Pre-Emptive Strike

A single manhole failure inspires Arizona utility to upgrade its manhole inspection protocol and lower infrastructure life cycle costs.
Pre-Emptive Strike
Ed Rosales operates the jet/vac controls while cleaning a sewer line.

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Alvin Robertson recalls his first look at a deteriorating i manhole, which inspired a new program to systematically map, inspect and rehabilitate his city’s sewer manhole network.

“It was 2003, and we were working on a lift station that was pumping into a manhole located in an intersection nearby,” says Robertson, sewer collections superintendent with the Municipal Utilities Department of Chandler, Arizona. “We decided to pop the frame and cover for a visual inspection and we couldn’t even see the manhole. All we could see was dirt — and then everything started breaking loose. We blocked off traffic and got the contractors in here to make a repair. That was an eye-opener for us. We brought in a CCTV camera and the 33-inch sewer was so full of dirt we could barely get it through. The manhole was precast, uncoated and unlined concrete, and the H2S had gotten happy and corroded away not only the manhole but the concrete sewer line too.”

The length of sewer pipe was sliplined with an Insituform product. Due to the severity of the deterioration, the manhole was replaced with a cast-in-place concrete insert. The utility could have treated the problem as a one-off, but the fact that the manhole was less than 20 years old set off alarm bells for Robertson, who realized that the surrounding road might have eventually collapsed.

“That manhole was our poster child for deterioration,” says Robert Buss, P.E., associate vice president with Carollo Engineers in Phoenix. “It was located at the end of a force main that discharges into it. Force main discharge is very turbulent and releases a lot of H2S, which eats away at cement and concrete.”

Wake-up call

Gina Ishida-Raybourn, P.E., is the acting utilities engineering manager with Chandler’s Municipal Utilities Department, but was working for Carollo at the time, giving her extensive firsthand perspective from two vantage points.

“That first manhole was a wake-up call to the city,” she says. “Chandler took a stand to head off the problem.”
Carollo was contracted to establish a wastewater master plan and a Capacity, Management, Operations and Maintenance program, which inspired Chandler’s multiyear sewer assessment program, beginning in 2005.

Sewer pipes in the system run from 6 to 66 inches in diameter. They’re made from a range of materials, including vitrified clay and PVC, with clay being the oldest, dating back to about 1950. Some of the larger-diameter concrete pipes are lined with PVC T-Lock liners by Ameron International. Sewer force mains are constructed of both ductile iron and HDPE.

“Overall, our pipes are in good condition,” says Ishida-Raybourn. “The manholes were our immediate concern. We could have taken a shotgun approach and inspected manholes all over town, but decided to be more methodical. We didn’t look at geographical areas as much as the material that made up the manhole. Brick was generally structurally fine after exposure to H2S, although the mortar might be affected. We looked primarily at concrete manholes that were most likely to be corroded by sewer gas, and preliminary inspection showed us that even uncoated concrete manholes on 8-inch pipe were performing well.”

The city counted just shy of 20,000 sewer manholes, which are mapped using ArcGIS by Esri. The goal of the Phase I inspection/assessment program was to inspect, assess and rehabilitate all interceptors 15 inches in diameter and greater and their associated manholes — approximately 105 miles of pipeline and 1,400 manholes.

Launching a pilot

Typical sewer construction, repair and rehabilitation are handled by outside contractors, so a contractor was hired to inspect manholes and adjoining pipes to identify urgent problems. Initial inspections of 100 manholes revealed more issues.

“We noticed that the first manholes off a trunk line were also in bad shape, because that manhole acted as a vent for the force main,” says Robertson.

Photos of the corrosion were used to successfully convince City Council that a larger multiyear rehabilitation budget was necessary.

Historically, the city responded to deteriorated manholes by cleaning the surface and applying an epoxy coating.

“It was labor intensive and required a lot of handwork,” says Ishida-Raybourn. “We had a worker strapped to a harness sporting a trowel and the epoxy coating. This approach required ventilation and confined-space entry and typically the coating only lasted for about 10 years. Also, for any manhole located within 100 feet of an intersection with a light, we had to set up traffic control using a police officer. Much of the cost of the work was going to traffic control.”

