Tucson Employs HDPE to Halt Contamination in Record Time
Drew L. Wilson
Tucson, AZ—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 undertaking.
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” and 24” 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 & filtration equipment.
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 infrastructure.
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” 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 material made.”
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.
“McElroy builds the Cadillac of fusion equipment,” said Ernst. “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.”
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.
McElroy PR and Marketing Department