Rehab in the Remote North

Innovative manhole rehabilitation technique reduces inflow and infiltration and proves value in extreme conditions.

Rehab in the Remote North

The Permacast system uses a bidirectional spin-caster that is lowered into the manhole and withdrawn vertically, so crews are usually able to avoid confined-space entry. The bidirectional motion eliminates concrete shadows caused by uneven bricks or other protrusions.

Carmacks is a remote village in the Yukon with a population of just 503. Infrastructure is relatively undeveloped, and repairs and rehabilitation present special challenges.

“Basically, the sewer system is the only infrastructure — even water is trucked in,” says Darrell Peters, project facilitator at Yukon Engineering Services.

As part of a review funded by the government of Yukon, Peters identified six manhole barrels that were leaking badly and overwhelming the community’s small wastewater treatment plant. “The system is near a big river, and main sewer lines are actually in the water table,” Peters says. “The manhole barrels were just part of the project, but they were a main source of infiltration — I could see water pouring in like a garden hose, and I had to dewater just to get a good look at the situation.”

At the time, Peters installed plugs to temporarily fix poorly installed seals and cracked barrels. For a permanent solution, he decided to try the Permacast system, developed by AP/M Permaform and installed by Edmonton-based Osco Mudjacking & Shotcreting. “I thought it could work here at a relatively low cost and would be a good choice given the remoteness of the location,” he says. “The nearest source of supplies is Whitehorse, about 200 km away.”

Combined solutions

The Permacast process Osco employs is based on a bidirectional spin-caster, which is lowered into the manhole from above and withdrawn vertically, so crews are usually able to avoid confined-space entry. The bidirectional feature is important in this regard; since the spray nozzle spins both clockwise and counterclockwise, there are typically no concrete shadows caused by uneven bricks or other protrusions. This eliminates handwork. The result is, essentially, a smooth, watertight, completely structural new manhole that tightly adheres to and stabilizes the existing manhole.

Permacast can be used to repair brick, concrete or even steel manholes. The material used is MS-10,000 — an ultrahigh-strength mortar based on ordinary Portland cement and fortified with microsilica and other densifying agents. Graded quartz sands and fibers are added to improve cohesion and flexural strength. The properties of MS-10,000 are much different than Portland cement, with twice the compressive strength.

“We set up over the manhole, lowered in the spin-caster, and withdrew it while spraying on MS-10,000. Sewer flow was not affected at all,” Peters says. Just a few passes were required to build up an inch of thickness. The process casts a new, joint-free pipe within the original manhole, and manhole diameter is not significantly reduced.

For added protection, ConShield — a concrete additive that inhibits microbial activity — was incorporated into the MS-10,000. Concrete treated with ConShield has been used for decades to prevent microbiologically induced corrosion. Poorly understood when many manholes were originally built and installed, microbiologically induced corrosion is now known to be a prime cause of concrete deterioration in wastewater systems. The organic waste, combined with increased temperatures, long retention times, turbulence and anaerobic conditions create hydrogen sulfide gas. This in turn creates a proliferation of Thiobacillus, a bacterium that feeds on the sulfur within the gas and then excretes sulfuric acid. And sulfuric acid, of course, attacks the concrete matrix and decomposes it into a crumbly white mass consisting primarily of gypsum.

ConShield — a water-stabilized silica salt — is a liquid added during the mixing phase that bonds molecularly with cement particles, forming a physical barrier to production of Thiobacillus. It is nontoxic to humans and animals, but it permanently inhibits single-celled organisms like Thiobacillus. “Using ConShield is a very good idea in any sanitary sewer, where organic material and turbulence lead to microbial damage,” says Don Moroz, Osco owner. “It’s a best practice, and we recommend it to greatly extend the life span of concrete sewer structures.”

Successful outcome

For Osco, the main challenge was weather; at the time of rehabilitation, nighttime temperatures were 5 degrees F, with daytime temperatures climbing all the way up to 41 degrees F. Aerotech Herman Nelson Int. heaters were brought on site and connected to 12-inch hoses that brought manhole barrels up to workable temperatures. Osco also had to contend with insulated frost lids, a unique feature of Yukon manholes used to prevent sewer freezing in winter.

Aside from those challenges, rehabilitation was routine. All six barrels, approximately 10 feet deep, were repaired in just two days on site.

Peters considers the project a success and says he is likely to use Permacast again. And given the extreme challenges successfully met by this project, it’s likely that the system will be used routinely throughout Canada, just as it is in the United States.



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