By using precast concrete products, a new project in California was completed quickly and efficiently with value engineering.
By Matt Werner
Combined sewer overflows are slowly becoming a remnant of the country’s early infrastructure. New wastewater and stormwater management projects have given cities the ability to treat wastewater effectively and safely without sending untreated overflows to nearby water sources.
In addition to replacing or supplementing these aging systems, many municipalities are investing millions in collection system redundancy to ensure their systems can handle capacity in the event of catastrophic events. One California town is using precast concrete structures as a critical component to its solution.
Roseville, Calif., just 20 minutes outside of Sacramento, has seen substantial growth – its population has nearly doubled in the past 20 years – and many businesses now call the idyllic town home.
Part of that growth is a new 500-acre development called Creekview, which includes more than 2,000 residential units, parks, space for commercial development and a school. Expanding infrastructure in the area is a natural part of that growth, including roads, utilities and wastewater collection infrastructure.
California-based HydroScience Engineers Inc. partnered with a civil development team and the City of Roseville to conduct the planning and engineering design of a 1-million-gallon-per-day sanitary sewer lift station to convey domestic wastewater into the city’s wastewater treatment plant.
Project Manager Jason Crowley, P.E., said that HydroScience Engineers designed a turnkey facility that would serve the residents of the Creekview Development and eventually become public infrastructure. Crowley and his team determined the facility hydraulics, evaluated the process and designed a pumping system that met the City of Roseville’s public standards. Crowley said Roseville has been forward-thinking with its engineering requirements, one of which is to build in four hours of emergency storage within the collection system itself. However, the city did not have public standards for how to design or construct an underground emergency storage basin to the capacity required.
“The whole point of the city’s 4-hour storage requirement is to mitigate a peak wet-weather flow event combined with an operational failure,” Crowley explained. “Four hours gives their operators ample time to respond. You never want something to overflow. The city’s method of calculating required storage, combined with conservative peak flow determinations, is a pretty bulletproof way to build redundancy into the system and eliminate overflow events that can cost public agencies in fines.”
Taking into account all the piping, manholes and other structures, HydroScience was able to calculate how much storage the underground basin would need based on several factors such as the city’s average rate of flow and multiplying it by the “peaking factor.”
“The hard thing about this design scenario is that the reservoir needs to be completely underground for automatic filling and gravity return. The elevation section is also driven in elevation by the lowest rimmed manhole in the collection system,” Crowley explained.
To meet the storage requirement, crews started looking for an underground solution that would give them 13,500 gallons of excess storage. The answer came in the form of a precast concrete box culvert system which includes top and bottom culvert sections and structural end caps to form watertight tanks.
Thanks to an existing relationship with Jensen Precast, Crowley reached out to see what solutions were available.
“We sat down with them prior to specifying our design and pitched our ideas to see if it was feasible,” Crowley said. “As a civil engineer, we regularly evaluate the feasibility of cast-in-place vs precast. For the underground structure, it made a lot of sense to go the precast route because it meant less time with an open excavation and less upfront engineering cost for the developer to bear.”
Jensen was able to engineer and design the basin, in coordination with HydroScience, which was one of the biggest benefits to Crowley.
“They, and a lot of precasters, have in-house engineers and civil guys who are in tune with water,” he noted. “All I needed to tell them was the length and how deep so they could figure out the approximate soil loadings, reinforcement design and structural requirements. That’s a huge benefit for us and the project owner.”
Not having to worry about all the codes, standards and requirements gave Crowley a lot of peace of mind. He knows Jensen specializes in this type of project and does it nearly every day, so it can expedite the design using templates, standards and lessons learned from prior projects.
“You know they’re building it in accordance with all ASTM, structural codes, seismic requirements and everything that’s project-specific,” he explained. “They are taking on that burden.
Seamless production and installation
Jensen Estimating Engineer Tim Pellegrini noted the past work with HydroScience as a driving factor in the project. Working with each other on successful projects in the past gave both sides a lot of familiarity and comfort. Jensen made one large tank comprised of box culvert sections and end caps.
“It was five overall pieces that we put together,” Pellegrini said. “That made it a lot easier for transport, and also made it easier for our setting and lifting capacity.”
The 10-foot-long-by-10-foot-wide-by-20-foot-high precast concrete tank holds 13,500 gallons and is buried 8 feet. Jensen also manufactured precast endcaps that with a tongue-and-groove design to fit together along with butyl gaskets for additional watertightness. The precast option came with many other benefits for Crowley. He appreciated having Jensen’s employees on the job site to provide additional quality control with installation. He also likes working with precasters because he can specify items such as access hatches and the precaster can integrate them into the lids and cast them into the structures.
“That’s a huge benefit,” he said. “With precast, as soon as we open up the hole, we can put it in the ground so you save a lot of time on-site, which saves you money too.”
Jensen installed liners to provide additional protection for the structure. Since these are overflow structures and are rarely exposed to, let alone filled with, wastewater, a larger interior surface of the tank could be exposed to very high levels of hydrogen sulfide gas over a long period of time. Various options are available to provide added protection in this scenario, and the team determined the liner system was the ideal option in this case.
Being able to do all of that in a quality-controlled environment was another benefit for Crowley, who appreciated the ability to apply it in the factory, rather than in the field where it would require applying something post-construction or adding a coating.
“It’s just a lot easier embedding the locking extensions into the concrete, and boom, you have your liner,” he said. “You don’t have to do some coating or anything post-construction. It’s just a matter of getting the joints welded, so it takes a lot off our on-site time.”
While Jensen’s work is finished, the system is still under construction along with the rest of the development.
All sides consider the project a success and hope to continue their work together in the future.
“We’ve been really happy working with their team,” Crowley said. “The project has gone great, they’ve met their deadlines and they produce great precast. And from an engineering standpoint, they’ve been great to work with.”
And with precast concrete playing a critical role, the homeowners and environment don’t have to worry about the system for many decades thanks to its durability and resilience.
Matt Werner is the managing editor of Precast Solutions magazine and is NPCA’s communication manager.