Size of concrete structures
The concrete structures consist of 3- to 5-foot-deep base slabs. Walls are as thick as 12 feet and as high as 50 feet, and suspended slabs measure 1 foot thick in order to accommodate the size specifications needed by the manufacturer of the treatment system. Currently, the base slabs have been poured and work continues on a portion of the walls.
According to Newman, the mass of the base slabs is greater than those at a typical wastewater plant due to the depth of the tanks, approximately 50 feet. Existing site conditions dictate this particular depth because it is intercepting a 72-inch reinforced concrete pipe that feeds stormwater into the pump station.
The majority of the vertical concrete is formed using a combination of handset Symons Steel Ply concrete forming system and crane-set Manto gang forms. Additionally, the suspended concrete decks will be formed using EFCO EZ Deck shoring.
“The large size of many of the placements required careful planning to ensure the concrete could be batched, delivered, tested, and placed efficiently,” says Newman.
The tolerances for the concrete slabs were 1/3 inch over 10 feet, according to Metcalf & Eddy’s design.
A shoring system is expected to be placed mid-September to support the formwork for the suspended concrete slabs and baffle walls. The shoring system towers will be built on their side, then lifted upright and placed with a crane. Once the concrete is placed and it gains the required strength, the shoring system will be lowered and wheels will be attached to allow the shoring towers to be moved under an opening for easy crane removal.
Facing the challenges
Currently, the project is on schedule, although concrete pouring for the pump station did not start on the scheduled date due to the challenges of the 50-foot-deep excavation.
Work on the existing chlorine tank incurred cold weather challenges, which affected the rate of hardening and strength gain of the concrete.
Additionally, Metcalf & Eddy was very strict about not allowing the use of accelerators due to the possibility for additional cracking in the massive concrete. Due to the absence of accelerators, placing the tall wall concrete in cold weather was more challenging due to the slow rate of hardening and therefore the slower permissible placement rates in order to avoid formwork blowouts.
Having faced difficult pouring conditions last winter, Newman Concrete Services scheduled the majority of the concrete pours to occur prior to the coming winter months in order to reduce the effect of cold weather.
“When you get into cold weather concrete, the temperature of the concrete must be maintained during the protection period,” says Newman.
Under the current schedule, some concrete is still likely to be poured in the early winter months this year. The upcoming winter months will provide an additional challenge because the concrete structures will be rising above the ground, therefore the concrete temperature must be maintained during the entire curing period. “This will likely involve blankets, tenting, and heating—the typical precautions necessary in the cold environment,” says Newman.
Corrosion and coatings
Despite the corrosive nature of a wastewater plant, the CSO retrofit at the Nashua Wastewater Treatment Plant is designed to handle only the stormwater overflow, and not everyday waste-water. Therefore, the rebar used for concrete reinforcement in the CSO project is normal black iron as the facility will only be used on an as-needed basis.
In order to prevent water intrusions into the concrete facilities, sheets of bentonite—a clay-based waterproofing blanket—have been wrapped around the entire exterior of the below-grade structures.
One of the considerations for using bentonite was the amount of sand found onsite, which results in high ground-water pressures. The dewatering system, used as part of the excavation for the pump station, is pumping thousands of gallons of water out of the ground. Once the concrete pour for the pump station is completed and the use f dewatering pumps halted, the structure will be encapsulated in water.
As a further measure, the pump station was carefully designed with a number of joints to make sure the contents of the pump station do not enter the groundwater—essentially a two-way waterproofing method on the concrete structure.
A key ingredient
Concrete is a vital component of a wastewater facility, primarily because of its ability to not only resist the corrosion caused by contaminants in the wastewater stream but to also contain the polluted stormwater during a rain event.
In the case of the Nashua Waste-water Treatment Plant, the inability to handle excess stormwater necessitated the CSO treatment expansion at the wastewater plant.
This expansion is crucial not only because CSOs are a danger in terms of polluting local rivers and streams, but the toxic mix of stormwater, sewage, and chemicals that pollute these waterways can have a profound impact on citizens whose health may be compromised by direct exposure to a CSO.
- Owner: City of Nashua, N.H.
- General contractor: Methuen Construction, Salem, N.H.
- Engineer: Metcalf & Eddy
