Formed in Place

A manhole rehabilitation system uses special forms to create structural, monolithic, watertight liners that stand up to wear
Formed in Place
Infratech project manager Ed Brown measures the diameter of a manhole to select the proper-size forms.

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Installed in the 1920s, the brick and block manholes in Waupaca, Wis., were failing from age. Some were in danger of collapsing. Most had no flow lines, enabling solids to accumulate and create headloss that reduced hydraulic capacity.

The city consulted senior engineer Taryn Nall of Kaempfer & Associates engineering firm. Nall recommended Infrastructure Technologies (Infratech) to rehabilitate the manholes using the Monoform system to create structural, monolithic, watertight concrete liners.

Street superintendent Roger Hansen asked Infratech regional manager Matt Huston to demonstrate the technology on a problem manhole.

“Sewage built up on the trench of this manhole so quickly that my crew had to clean it monthly,” says Hansen.

For the demonstration, the public works department provided a new casting and lid from Neenah Foundry. Two Infratech technicians installed the concrete liner and constructed a bench one inch above the crown of the pipe.

“I was extremely happy with their work,” says Hansen. “It turned out so well that the structure looks like one cast by a manhole company.”

The crew now cleans it annually as part of the sewer maintenance program, and the department budgets $30,000 per year for manhole rehabilitation. In 2011, it rehabilitated 10 structures with the Monoform system.

 

Manholes of the year

Many manholes remain to be located on GIS as the department transitions from paper to digital files. The cleaning crew helps identify the worst manholes. Hansen believes the city has rehabilitated all manholes without flow lines.

Lining manholes is a custom job. Workers first measure the diameter at the bottom of the structure, the height of the chimney, and the depth, enabling the Monoform crew to select the appropriate size wall, reducer, and chimney forms. Most of the manholes are 48 inches at the bottom and taper to 28 inches at the top. Depths vary from 4 to 25 feet.

“The chimney must be knocked off to make room for the concrete forms,” says Hansen. “The taller the chimney, the higher the demolition cost.”

Going around or abandoning in and out pipes also affects price.

Before the Infratech team arrived, the city cleaned the manholes to be rehabilitated using a 2003 Vactor 2115-J6 combination sewer cleaner with a 15-cubic-yard debris body, four 625-gallon freshwater tanks, two-stage fan with fluid coupler pulling 450 gpm, and dual-action water pump delivering 80 gpm/2,500 psi.

Infratech project manager Ed Brown and technician Jim Peltier parked their fifth-wheel truck and job trailer in the center of the street next to the manhole, leaving enough space for traffic to pass on either side, and marked the work area with traffic cones.

“Except for one instance when we resurfaced a street, we have never closed a thoroughfare for them to work,” says Hansen. “We have always kept live traffic on it.”

 

Smooth process

The Infratech team rehabilitated three or four manholes every two days. For snowplowing purposes, they sawed a square cut through the asphalt, but with the corners pointed toward the curbs and traffic flow. The Infratech personnel checked for gases before entering manholes and followed OSHA confined-space entry standards.

Using a Vanair Viper air compressor driving the jackhammer, Peltier removed the asphalt and soil from around the manhole. Before demolishing the chimney, Brown covered the bottom of the hole with a tarp and a special parachute attached to lift lines to catch falling debris.

“They remove as much chimney as necessary to achieve a uniform diameter with a 4-inch space between the form and manhole wall,” says Hansen. “That eliminates thin spots in the concrete liner, especially important at frost depth.”

Brown and Peltier cut a 36-inch Sonotube concrete form from Sonoco Construction to the proper depth, then grouted it to the manhole’s rim. After removing the tarp and parachute, Brown scrubbed the lower areas of the hole to remove remaining dirt or bits of brick and mortar.

 

Inside job

To build the manhole liner, Peltier handed down the custom-made sections to Brown, who fastened them with C-clamps to form a 40-inch ring, then mudded it in place. The rings tapered to 26 inches as the column rose.

“They form manholes the first and second day, then pour concrete in the afternoon,” says Hansen. “At first, I wasn’t positive that I liked the idea of pouring concrete surface patch with the asphalt. I’m okay with it now, as we haven’t seen any settling or joint problems.”

A pouring cap on top of the chimney form kept 7-bag 4,500 psi concrete mix from entering the manhole. With the pour completed, Brown entered the manhole and tapped the wall to settle the concrete into any voids. A sheet of plywood covered the hole while the concrete cured. After it had hardened, the crew removed the forms, applied urethane sealants at the pipe connections, and placed the new casting and lid.

Hansen appreciates that the work is flexible.

“If we find a critical manhole not in this year’s allotment, I ask Ed and Jim to take the measurements,” he says. “Matt faxes a quote and allows us to change which manholes we want done.”

Hansen estimates that excavating and replacing the manholes would cost $8,000 to $10,000 each.

“The Monoform system averages $3,000 per manhole, saving us $5,000 to $7,000 a unit. That’s a substantial amount of money for any city.”



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