Form Fitting

The Monoform manhole rehabilitation system uses poured-in-place concrete to create a brand-new, leak-free structure inside the old

Deteriorating manholes: to repair, restore or replace? And how? At what cost?

These nettlesome questions face many municipal sewer system managers, especially in older cities where infrastructure is long in the tooth.

One method of manhole restoration is to fit a plastic conical form inside the existing structure and pour a concrete liner around it — essentially providing a new manhole wall. The concrete restores the structural integrity of the manhole while forming a seamless barrier against future infiltration and inflow.

Infrastructure Technologies Inc. (Infratech) of Rogers, Minn., markets and installs the technology known as the Monoform manhole liner-forming system. The procedure includes preparing the existing structure to accommodate a custom concrete form and pouring ready-mixed concrete to form the new manhole walls.

All necessary bottom work is performed, including new bench and inverts providing proper flow conveyance. New castings and chimney seals are provided as needed.

Founded in 1994, Infratech focuses mainly on wastewater collection system inspection, maintenance and repair using various trenchless methods. The company has 20 service crews employing about 50 people and serving municipal and industrial clients throughout Minnesota, Wisconsin, Iowa, Upper Michigan and the Dakotas.

Of these 20 crews, three two-man crews are fully dedicated to the Monoform process, often in coordination with collection system rehabilitation and pipeline restoration. The city of Menasha, Wis. (population 17,000), hired Infratech to restore 110 manholes during the 2008 construction season. The contract was worth $312,000, about half what it would take to tear out existing manholes and replace them, according to city estimates.

The city purchased new manhole castings and lids separately from Neenah Foundry, Neenah, Wis. The project followed an extensive visual review of all manholes in the system by Kaempfer & Associates Inc., an engineering firm in Oconto Falls, Wis. Kaempfer worked with the city to develop a restoration plan and identified manholes needing work. Many are between 50 and 80 years old and most are of brick construction.

“While there were some manholes beyond repair, we felt the Monoform system was the way to go where we could salvage the structure,” explains Taryn Nall, senior engineer with Kaempfer. “It provides a complete seal against I&I, as opposed to pre-cast structures that contain joints every few feet.”

Walk-around

On Wednesday, June 25, a manhole restoration took place on Appleton Street in a residential neighborhood of middle-class homes, parks and schools. The hole was 48 inches in diameter at the bottom, 26 inches at the chimney, and 7 1/2 feet deep. At a nearby location on Tenth Street, Infratech finished an earlier restoration to show the process from start to finish.

The Infratech crew consisted of Ed Brown, project superintendent, and Dave Thompson, technician. Matt Huston, marketing director and part owner of Infratech, Nall of Kaempfer, and Charlie Hutter, city engineering aide, observed.

Two Infratech vehicles occupied the center of the street next to the manhole, providing enough space on either side to allow cars and trucks to pass. Traffic cones marked the work area. The crew used a one-ton dumper to carry off debris from the manhole. The second vehicle, a 4x4 hooked to a 26-foot dual-axle trailer, housed various pieces of equipment, including a Van Air Viper compressor that would drive a jackhammer. The rear section held additional equipment and supplies, among them a concrete saw, bags of high-strength, non-shrink grout, pails and a DeWalt mixer.

In between, wood racks held multiple sections of Monoform plastic core-forming sections, in diameters from 26 to 48 inches, and rise heights of 6 and 12 inches. “We carry enough panels to match just about any configuration we encounter in the field,” explains Huston. “And since we have our own fabrication shop, we can make custom panels as needed.”

Operation

In the first step in the process, Brown scribed a 44-inch square on the street surface around the manhole, using a template. He angled the square so it was actually diamond-shaped, with the corners facing up and down the street and curb-to-curb. “We do this so the snowplows don’t hit the sidelines of the cut and damage the street surface,” he explained.

Next he and Thompson rigged a canopy over the hole, and Brown cut around the diamond with the cement saw. After firing up the jackhammer, Brown and Thompson carefully dug out and removed the asphalt and compacted soil from around the manhole, placing the debris in the dump truck.

The two then removed the manhole cover and pulled out the old casting, which was badly corroded around the rim in a couple of places. “We could reuse and reset the casting if it were in good shape,” explained Huston. “It depends on the specification.”

With the hole exposed, Huston took air quality readings with a Biosystems PhD Lite air monitoring system. The numbers looked fine, so the crew rigged a confined-space entry tripod and Brown entered the hole, taking various measurements of the diameter and inspecting the sidewalls, bench and inverts. “Two old laterals down here,” he shouted, “but they’re both blocked. That’s good!”

Infratech uses a tarp, held down by boards and a special “parachute” attached to lift lines, to cover the bottom of the hole and prevent debris from getting into the sewer line. Thompson dropped those items down to Brown, who positioned them around the bench. With a hammer, he also broke off or bent back a few steel rod steps that remained on the manhole walls.

Then it was back to the jackhammer. Brown drove the blade into the upper sections of brick, breaking out the top seven courses. As he did that, Thompson returned to the trailer and began assembling the plastic core panels, bolting circular sections together with wing nuts.

