Monolithic Bases Seal Manholes From the Start

Rohnert Park sewer expansion project uses precast manhole bases for watertight construction.

Monolithic Bases Seal Manholes From the Start

The Rohnert Park sewer expansion project called for open-cut construction, with the use of monolithic base inserts in the manholes.

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Years of problems with inflow and infiltration in its sewer collections system inspired Rohnert Park, California, to adopt new standards when it came to designing and constructing a trunk sewer expansion. 

Specifically, they rethought the way they were building up manholes. Previous projects had used standard cast-in-place processes, but now, they wanted to invest in I&I-inhibiting construction technology: monolithic base inserts. 

“You can’t do too much better in our industry to prevent I&I from getting into your system than to use monolithic pieces,” says Matt Winkelman, project manager with GHD, the consulting firm on the project. 

For this project, Rohnert Park used PREDL Systems North America precast base inserts. The dual-containment system consists of fiberglass-reinforced polymer (FRP) bonded to an HDPE transition strip, which mechanically locks to the surface of both the FRP base liner and the HDPE wall liner. Integral compression gasket fittings connect the sewer pipes to the manhole base. All these elements seal the manhole as one cohesive unit, virtually eliminating infiltration and H2S corrosion, saving space and providing construction efficiency.

The manhole base insert is prebuilt at PREDL’s ISO 9001-certified facility, and shipped to a concrete precaster — in this case, NCPA-certified Jensen Precast — who can simply precast then ship the completed base to site instead of building a system from the ground up on site.

“The advantage is in having more assurance of the quality before it’s shipped to the site, and once the manhole base is received at the site, it really does cause more efficiency in construction,” Winkelman says. 

Monolithic manhole bases weren’t originally part of the city’s design for the new trunk system, but after a fortuitous call from PREDL, the design team and contractor decided it fit the bill. 

“There were a number of community factors and potential public impacts that were also to be considered … and I’d say having the manholes designed as they were, especially with the fiberglass-reinforced base inserts, helped speed construction along. It kept things efficient,” Winkelman says. “Everyone was eager to see the benefits come to life.”

A new leaf

The project stemmed from a collections system analysis in the early 2000s. It showed a number of hydraulic deficiencies, I&I included, that necessitated infrastructure growth. Rohnert Park began planning for the new trunk sewer using traditional construction methods. 

“I&I was prevalent enough that it had to be part of the solution … to convey those peak flows from I&I and to accommodate for growth,” Winkelman says. “Not wanting this new infrastructure, these new assets, to eventually turn into the same concerns that they have with their older infrastructure. The city wanted a longer design life for the new trunk sewer, including their manholes, and they wanted the manholes and pipelines to be as watertight as they could reasonably get them. 

“This being a client of ours that has set a very high standard for quality of construction materials … in keeping I&I out of their critical assets and this being a new trunk sewer — a big investment — they were very firm on that point.” 

Because of these factors, when PREDL called at a critical moment as the design team was reanalyzing manhole parameters for the project, the city decided to go with the fiberglass base insert — despite a slight increase in upfront cost. 

“With the PREDL system, we’re able to get a smaller manhole with a larger-diameter pipe, so you have a smaller footprint and the cost savings of using a smaller barrel section,” says Warren Naumann, supervising maintenance worker for sewer and stormwater in Rohnert Park. “The cost (of the insert) was negligible, and the end product was much nicer than the product specified at that time.” 

The city was so happy with the inserts that they decided to sole-source the product for the remaining phase 3, which was completed in 2016.

Ultimate prevention

A monolithic system is a seamless, one-piece design. PREDL’s manhole base insert is FRP bonded to HDPE, built out at PREDL’s facility. That insert is sent to a precaster, where concrete is poured around the prebuilt shape to fill out the form.

In typical manhole construction, the bench would be built up separately from the pipe and connections and all the pieces combined on site using coatings or CIP liners to mimic the one-piece concept. 

“It took some of the guesswork out, and it took some of the effort out of the field and put the quality control into the shop,” Winkelman says. “That’s a unique aspect of these monolithic systems — the ability to create a shape that’s monolithic, that can be quality controlled in a shop, reviewed through shop drawings, and then brought out to the field and be ready to go.” 

The project used bell and spigot connections, but Winkelman says the base inserts can be designed to pretty much any shape to fit the system. 

Construction on phase 3 concluded in 2016, and a year later, the city has only good things to say. 

“The whole process went very well,” Naumann says. “A couple of the engineers/contractors who I’ve talked to say it’s the best system they’ve ever seen.”



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