90-inch Hot Tap and Line Plug

Project owner: Northwest Suburban Municipal Joint Action Water Agency
Profile: Provides 10 billion gallons of water yearly to 500,000 people in northwest suburban Chicago. Maintains main and booster pumping stations, delivery structures, 20 million gallons in storage, and 55 miles of transmission main from 12 inches to 90 inches in diameter.
Employees: 14
Project budget: $8,159,000
Project cost: $8,193,000
Project timeline: February 2015 – June 6, 2016
Project manager: T.D. Williamson Inc. (TDW)
Structural design, construction management, field engineering, materials QA/QC, schedule monitoring, site safety: Graef
Thrust-restraint system: Benchmark Construction Co. Inc.
Safety consultant, construction scheduling: J.A. Watts Inc.
Materials testing: GeoServices Inc.

In 2014, the Illinois Tollway Authority kicked off a $2.5 billion project to widen and/or rebuild 62 miles between Rockford, the state’s third-largest city, and the world’s fourth-busiest airport outside Chicago.

Built in the 1950s and 1960s, the Jane Addams Memorial Tollway (I-90) enabled northwest suburban Chicago to grow and prosper. After more than a half-century of wear and tear, its owner is ensuring the road continues supporting prosperity. Some portions are being expanded to six lanes, others to eight. Inside shoulders are being widened to accommodate public transit and interchanges are being reconfigured.

Inevitably, other infrastructure agencies are being affected by the work.

One is Northwest Suburban Municipal Joint Action Water Agency in Elk Grove Village. Since the 1980s, the agency has provided seven municipalities in northwest suburban Chicago with potable water. The City of Chicago Department of Water Management pumps finished water 16 miles to the agency’s pump station near O’Hare International Airport, where additional sodium hypochlorite is added to maintain chlorine residuals. The agency then pumps the water to the municipalities’ distribution systems via 55 miles of transmission main ranging from 12 inches to 90 inches in diameter.

Many of those miles parallel the tollway.

Maintaining 125 psi to keep faucets running

The agency is a governmental corporation overseen by a board consisting of the seven member municipalities. It collects no tax; all revenue comes from consumer charges contributed by the member municipalities.

Agency managers had dealt with operational issues related to recent runway expansion at the airport, but that project was smaller and less complex. Now they had to figure out how to deliver water while almost seven miles of transmission main were relocated.

Traditional relocation methods would have required significant rerouting of the main, which would result in longer reconstruction lengths and the abandonment of a serviceable main. T.D. Williamson Inc. (TDW), a pipeline solutions provider in Tulsa, Okla., and Chicago engineering firm Graef teamed up to deploy TDW’s STOPPLE pipeline-isolation system instead.

Anyone who works for a public water system knows what hot tapping is: isolating a section of pipeline by cutting and then plugging two holes in the pipe. The concept of hot tapping and line plugging to accommodate bypass lines isn’t new and the STOPPLE system is 60 years old. However, this is the third time globally the system was used with a pipeline of this size (90-inch diameter) and composition (bar-wrapped, steel-cylinder, reinforced concrete pressure pipe) under 1.3 million pounds of force on the plugging heads.

In addition to requiring fewer additional pipe segments and conserving water by draining and treating a smaller section of pipeline, the STOPPLE system eliminated a year of construction time – a crucial benefit when working next to a high-volume tollway.

Protecting joint integrity during tapping

Working with pipe this large in such a dangerous location presents unique design and constructability issues. Throughout the project development phase, TDW and Graef met with tollway and agency managers to discuss the team’s approach to executing the scope of services required. They covered design constraints, construction pitfalls, and potential courses of action if planned conditions weren’t met in the field.

In addition, water agency managers were familiar with hot tapping, having been through a similar procedure during O’Hare’s runway expansion. But the process is outside normal business operations for tollway managers, so TDW/Graef maintained open lines of communication to share knowledge, address concerns, and provide updates on progress, execution, and performance.

Several major challenges required planning and engineering workarounds to execute the tap within the time allowed by the agency’s demand cycle.

Lengths of bar-wrapped, steel-cylinder, reinforced concrete pressure pipe are joined with steel spigot rings fitted with rubber gaskets and grouted to seal the connection. Through review of installation schedules and joint details, it was believed that deflection of more than 1/16th an inch at the tapping location would compromise the joint’s seal. More than that could create a leak that would be extremely difficult, if not impossible, to control. Thus, preserving joint integrity during tapping was paramount.

To create a bulkhead that arrested plugging head forces during tapping, the TDW/Graef team designed and built a thrust-retaining system (TRS) comprised of caissons placed on either side of the pipe and encased in concrete. The TRS was adopted as a design criterion to create a zero deflection system to preserve joint integrity.

Working around the inevitable: unforeseen field conditions

Scheduling was another major concern. Because shutting down the main wasn’t an option, TRS construction and manufacturing and installation of tapping fittings were closely monitored.

Demand by the agency’s seven member municipalities varies seasonally. They offer some capacity or system redundancy via reservoirs and smaller interconnected mains, but there was no way to replicate the volume delivered through this pipeline should the tap fail or a joint rupture.

Agency managers gave community distribution system managers a heads up in advance of the tapping so they could fill their reservoirs and plan for intermittent service outages while the pipe was cut and the line stop inserted. After verifying the sealing element was holding, the pumps were returned to normal operation once the bypass went live. Water service to hospitals, schools, restaurants, fire departments, and homes wasn’t interrupted.

Once the bypass system was designed and tested, crews moved ahead with the tapping plan.

During all critical fitting installations, standby crews were available onsite with additional parts and shipping crews were available to transport pieces that may have required modification to TDW’s manufacturing facility in Tulsa. When the inner pipe cylinder was found to be out of round, several pieces were shipped back to Tulsa and modified, returned, and reinstalled to create the proper sealing to allow the pipe to be tapped and bypassed.

Meanwhile, agency managers worked to avoid service interruptions and/or delays while meeting TDW/Graef criteria for a successful operation. Flows and pressures had to remain constant at 125 psi. A main break or fire in one of the seven member municipalities could compromise the entire procedure, so employees were on duty around the clock in the field and at the main pump station to monitor the system for deviations and oversee the tapping procedure.

At $8,193,000, the final construction cost came within 1% of the $8,159,000 estimate. Even though several extensions were issued due to delays related to other work in the corridor, the project was completed earlier than scheduled with all milestones met by the contractually obligated dates.

Most people have no idea about the risks involved in relocating pressurized pipelines. No one would have known about the project unless something went wrong. The fact that they nothing did earned the project an American Council of Engineering Consultants Water Resources Honors Award in February 2017.