Trusting the Data: Condition Assessment Helps Arizona Town Extend the Life of Critical Main

Black & Veatch measures the extent of damage on this excavated bar-wrapped pipe to plan repairs.

By Jason Montgomery, George Schaaf, Britt Klein & Bethany McDonald

On a sunny day in May 2022, crews in Gilbert, Arizona, unearthed the crown of a critical water transmission main. Removing cracked concrete mortar from the outside of the bar-wrapped pipe revealed what the team expected to find. The pipe’s reinforcing bars were intact, but there was significant corrosion on the steel cylinder.

The team knew where to dig and what they might find thanks to planning, collaboration, and data collected by a robot. The Town of Gilbert leveraged innovative technology to see the invisible — the condition of their buried infrastructure. For Gilbert, infrastructure integrity is a key priority when planning for their community’s water future.

Assessing for the Future

Gilbert was known historically as the hay shipping capital of the world. This once agricultural community is now a fast-growing suburb located southeast of Phoenix. Water is precious in the Sonoran Desert, especially with current water shortages on the Colorado River. The Town of Gilbert is committed to sustainably managing water resources and infrastructure so their community can continue to thrive.

The Town’s 1,500 miles of pipelines are essential to delivering a reliable supply of water to customers across four pressure zones. Gilbert’s Water Distribution Division proactively manages these buried assets to reduce main breaks and protect continuity of service. Their asset management program relies on measuring risk and using that information to prioritize capital and maintenance projects throughout the distribution network.

The Town of Gilbert proactively addressed an issue with joint separation after identifying a water leak at this pipe joint.

The Zone 1 Transmission Main is one of Gilbert’s most critical pipelines, providing approximately 23 million gallons per day to northwest Gilbert and its vibrant Heritage District. In 2019, the Town began a capital improvement project that would enhance reliability and maximize the life of this critical asset. The project included immediate improvements along 4.5 miles of the main as well as a comprehensive condition assessment that was a first-of-its-kind effort for Gilbert.  

Laying the Groundwork

The Town of Gilbert and engineering firm Black & Veatch planned the project starting with a series of tests and improvements to protect the Zone 1 Transmission Main, benefit operations, and make it easier and safer to access the pipeline for future cleaning, maintenance, and inspection.

The team analyzed the main’s thrust restraint design and evaluated the need for a cathodic protection system. The Town’s contractor, Achen-Gardner Construction, completed improvements including replacing isolation valves, air release valves, and drains while adding new system interconnects and manways in strategic locations.

The window of time the Zone 1 Transmission Main could be out of service was limited due to system demands, so the Town of Gilbert shut the pipeline down in stages to facilitate the project. Using real-time pressure monitoring data, Black & Veatch built and calibrated a model to ensure the water system could accommodate the shutdowns while maintaining adequate system pressures and fire flows. The Town conducted extensive outreach to notify the public of the project and water cloudiness that can occur during shutdowns. They also proactively flushed the system as needed to minimize any discoloration issues.

With the line out of service, it was the ideal time to complete a full condition assessment and make any necessary repairs. The Town of Gilbert and Black & Veatch partnered with Xylem to inspect the Zone 1 Transmission Main for leaks and pipe wall deterioration over three phases between 2021 and 2022.

The Zone 1 Transmission Main is constructed of bar-wrapped pipe of several different diameters, including 24-inch, 36-inch, and 48-inch pipe. Bar-wrapped pipe is a composite material that consists of an inner mortar lining, a steel cylinder, steel reinforcing bar wraps, and an outer mortar coating. Together, the steel bars and cylinder provide the pipe’s strength, and corrosion of these structural elements can lead to failure. Bar-wrapped pipe failures often begin as leaks.

Listening for Leaks

Using the free-swimming SmartBall® platform, Xylem inspected the transmission main for leaks while the pipeline remained in service. The project team prepared for the inspection by modeling the water system and performing trial runs. This extensive planning and coordination helped ensure system demands were met and a consistent flow rate was achieved to both propel the tool and collect quality data.

