Construction of a $168 million expansion of a water treatment plant in Mesa, Ariz., hit a major milestone to start the year, but not before McCarthy Building Cos. overcome a possible blockage.
The builder began work on the first phase, which includes building a second on-site reservoir, in January 2024, according to a press release sent to Construction Dive. The new 8-million-gallon tank measures 234 feet by 234 feet and is 25 feet tall and sits just south of the facility’s existing reservoir, which McCarthy also built nearly two decades ago.
But due to the soft nature of the soil in the surrounding area, the general contractor based in St. Louis knew the project could lead to potential settlement problems and leaks down the road.
To overcome this, Black & Veatch, the engineer of record, McCarthy and Overland Park, Kansas, adopted a leak detection system under the reservoir. This meant the implementation of several proactive strategies on site, according to the press release.
For example, crews placed a layer of No. 57 rock under the entire reservoir footprint and installed four-quadrant drill pipes tied to a reservoir. The work will allow the Mesa City Council to identify the area of origin if any leaks are detected in the future.
This sub-slab system is a marked improvement over traditional tank designs, according to McCarthy.
“Our team was able to anticipate many of the project’s challenges, but there were still some very unique circumstances for the project team, which would not have happened successfully if not for the cooperative and collaborative relationship between all project participants,” Gray Wensley, senior superintendent of McCarthy’s water team, said in the statement. “From deep excavation and complex shoring to the leak detection system and raised deck, the team found solutions and delivered a high-quality structure in a very tight location while keeping the plant fully operational throughout.”
According to McCarthy, effective preconstruction planning accelerated the construction schedule by two to two and a half months. The team also used space in a nearby city park to store approximately 20,000 cubic meters of excavated material, which avoided the costly export and re-import of backfill.
“This reservoir is a critical piece of our long-term water operations strategy for the East Valley, and it has been delivered with the kind of care and innovation we expect in a project of this importance,” Chris Hassert, director of water resources for the city of Mesa, said in the statement. “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 job required about 8,300 cubic yards of concrete, which McCarthy’s concrete experts poured into 40-foot by 40-foot squares and sequenced diagonally to allow for expansion joints. McCarthy hopes to complete that portion of the project in October.
Construction of a new reservoir next to an existing one
McCarthy and Black & Veath used virtual design and construction modeling to plan shoring, ground anchors, slopes and crane locations, according to the release. The excavation phase extended approximately 30 feet below grade, with approximately 12 feet of separation between the new and original reservoirs. Crews also built a bank of live fiber optic conduits between the two bodies of water.
The second phase of the project will double the plant’s treatment capacity from 24 million to 48 million gallons of water per day, according to the release. The planned work includes the addition of a flocculation system with mirror sand ballast, ozone generation and sodium hypochlorite generation for disinfection, according to the release.
McCarthy expects to complete the second phase of the project by early summer 2027, according to the contractor.
