Identifying Problems and Solutions to Leaky Manholes
How municipalities can prevent inflow and infiltration
Stormwater that finds its way into manholes through manhole covers or soil infiltration is a major problem for municipalities. Street damage, higher operating costs for sewage treatment plants, large construction and maintenance costs for existing facilities, and wasted sewer capacity that can result in no new service to industrial and residential areas, are four main problems caused by stormwater infiltration.
The term should be more precisely divided into stormwater inflow, which refers to stormwater that directly enters a sanitary sewer system through sump pumps, roof drains, catch basins and commercial discharges; and stormwater infiltration, meaning stormwater that permeates the soil and enters the system through pipes, laterals and manholes. Key leakage sources include rainfall, snowmelt, surcharged storm structures and high groundwater.
The U.S. Environmental Protection Agency estimates that 25 to 60 percent of all flows into sewage lines are due to inflows and infiltration (I/I) The rubber hits the road, so to speak, at the sewage treatment plant, where all this extra water needs to be treated. The cost can be determined by simply multiplying the treatment cost by the plant capacity. For example, a 10-million-gallons-per-day plant treating infiltration will incur additional costs of $3,000 to $8,000 per day, adding up to over $1 million a year.
Additional problems caused by stormwater I/I include flooded basements, citizen complaints, sewer overflows and bypasses, and even lawsuits. Extra costs will ultimately need to be borne by taxpayers, making the problem everyone’s business.
Identifying leaky manholes
William Goff, national sales manager with Sealing Systems, Inc., identified problems with manhole infiltration and possible solutions during a presentation in January at the Underground Construction Technology conference in Houston, TX.
Manhole leaks can be identified through dyed water testing, vacuum testing, flow monitoring, smoke testing, routine inspections, and inspections during or after a rain event. Physical evidence is usually abundant as well.
“Stains indicate that water is going in through the chimney area. Further investigation around the casting itself, if it’s in a road, you’ll see a cracked, circular depression, probably a depression in the asphalt. In soil areas, you’re going to see a depressed area in the soil,” said Goff. “Inside the manhole, in the bench area, you’re probably going to see silt, and in worse cases, you’re going to see grass. The grass doesn’t grow in the manhole. It’s usually infiltrated into it.”
According to Goff, while flowmeter measurements during and after rain events can reveal correlations between the event and wastewater treatment flows that point to infiltration, defects can also show up as broken or cracked frames, cracked pavement, and street damage, where infiltration into a catch basin washes away the soil, causing significant damage to the pavement.
These defects are caused by frosts lifts, the pavement expanding and contract- ing due to fluctuating temperatures, traffic loads where vehicles are driving over the casting, ground movement (referring to soil not seismic) and poor construction methods or materials.
Cracks in the concrete
To correct the deficiencies, municipalities often pour a concrete collar around the manhole, thinking it will correct the depression in the failed asphalt. However, Goff explained that unlike cast iron, which tends to expand and contract with changing temperatures, concrete does not at the same rate. That causes cracks for stormwater to easily find its way into the manhole.
“So instead of having infiltration coming into the chimney area, you now have inflow, because you’ve got water coming off the asphalt, going into a concrete ditch, and into the chimney,” he said. “This has exacerbated the problem. In colder climates, where you’re getting the cracks, it’s actually becoming a worse situation.”
Those cracks can cause municipalities big money. According to correlated data from pump manufacturer Gorman-Rupp, a one-sixteenth- inch crack around the circumference of a 27-inchdiameter manhole chimney lets in 4,517 gallons per day. Using 100 rain event days as a baseline, that is 451,708 gallons a year, for one manhole, at a treatment cost of $1,355 annually.
Widen that crack to an inch, and a municipality is looking at 1.1 million gallons per day, or 115.3 million per year, at an annual cost of $346,106. Again, this is for one manhole.
Given the high cost of extra stormwater treatment, how can municipalities go about preventing inflow and infiltration? According to Goff, there are five areas to look at: manhole cover inflow; grade adjustment ring deterioration; manhole frame-chimney area; manhole joint sections; and manhole wall leaks.
Gaskets can be used to prevent inflow between the manhole cover and the frame. The manhole cover can be replaced with a concealed-pick hole, self-sealing gasketed lid, or with a watertight rain cover. A manhole cover insert, manufactured from stainless steel or HDPE, is another option. These are custom-designed to provide an enhanced fit and seal, and can be installed inexpensively.
To prevent grade adjustment ring deterioration, rings made from recycled rubber tires eliminate vibration and will not crack. Recycled rubber riser ring standards are strong and impact-resistant, with the same compression ratio as asphalt. Mortarless assembly can be done by one man in minutes, although polyurethane sealing must be applied to both sides of the ring.
In the manhole chimney frame, problems include deteriorated grade rings, an offset manhole frame, gaps in the frame/chimney joint, cracked or broken grade rings and deteriorated mortar.
This brought Goff to his two recommendations for manhole frame chimney seals: Flex-Seal Utility Sealant, an internal aromatic urethane seal; and Infi-Shiled Uni-band, an external rubber seal.
Both the Flex-Seal kit and the Infi-Shield Uni-Band cover 12 vertical inches. Ideal for new installations, the external seal is easily installed – typically it goes on in under 10 minutes, however, the manhole must be excavated.
For the internal seal, surface preparation is critical. Unlike the external seal, excavation is not required, and the other advantage is it conforms to an irregularly-shaped manhole.
Goff used the example of urethane seals to demonstrate how quickly one municipality covered its costs of installing the seals.
Calculating that it was going to cost them $5,250 per rain event day to treat chimney-area infiltration and inflow, the city showed that after 43 days, installing the seals saved them about $226,000 in treatment plant costs.
Considering that chimney frame leaks are one of the most significant sources of extra stormwater into the sewage system, Goff concluded that installing a chimney frame seal should markedly reduce the amount of clear water that enters the system.
“It will reduce or prevent sanitary sewer overflows, combined sewer overflows, system surcharging and backups,” he said. “It will reduce your system maintenance. It will reduce the pavement settling around manholes. And most importantly, it will increase system capacity and allow for new development.”