A significant step in the planning process was to identify and test all side outlet valves along the inspection route. This enabled the project team to successfully time the opening and closing of each valve to propel the equipment and keep the free-swimming tool on its intended path.  

The team inserted SmartBall into the pipeline through a check valve at a booster pump station within the water treatment plant. The pumps were turned on with field crews standing by, and the inspection was underway. The team tracked the tool as it rolled through the pipeline with the flow for just over 7 hours and extracted it from a fire hydrant 4.5 miles downstream.

The SmartBall platform is extracted from the Zone 1 Transmission Main after inspecting the pipeline for leaks. 

The SmartBall platform uses acoustic technology to listen for leaks. Analysis of this audio data indicated a small leak, less than 2 gallons per minute, on the 48-inch pipeline within the water treatment plant. Black & Veatch investigated the leak in the field and found a circumferential crack in the mortar diaper and separation at the joint.

The Town of Gilbert decided to investigate additional joints upstream and downstream of this location to determine if thrust forces were causing joint separation. Black & Veatch performed an internal visual inspection of the area. They identified multiple broken welds at downstream joints and recommended a series of repairs to address the issues.

Inspecting the Pipe Wall

The PureRobotics platform is inserted into the Zone 1 Transmission Main to locate and quantify pipe wall deterioration.  

In coordination with planned shutdowns, Xylem assessed the structural condition of the pipeline using its PureRobotics® platform. The tool uses electromagnetic technology to identify pipes with steel cylinder corrosion and/or broken bar wraps. The robot’s high-resolution camera also captures visual defects inside the pipeline.

The PureRobotics platform identified five pipes with deterioration — less than 1% of the 769 pipe segments inspected. One pipe with cylinder wall loss and broken bar wraps was flagged as an immediate risk. The pipe was verified to have a failed mortar coating that revealed significant underlying structural damage, including broken bar wraps and a through-wall defect in the steel cylinder where only the liner remained intact. Due to the extent of the damage, the Town of Gilbert decided to replace the pipe while it was excavated. Quick action by the Town and project team likely prevented a catastrophic failure adjacent to the Western Powerline Trail and commercial and residential properties in the popular Heritage District. The Zone 1 Transmission Main supplies the majority of water to the largest pressure zone in Gilbert. A failure on this pipeline would have placed extreme stress on the water system and on operations staff responding to the emergency, likely impacting service and public trust.

Black & Veatch validated inspection results on the four other deteriorated segments. These locations were repaired by welding a new steel cylinder around the outside of the pipe, commonly known as a butt strap or patch plate. 

The Town of Gilbert made a series of proactive repairs, including this butt strap repair, to extend the life of the Zone 1 Transmission Main.

Trusting the Data

The Zone 1 Transmission Main is a critical asset that supplies drinking water to a growing community. The Town of Gilbert leveraged pipeline condition data to identify immediate concerns and complete proactive repairs while the pipeline was shut down. By inspecting and addressing at-risk pipes, the Town prevented water loss, service disruption, and emergency repairs while extending the life of the pipeline and saving ratepayer dollars. It would have cost an estimated four times more to perform emergency repairs had the deteriorated pipes failed. This project also provided valuable lessons learned as the Town of Gilbert further develops their transmission main assessment program and potentially expands their water reuse distribution system.

Inspection data indicated that the vast majority of the Zone 1 Transmission Main was in good condition and could continue to operate into the future. Knowing the current condition of their buried infrastructure, the Town of Gilbert is better able to prioritize capital investments that preserve critical infrastructure, sustain operations, and continue to provide a high level of service to customers.

Jason Montgomery is Senior Project Manager with the Town of Gilbert, Arizona; George Schaaf is Senior Project Manager at Xylem; Britt Klein is Business Development Manager at Xylem; and Bethany McDonald is Condition Assessment Practice Lead at Black & Veatch.

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