Once Brown had removed the upper sections of brick, he continued to use the hammer to take out additional sections of the chimney to achieve a uniform diameter up and down the hole. He explained this was necessary to make sure the space between the form and the manhole wall was at least 4 inches thick. “We don’t want thin spots in the concrete liner,” he said, “especially at frost depth.”

A section of 36-inch-diameter sonotube sits on top of the form. Brown and Thompson cut a piece of tube to the proper depth, then positioned it onto the brick rim of the existing manhole and mudded it into place using the grout Thompson had mixed. The two removed the debris parachute and tarp, and Brown scrubbed the lower areas of the hole, and bench and invert, taking care to remove any remaining dirt or pieces of brick and mortar.

Then the crew started to build the interior “cone.” The bottom ring must be absolutely level, so they took extra care to shim it to the exact position. When it was perfectly level, Brown placed plastic sleeves over the inverts and mudded the bottom ring to the bench area.

Meanwhile, Thompson prepared the additional ring panels and handed them down to Brown, who in turn used C-clamps to fasten them to the ring below. When fully assembled, the cone rose to within 15 1/2 inches of the street surface and was exactly 4 inches inside the circumference of the existing structure at the bottom, and 5 inches inside around the top area.

Now they were ready for concrete. While Brown called the local supplier, Thompson cleaned and straightened the area. The load arrived about 45 minutes later. After putting a pour cap over the top form to keep concrete out, Brown and Thompson guided the concrete chute into place and pushed the concrete down the space between the existing manhole wall and the interior cone form. “It takes about a cubic yard of concrete for a hole this deep,” said Brown.

They removed the cap and Brown went into the hole again to tap the inside wall and settle concrete into any voids. Then they smoothed the concrete flush around the sonotube rim, placed a plywood section over the hole, and called it a day.

The next morning they returned to remove the cone (“very carefully — that’s key,” explains Huston), place the bed joints and grade rings around the top of the form, and finally place the new casting and lid.

Infratech employs a unique circular rubber seal system to prevent any infiltration of water at the joint between the casting and the concrete. An inner seal “skirt” is wrapped around the structure and banded tightly. Then a secondary seal goes over the external joint. The seals are manufactured by Adaptor Inc. of West Allis, Wis.

Finally the crew placed rebars, cemented the excavated street surface area around the finished manhole, and set cones to protect the area from traffic until the concrete cures.

Observer comments

This is a labor-intensive process: Preparation and pouring of a manhole retrofit takes several hours. The Infratech team started working on the Appleton Street manhole about 10 a.m. and was finished with the concrete pour at 5 p.m.

The next morning they came back to place and seal the casting and lid, and mud in the surface excavation. According to Huston, his two-man team can do four manhole retrofits per week. One wonders if some of the process could not be automated and some of the manual steps replaced by machines.

Infratech promotes its process as unobtrusive, and while the cement sawing and jack hammering produce loud noise, neighbors did not seem to mind. One woman, Donna Dutter, said the project didn’t bother her at all. “We are able to get in and out,” she said. “And I think the city does a good job of keeping things in good shape.”

It was obvious that skilled craftsmen and communication between workers are keys to a quality job. Brown was a virtual surgeon with the jackhammer, able to knock off precise sections of the old brick manhole walls and dig out rock and compacted soil right up to the edge of the sonotube. And while the process took several hours, the communication between Brown and Thompson was professional and efficient and greatly facilitated progress.

Thompson on the surface seemed to anticipate every request from Brown down in the hole, and Brown responded with a courteous “thank you” each time Thompson delivered a line or a bucket.

Another question is potential damage to the “good” part of the manhole through all the jack hammering above. And does the team ever encounter rock or material they can’t readily break up and remove?

Manufacturer comments

Huston says the company has considered using light machinery, such as a mini-excavator, for use in casting and chimney removal. However, he explains, chimney removal requires an accuracy that would be difficult to duplicate with a machine. Because of varying chimney depths and often irregular-sized openings, the jack hammer provides much better control, he believes.

“If one were to consider having a machine onsite, they would also need to increase the ability to form, pour and finish multiple structures at once,” he explains. “This would require more personnel, more forms and all related tools, including trucks and trailers for transport.”

Huston adds that damage to the existing brick below as a result of the jack hammering is not a concern, because the new liner does not require a bond or rely on the condition of the existing substrate or wall section: It is a completely stand-alone, full-structural liner.

“Depending on the era of construction and type of material used at the time, some manholes require a bit more effort during the chimney bust out,” he explains. “Some manhole construction styles also include deeper chimney sections. This type of information is taken into account when performing site inspections before the construction process.

“Essentially, each and every Monoform manhole restoration is a custom project in and of itself,” Huston says. “In most cases, chimney size is actually increased from its existing size after the Monoform liner is installed.” He emphasized that owners considering a sewer rehabilitation project should thoroughly evaluate the condition of their manhole structures.

Lastly, Huston noted that in addition to the standard manhole-forming components, custom forms are available for lift station retrofits and rehabilitation. The Infratech in-house fabrication shop will custom make forms of different shapes and sizes to fit each application. The Monoform service can be provided by Infratech crews, or end-users can purchase the forming system with a personalized training package.



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