For at least 25 years, Nashua, N.H., has been replacing 7,500 triangular manhole covers with round ones. Like a circle, a Reuleaux triangle's constant width prevents the 150-pound cover from falling into the manhole. Even better, the covers point in the direction sewage flows.
These were probably compelling arguments a century ago when public works ordered the covers, which are 25½ inches at their widest point, from local manufacturer Nashua Foundries Inc. The automobile was relatively new, so the U.S. Occupational Safety & Health Administration had yet to require a cover to support at least twice a vehicle's applied axle load, or that sewer workers arm themselves against injury or death. Today, getting into those manholes is virtually impossible.
Street Superintendent Roy Sorenson told the local newspaper last summer that his team replaces 100 or 200 covers every year in the course of regularly scheduled maintenance and repaving. With an estimated 2,000 left to go, he and his team have at least a decade more work.
U.S. EPA estimates the number of sewer manholes nationwide to be 12 million. Most coincide with the typical lengths of city and suburban blocks — about 300 feet apart — but distances between structures range from 100 to 500 feet. Regardless of their relationship to each other, though, 80% are in some state of disrepair.
It's tough to calculate the true cost of raising a manhole, but to help current and potential customers compare the life-cycle costs of various options riser manufacturer American Highway Products Ltd. of Bolivar, Ohio, analyzed interviews with 30 paving maintenance supervisors and contractors to develop a checklist:
Material. Irrelevant unless labor's included. Precast concrete risers, for example, are relatively inexpensive but equipment- and labor-intensive.
- Labor. Look at time logs and maintenance reports. For example, at one time Shoreline, Wash., would pave over manholes, then go back and jackhammer the new pavement to dig out and raise the frame with precast grade rings. The wastewater manager analyzed three years of maintenance records to calculate per-structure material and labor costs of $500.
In a similar scenario, the City of Ontario, Calif., found that per-structure labor costs were $360 when using pre-cast grade rings.
Liability claims. Bent rims; broken wheels, axles, struts, springs, steering linkages, etc.
- Pavement damage. The chance that a hot-mix patch doesn't bond well or rests on insufficiently compacted fill, setting the stage for subsidence that causes pavement cracks and uneven manholes.
- Inflow and infiltration. A decade ago, EPA estimated that upgrading treatment plants to eliminate the nation's 40,000 sewer overflows every year would cost $88 billion. The agency believes about one-third of the rain that causes them enters wastewater systems through the top of the manhole around the cover.
- Sustainability. Digging up and resetting utility frames unearths 500 to 1,000 pounds of material that, if contaminated, must be hauled to an appropriate disposal facility. Other energy costs: jackhammering, infilling with new asphalt, riser shipping.
- Lane closures: Any procedure that extends the duration of or requires future closures inconveniences the public. (And you know what that means.)
- Employee safety. Raising manholes during paving lessens the chances for traffic-related injury. Also, look at worker compensation claims related to lifting, crushing, or pinching when risers are delivered, racked for storage, loaded into trucks, and when carried to the utility frame and set into place.