Construction on the $168 million expansion of the Signal Butte Water Treatment Plant in Mesa, Arizona, has reached a milestone with the completion of a new 8-million-gallon potable water reservoir. The new infrastructure is a vital part of the ongoing effort to increase the plant's capacity to meet the growing water needs of the East Valley.
McCarthy Building Companies Inc. is leading the construction, with Black & Veatch serving as the engineer of record for the multi-phase expansion project. The plant, located on 84 acres near Elliot Road and Signal Butte Road, is undergoing a two-phase construction process designed to keep the existing plant operational throughout the expansion.
Phase I, which began in January 2024 and is expected to conclude in October 2026, includes the addition of the new reservoir, as well as critical system redundancies to improve reliability. This phase will also lay the groundwork for the Phase II expansion, which is slated for completion in the summer of 2027.
The new reservoir, measuring 234 feet by 234 feet and standing 25 feet tall, sits adjacent to an existing reservoir McCarthy built nearly two decades ago. Constructed with approximately 8,300 cubic yards of concrete, the project presented a number of challenges due to the proximity of the existing infrastructure. To address these challenges, McCarthy and Black & Veatch used virtual design and construction (VDC) modeling to plan for excavation, soil nails, and crane placement while minimizing disruptions to the surrounding area. The excavation, which extended roughly 30 feet below grade, was complicated by the narrow 12-foot gap between the two reservoirs, with a live fiber-optic duct bank running between them.
The new reservoir, measuring 234 feet by 234 feet and standing 25 feet tall, sits adjacent to an existing reservoir McCarthy built nearly two decades ago
“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.
The soft soil surrounding the area, known to cause settlement and leakage issues, led to the implementation of a forward-thinking leak-detection system. The system includes a layer of #57 rock beneath the entire reservoir footprint and perforated pipes in four quadrants, which are connected to a sump. This innovative approach enables the City of Mesa to quickly identify any leakage areas.
“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.”
The new reservoir's completion ahead of schedule, by an estimated 2 to 2.5 months, means the City can begin using it sooner than anticipated. The construction team also managed to reduce costs by using a nearby City park to stockpile approximately 20,000 cubic yards of excavated material, avoiding the need to export and re-import backfill.
Looking ahead, Phase II of the expansion will increase the plant's treatment capacity from 24 million gallons to 48 million gallons of water per day. The planned upgrades include new sand-ballasted flocculation systems, ozone generation, sodium hypochlorite disinfection, additional filters, and improvements in solids handling.
