Oct 28, 2011

Life Expectancy Up

Janesville, Wis., is a city that actively looks for ways to improve its underground infrastructure. Located in a northern climate, Janesville has experienced the damaging effects of freeze-thaw cycles on its sewer systems, typically observed as damage to the chimney section of manholes. The movement in roadways and pressures created by freezing water can create cracking and separation in the concrete grade rings that the city traditionally has used to adjust the manholes, as well as rapid degradation in the mortar and shims that are used to fine-tune the casting height and slope to match the road level. 

Precision in the adjustment is crucial, as castings set too high will be at risk of being struck by snowplows, causing damage to the equipment and the structure. Castings set too low create a pothole, which can speed up the breakdown of the manhole by increasing the impact forces and allowing water to pool during the freeze-thaw cycles.

Damage to the manholes creates additional problems for the system, allowing for infiltration of water during and immediately after storm events, as well as dropping the height of the manhole cover, which negatively impacts driving conditions. 

The city’s senior engineer, Steve Sage, had all of these things to consider when creating the specifications for a recent street overlay project in the Arbor Ridge neighborhood. Several years after the initial construction of the neighborhood, a final coat of asphalt was scheduled to be applied that would require all of the sanitary and storm sewer manholes to be readjusted to match the new road height.

Solutions Selection

Sage reviewed his options as he wrote the specification. He investigated a product he had seen at a recent Wisconsin American Public Works Assn. conference as an option for raising the manholes to grade. Product durability was one of the attributes that had become a focus of the investigation, as the cost of doing repairs over time was always higher than the initial job cost. Another key feature was precision adjustment without adding any materials that would shorten the life of the manholes. Ultimately, the city decided to include Pro-Ring in the specification for the job as it went out for bid.

The city also elected to use an external mechanical chimney seal on all manholes. This has been a recent standard for construction and rehabilitation of manholes in Janesville. The externals are applied whenever a manhole chimney is being constructed to prevent all inflow and infiltration between the frame and the cone section, while still allowing for movement of the roadway during the freeze-thaw cycles. For this project, an X-85 lightweight external seal was selected.

Manhole Adjustment

The task of adjusting the manholes was awarded to E & N Hughes, supported by Ferguson. The installation workers had performed many adjustments in the past using concrete grade rings, but this was their first experience with a lightweight alternative. They quickly were impressed with the ease of handling these rings, as the concrete rings weighed almost 20 times more. That made the job go more quickly than it would have if they were using concrete, and it also reduced the risk of injury from lowering concrete into position. 

“[We]…give them a thumbs up on overall ease of installation,” said Todd Hughes, vice president of E & N Hughes. “The Pro-Rings are lightweight but sturdy and will most likely outlast the manholes they are installed on. The adhesive system and the many sizes, including the sloped rings, made our installation much faster.” 

The project team also had a chance to witness the impact of the freeze and thaw damage in the manholes, as many of the previous adjustments using shims and mortar did not hold up well. The project ultimately required some of the previous adjustments to be removed because the damage was so extensive—an unforeseen cost to the city and the contractor that can be associated with mortared adjustments.

One of the attributes of the neighborhood that became apparent as the work progressed was that the geography of the terrain allowed most of the manholes to be positioned in sloped roadway. That led to increased use of mortar and shims during the initial construction in order to ensure the castings matched the roadway slope. Because the degradation that had occurred primarily was in the mortar used, the Pro-Ring system brought out an additional advantage of offering sloped rings that are able to be adjusted to match any slope, eliminating the need to use mortar. In fact, every casting now could be adjusted to within 1/8 in. of the road surface with an exactly matching slope.

Ultimately, the job was completed by  adjusting 63 manholes with Pro-Ring and X-85 seals. The city now is able to plan for a 50-year life expectancy from these manholes, eliminating the repair and maintenance costs over that time period. The project represents success for the city in its efforts to improve its infrastructure, both in current performance and lifetime costs. As a result, Janesville can plan for its future generations.

 

About the author

Eric Dickson is general manager for Cretex Specialty Systems. Dickson can be reached at [email protected].

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