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Any time you set out to build a large-diameter wastewater pipeline that is nearly 5 miles long and as deep as 20 feet, you are bound to face some challenges. But at the $41.2 million Plant Interconnect project in Pima County, Ariz., the design and construction team met those challenges and completed the big gravity-flow pipeline with no major problems.
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Pima County’s wastewater reclamation system serves the greater Tucson area, with a population of more than 1 million, and covers approximately 2,000 square miles. A portion of that area — 473 square miles — is served by two wastewater reclamation facilities (WRF). One of them, the Ina Road WRF, stands 4.6 miles north of the Roger Road WRF.
The Plant Interconnect pipeline connects the two reclamation plants. Construction of the big pipeline is part of Pima County’s Regional Optimization Master Plan, which includes an expansion of the Ina Road WRF to 50 million gallons per day (mgd), construction of a new 32-mgd reclamation campus at the old Roger Road WRF, odor control facilities, and decommissioning of the old 41-mgd Roger Road WRF.
The Plant Interconnect pipeline consists of 17,600 feet of 72-inch-diameter pipe and 6,600 feet of 60-inch-diameter pipe. The pipeline will carry an excess flow averaging 36 mgd (peak 81 mgd) from the new Roger Road facility north to the Ina Road plant. The county chose Hobas pipe — a centrifugally cast, fiberglass reinforced polymer mortar pipe — for the project. The Plant Interconnect project brings the total number of installations in the Tucson area in recent years to nine with this product.
“We briefly explored PVC pipe, but they don’t make it that big,” said Jaime Rivera, project manager for the Pima County Regional Wastewater Reclamation Department. “So that narrowed our search to two choices: a PVC liner for concrete pipe or Hobas. We had some issues with constructability of the PVC liner, so it was less risky to use Hobas. It is corrosion resistant with no added-on lining.”
Time-saving project delivery
By using the Construction Manager At-Risk (CMAR) delivery method, the county was able to slash four to six months from the time it takes to go from design to construction. With the CMAR process, Pima County selected the engineer — Brown and Caldwell — at the same time it chose a construction manager, the Sundt-Kiewit joint venture. Sundt-Kiewit subcontracted the pipeline construction to Don Kelly Construction, Bozeman, Mont.
Rivera said the conventional design-bid-build process takes four to six months just to advertise the project for bids, select the low bidder, get the low bidder approved by the County Board of Administration, award the contract, and issue a Notice to Proceed. Instead, the county selected the designer and construction manager at the same time and set a maximum amount it could spend to build the Plant Interconnect. The construction manager set $25.7 million as its guaranteed maximum construction cost — and, in fact, completed the project for less than that amount. (The $41.2 million cited earlier includes non-construction costs such as right-of-way, design costs, and program management.)
During the design stage, Hobas engineering staff assisted in preparation of lay drawings to identify the locations for fittings and deflected pipe joints to maintain the designed alignment, said Rene Garcia, EIT, senior engineering associate at Hobas. “We also provided input to the proposed installation method, and suggested trench details that would be easily constructible and provide the needed support for the pipe,” Garcia said.
Design was completed in January 2009, and the construction Notice to Proceed was given that April. Construction began shortly thereafter, and reached substantial completion by December 2010.
The project presented challenges in the form of two washes — river beds that are dry most of the time but flood occasionally and erode away the channel over time. The big pipeline had to cross under those washes with sufficient cover that floods would never reach the pipe’s crown. James Nichols, project engineer for Brown and Caldwell, said the county had two choices for building the wash crossings.
For one, it is possible to build an expensive grade-control structure directly over the pipeline. This structure would hold sediment upstream of the pipeline and would only allow channel degradation downstream of the structure in order to protect the pipeline. That structure exceeded the county’s budget. Therefore, it was decided to use two siphons to carry wastewater under the washes.
When a siphon is installed to go beneath an obstruction or river channel, Nichols said the flow goes from a gravity flow with the pipe partially full to a pressurized flow with the pipe completely full. The velocity of the wastewater needs to be high enough to move solids up the slope on the downstream leg. The siphons — constructed to depths of about 35 feet — consisted of dual 42-inch steel pipelines with welded joints and special linings and coatings for corrosion protection.
Gravity flow carries the wastewater from south to north. “At the very northerly end, we connected to a pipe elevation that was fixed,” Nichols said. “So we could not go any deeper at that point. We had to connect into that starting point since the county did not want to consider installing a pump station.
“In order to go beneath the washes with a total gravity system, we had to use a very flat slope on the pipe going back toward the beginning point on the south end,” he added. “When you have a very flat slope, the pipe has to be larger to carry the same amount of flow. The required pipe diameter was about 84-inches for the gravity system. We were able to get under the two washes, but with only a few feet of cover.
Ease of installation
Don Kelly Construction began construction at the north end, and used two crews to work southward, digging the trench and burying the pipe.
Kelly used 28-foot-long trench boxes stacked as necessary for the depth. Production in the sandy silt soil averaged about 250 feet per day per crew over the length of the project. “There were days when we worked the shallower reaches where a crew would average 400 feet per day,” said Jeff Hagen, Kelly’s general superintendent for the project.
The Hobas pipe fit together well, Hagen said. “We generally lowered pipe into the ditch with a strap. Depending on the situation, you can pull it home with that strap to make the joint, or you can push on the end of the pipe with the excavator to join them together. It takes a bit of force because you want to make sure you get a good seal. The 60-inch pipe takes [about] 6,000 pounds of force to join the two pipes together.” Hobas uses gasket-sealed FWC couplings to join the pipe.
Rivera noted that for reaches that could have steeper grades, the engineer could specify the smaller 60-inch pipe instead of the 72-inch pipe because the water is flowing faster.
Every joint in the Hobas pipe passed an air test of 5 pounds per square inch (psi) as required by the Arizona Department of Environmental Quality, Rivera said.
Hagen said the larger Hobas pipe was tested for deflection by using a wooden cross to measure any deflections. Smaller pipe could be tested with a mandrel. All pipe passed a 5-percent deflection test. And finally, due to the tight grade requirements, the county ran a closed-circuit television camera down the pipe, following some water poured into it, to measure dips and peaks. “We have to make sure there are no excessive dips in the line,” Rivera said. “Our standard is 1.75 inches and all pipe passed that test as well.”
The pipe was bedded in 6 inches of imported sand and backfilled with sand as much as 1 foot over the crown. Kelly compacted the sand backfill alongside the pipe with a tamping compactor to provide side support. “The sand works well to get compaction so that those pipes will not deflect when you finish backfilling over the top of them,” Hagen said. “As long as you have side support, the Hobas pipe will not over-deflect on you.”
Daniel C. Brown owns TechniComm, a Des Plaines, Ill.-based communications business specializing in engineering and construction topics. He can be contacted at danbrown4@msn.com.