A life cycle approach

Working with Carollo, Chandler developed a new approach to rehabilitating corroded manholes, which considered overall life cycle costs. On a 5-foot-diameter manhole, the cost of fiber-reinforced plastic liner inserts had narrowed to just about one-third above the cost of epoxy coating. However, with a 40-year life span, the projected life cycle costs of FRP falls to less than one quarter of epoxy.

“We now rehab with FRP rather than coat with epoxy products,” says Ishida-Raybourn. “FRP inserts were also becoming a better economical choice over coatings because of the higher number of contractors certified to perform the work that were flooding the Arizona market.”

To install the FRP liners, the cone is removed and the bench is cleaned up, rebuilt and coated with epoxy, then the FRP insert is installed within the existing manhole. Flowable grout is poured into the annular space and slurry is poured around the cone of the insert. The pavement section is then replaced, along with the frame, cover and concrete collar.

Specific geographical areas requiring FRP liners are bundled into bid packages along traffic arteries to ensure economical bids. Contractors choose their own liner product for installation, although they’ve primarily been supplied with products from Containment Solutions Inc. and LF Manufacturing.

Camera speeds inspections

Historically, manhole inspections were carried out by workers lowered on straps to perform both visual and CCTV inspections with hand-held cameras.

In order to speed inspections, Carollo recommended the use of the 360-degree Digital Imaging IBAK Panoramo CCTV system, in which a camera is lowered into the manhole from the surface. The camera and software were purchased in 2015 by the Arizona branch of Pro-Pipe to work on the Chandler contracts. Since then, the IBAK has cut per-manhole inspection costs in half while more than doubling inspection efficiency to 25 to 30 manholes per day. Up to 400 manholes are now inspected each year.

“The current approach is to use the IBAK on a bulk number of manholes as a first pass,” says Ishida-Raybourn. “Upon review of the camera record, if we have questions about the data, a manned-entry inspection is performed. The only thing we wish the camera could do is physically probe the concrete. When it’s mushy it can often look fine, but will start to come apart when you stick a screwdriver into it.”

Another challenge to manhole integrity is the widening of roads. Some manholes that might once have been located away from traffic now see heavier traffic impacting manhole surfaces.

“The lids and covers are getting pretty beat up,” says Ishida-Raybourn. “We’ve had to go in and replace the frames and covers on some of those.”

To date, 116 deteriorated manholes have been replaced with FRP inserts and the utility is also looking at using FRP or polymer concrete manholes for new construction.

“We’re on the way to being bulletproof,” says Robertson.

Underused sewers can be a major headache

The city of Chandler, Arizona, has experienced explosive growth, with its population more than quadrupling from just over 50,000 in 1985 to almost 250,000 today. A growth pause has caused some temporary headaches for the city.

“When Arizona had a housing boom, developers were going crazy to get these big pipes to all of the subdivisions popping up,” says Gina Ishida-Raybourn, acting utilities engineering manager with Chandler’s Municipal Utilities Department.

“We sized the pipes for the build-out, but in 2007-‘08, residential construction flattened out. With pipes only half full, it’s resulted in less-than-ideal flow. With some flatter-than-ideal slopes, the solids are turning septic and that’s causing both odor and corrosion of concrete.”

City crews use a pair of Camel jet/vac trucks to maintain the system, with a goal of cleaning every pipe over five years. A thorough flushing helps to move accumulated solids along the underutilized pipes.

Crews must also respond to another complaint.

“We sometimes get calls from homeowners about cockroaches coming into homes through sewer laterals,” says Ishida-Raybourn. “With low flows, they can survive on the grease and solids in the half-full pipes. Painting insecticide inside the sewer manholes does the trick.”

However, growth projections for the city predict that Chandler’s population will rise to about 300,000 people over the next decade.

“When growth resumes, this temporary problem should resolve itself,” she says.


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