Construction on the $168 million expansion of the Signal Butte Water Treatment Plant in Mesa, Ariz., has reached a major milestone with the completion of a new 8-million-gallon potable water reservoir. McCarthy Building Companies Inc. (McCarthy) is serving as the general contractor and Black & Veatch is the engineer of record on the multi-phase plant expansion project.

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Located on 84 acres north of Elliot Road and east of Signal Butte Road, the project is being delivered in two phases to keep the existing plant fully operational. Phase I, which began in January 2024 and is expected to complete in October 2026, includes construction of a second reservoir, key system redundancies to enhance reliability, and provisions that will support the plant’s future Phase II expansion, which is expected to complete in early summer 2027.

The new 8-million-gallon reservoir sits just south of the facility’s existing reservoir, which McCarthy also constructed nearly two decades ago. The new structure is approximately 234 feet by 234 feet and 25 feet tall, requiring approximately 8,300 cubic yards of concrete for McCarthy’s concrete experts to pour in 40-foot by 40-foot squares and sequenced diagonally to allow for the expansion joints. Building the new reservoir adjacent to an existing reservoir presented several unique challenges:

  • McCarthy and Black & Veatch used virtual design and construction (VDC) modeling to plan shoring, soil nails, slopes and crane locations so the team could safely excavate without impacting the existing infrastructure or nearby park and roads.
  • The excavation extended roughly 30 feet below grade, with only about 12 feet separating the new and existing reservoirs and a live fiber-optic duct bank in between.
  • Specialized soil-nails with shotcrete shoring and engineered beam supports were installed to protect the duct bank while crews tied in new piping below and connect it to the facility.

“Having had the experience of building the first reservoir, our team was able to anticipate many of the project challenges, but there were still some highly unique circumstances for the project team to address, which would not have happened as successfully had it not been for the cooperative and collaborative relationship between everyone on the project,” said Gray Wensley, senior superintendent for McCarthy’s Water team. “From the deep excavation and complex shoring to the leak-detection system and elevated deck, the team found solutions and delivered a high-quality structure on a very tight site while keeping the plant fully operational the entire time.”

Due to the soft nature of the surrounding area’s soil that has historically caused settlement and leakage concerns, the City and engineer also adopted a forward-thinking leak-detection system beneath the reservoir and several proactive strategies were implemented:

  • A Layer of #57 rock was placed under the entire footprint of the tank.
  • Perforated pipe was installed in four quadrants and tied back to a sump, allowing City staff to pinpoint the source area if any leakage is ever detected.

This sub-slab system, which required civil and concrete crews to place and shape rock and geotextile fabric largely by hand, is a notable enhancement compared to traditional reservoir designs.

Additionally, due to effective preconstruction planning, the construction team was able to accelerate the reservoir schedule by an estimated 2 to 2.5 months, allowing the City to put the reservoir into service earlier than originally anticipated. The team was also able to utilize space at a nearby City park to stockpile roughly 20,000 cubic yards of excavated material, avoiding the costly export and re-import of backfill. 

“This reservoir is a critical piece of our long-term water operational strategy for the East Valley, and it’s been delivered with the kind of care and innovation we expect on a project of this importance,” said Chris Hassert, City of Mesa Water Resources Director. “The team found ways to save time and money while adding new tools, like the leak-detection system, that will help us manage this asset for decades.”

As work continues on the broader Signal Butte Water Treatment Plant expansion, the completion of the new reservoir marks a major step forward in ensuring Mesa can reliably serve current customers and accommodate future growth across the East Valley.

Phase II of the project will double the plant’s treatment capacity from 24 million to 48 million gallons of water per day. Planned work includes adding a mirrored sand-ballasted flocculation system, ozone generation and sodium hypochlorite generation for disinfection, six additional filters as well as solids handling improvements. In addition to increasing treatment capacity, Phase II will further enhance plant operations and resiliency for Mesa’s growing East Valley service area. 

McCarthy’s National Water Group constructs large-scale, complex collaborative delivery water and wastewater projects in communities throughout the U.S., including numerous projects in the East Valley. McCarthy’s Water Group partners with municipal, governmental and private clients to help them process wastewater, address plant upgrades and deliver cost-effective clean water to the communities they serve.