Top: Before installing high-density polyethylene rings around a manhole to prevent infiltration, a worker measures the height from the top of the cylinder to grade. Bottom: Thanks to their lighter weight, plastic rings are easier to install than their concrete counterparts. Photos: Richard Kronick
There are about 20 million manholes in America, and all of them have essentially the same construction: a massive cone section rising up from the sewer. Until the late 1920s, these cones were made of bricks. In subsequent years, precast concrete arrived on the scene. Above the cone section is a short section called the “chimney,” which is capped by an iron frame and cover.
The chimney is a tricky necessity. Each one must be custom-built to the right height and slope for its location so the cover and frame are flush with the road surface. And while each concrete chimney is unique, they all share one characteristic: they're a headache.
“The rings break, so you get silt, rocks, and stormwater flowing into the sewer,” says Roger Glanzer, utilities supervisor for the Minneapolis suburb of Edina, which has a population of 47,000. “That means you're paying for treating water that doesn't need to be treated.”
Further, the chimney continues to deteriorate over time. The deterioration, in turn, causes the roadway above to become prone to collapse, which leads to potholes around manholes. The whole process leads to a lot of labor-intensive, expensive road repairs.
There are three main causes of manhole chimney deterioration:
- Freeze-thaw cycle: This culprit is especially prevalent in the northern United States.
- Microbes: In sanitary sewers, they metabolize hydrogen sulfide gas, transforming it into sulfuric acid. The acid gradually turns the mortar between the rings to dust.
- Improper construction: Building a concrete chimney requires skill, but the job all too often goes to a worker with little or no masonry construction experience.
In the late 1990s, Glanzer decided to use rings made of high-density polyethylene (HDPE), manufactured by Ladtech Inc. of Lino Lakes, Minn. He's happy to report he's had good results.
“We put some of them in areas where we periodically go back and look at them to see if there's any sign of infiltration or degradation,” says Glanzer. “It's been about eight years, and we've had no problems. When sealed properly, they don't allow infiltration.”
The rings are easier to handle because they're virtually unbreakable and weigh a fraction of their 85-pound concrete counterparts. In addition, the plastic rings can be installed more quickly because they interlock with each other and are self-aligning. Workers seal the rings together with a butyl rubber compound. Both HDPE and butyl rubber are highly resistant to acid.
“The only problem we've had was getting the engineering superintendent to change his specs,” says Glanzer. “He didn't think the plastic would hold up, but it does, and the crews like working with them.”
Glanzer has partnered with Lad-tech's distributor to train crews on the ins and outs of installing the rings. Overall, they've found the process to be fairly straightforward. Glanzer calculates that each installation (requiring a new cover and frame, and post-installation asphalt repair) costs about $700, which is paid for by sewer assessments.
It appears to be money well-spent.
— Richard Kronick is a Minneapolis-based business writer.