate," said Ismael Diaz, Engineer and Project Manager for
Rain For Rent, the general contractor for the project. "We started slowly
using HDPE in the last couple of years for bypass operations and now with this
job, we have jumped in with both feet."
Rain For Rent is based in Bakersfield, California with
offices across North America. The company began as an oilfield supply company
in 1934 and has evolved into a liquid, and solids handling business that also
handles pumps, liquid storage tanks, pipe, roll off boxes, and separation and
The greatest benefit of HDPE is its resistance to chemical
attack and an expected life of over 100 years. HDPE is the only piping material
in existence with a leak free rating and many experts in the industry feel it
is the solution to the industry-wide problems associated with aging and failing
Potable water infrastructure is also experiencing increased
failures associated with age and water leaks can be even more costly. Leaking
water infrastructure reduces flow rates, increases electricity costs for
pumping, cuts profits, increases water treatment costs, creates back-siphon
contamination risk and requires continual maintenance and monitoring.
According to an article published in the July 1994 issue of
Opflow, by American Water Works Association (AWWA), 10'15% water loss in
typical municipal water systems is the accepted standard.
With 47 states experiencing water shortages this year due to
drought situations, this acceptable standard is becoming a problem.
The city of Tucson has used HDPE on two large-scale water
projects in the past. Sixty thousand feet of 16, 24, 42, 48 and 54-in. HDPE
were used to bring water from several wells to a reservoir.
"We hope that the success HDPE has had on the bypass
job will open the door for more polyethylene use in the city," said
Denise Ernst, SW regional HDPE manager of Maskell-Robbins (MR), the pipe
supplier and subcontractor orchestrating the fusion process. "Unless it
is a real high temperature or high pressure application, it is the best piping
Along with the time restraints of the Tucson bypass project,
a challenging aspect was the limited amount of space the pipeline would be able
to utilize. An asphalt bike path that runs along the river was used as the
right-of-way for the pipeline. The four pipelines traveled across four
pedestrian bridges and underneath four roads in tunnels built for bike travel.
The crews were also under strict stipulations not to damage the plant life on
both sides of the path.
"HDPE is the only pipe material that could have been
used for this operation," said Adam Rivas, fusion technician of MR.
"It is flexible enough to handle the curves of the path and follow the
contour of the land. It is also the only material of this size that could been
joined together in this short amount of time."
HDPE is heat fused together and the resulting joint is
stronger than the pipe itself in both tensile strength and pressure conditions.
Unless there is third party damage, there is no chance for the pipe to fail and
further contaminant the area. MR supplied a total of 10 McElroy fusion machines
to fuse the pipe together.
"I am very pleased with the way all of the fusion
equipment performed on this fast-track job because any failure of equipment
could have postponed the entire operation," said Ernst.
McElroy's TracStar 500 was of particular importance on
the location. The TracStar 500 is a mobile fusion machine mounted on a track
system with an onboard generator.
"Without the TracStar machines I don't see how
this job could have stayed on schedule," said Rivas. "It has almost
double the gauge pressure of a conventional machine and the added pressure
coupled with the power of the tracks gives you the force needed to overcome
drag and achieve proper fusion pressure."
For now the city of Tucson can relax with the knowledge that
there sewer catastrophe is being repaired in record time. In many parts of the
country HDPE is being used more and more to repair faulty infrastructure before
huge ruptures cause environmental damage, inconveniences to the public and
large repair bills.