St. Louis Wastewater Tunnel Rehab: The Proof
Is In The Concrete

By Angus Stocking

Photos courtesy of Con Shield Technologies, Inc.

The Metropolitan St. Louis Sewer District (MSD) was facing a problem familiar to many big city wastewater departments managing underground assets. A large (72-inch/182.88cm) sanitary sewer needed to be replaced with an even larger line to handle wet weather flows and eliminate sanitary sewer overflows (SSOs).



“We’re handling the Coldwater Creek project in three phases,” explains MSD Principal Engineer Greg Tolcou, P.E., “and this phase is the shortest. It is a 2,300-foot (701.04m) stretch that passes (at an average depth of 20-25 feet/6.1m-7.6m to flow line) under Lindbergh Boulevard. It also passes under a 20-inch (50.8cm) gas line, a 24-inch (60.96cm) water main, a condominium complex lake, the parking lot and the improvements for a driving range. The projected cost to restore surface disruptions is getting so high on these kinds of projects that tunneling is becoming more and more cost-effective.”
In fact, tunneling made so much sense on this phase of the project that the bid-winning contractor, SAK Construction, LLC, decided to use 96-inch (243.8cm) pipe, rather than the specified 90-inch (228.6cm), in order to use a tunnel boring machine (TBM). But choosing the right material for 2,300 feet (701.04m) of very large diameter pipe created its own set of challenges.


Concrete -- A Good Choice
“PVC pipe might have been a good choice, but it’s not available at that size,” says Tolcou, “and fiberglass wouldn’t work at this depth — the extra shoring and bedding required, along with its cost, made it too expensive for this project.”

That left concrete. It could stand the stresses of being jacked in behind the TBM; it didn’t need special shoring or bedding, and it is inherently inexpensive compared to other options. And besides, “St. Louis is a concrete town,” says Tolcou: because the city is on the Mississippi River, and near sources of cement, limestone, and sand, concrete pipe makers are easy to find in St. Louis.

That didn’t mean concrete was an easy choice. “There’s a perception about concrete pipe in sanitary sewer applications,” explains Dan Swidrak, P.E., a product engineer at Independent Pipe, which cast the Coldwater Creek pipe. “People worry that the line may corrode.”

Corrosion in sanitary sewers is a very real problem. Warm temperatures, turbulence, organic waste, and low oxygen levels create hydrogen sulfide gas and breed aerobic Thiobacillus bacteria. The Thiobacillus colonize in the concrete pipe and begin converting hydrogen sulfide into sulfuric acid. The acid attacks the concrete matrix. In the right — or rather, wrong — conditions, microbiologically induced corrosion (MIC) can quickly destroy unprotected pipe.
Fighting MIC is tough. Chemicals like potassium permanganate, chloride, and oxygen can be injected into flows to combat hydrogen sulfide build up, but regular chemical addition is quite expensive.

Concrete pipe can also be lined with vinyl but, “It’s very labor intensive,” Tolcou says. “Each piece has to be welded at the joints by hand, and it’s subject to nicks and pinholes, which allow gas penetration.” If linings are compromised, bacteria and gas can easily work together in the space between the lining and pipe, causing serious deterioration, which is hard to detect with TV inspection until there is a catastrophic failure.

A New Product For An Old Problem
MSD specified the use of ConmicShield, an anti-microbial additive that is mixed directly into concrete and permanently inhibits Thiobacillus growth. “I learned about ConmicShield from Independent Pipe and from MSD’s own materials committee,” says Tolcou.

MSD Materials Engineer Ron Moore said that MSD first used ConmicShield eight years ago with excellent results on the Maline Drop Shaft, near Chain of Rocks Bridge. MIC had corroded more than five inches of the walls in the lower third of the 50-foot (15.24m) deep structure.

Moore, supervisor of the restoration, says that it was a successful project: “We used ConmicShield to repair the drop shaft, with severe deterioration in the bottom third — it was completely eaten by gases — and we’ve been monitoring it ever since. Eight years later, I’m happy to say there’s no deterioration whatsoever.”
Tolcou was encouraged, but still, casting 2,300 feet (701.04m) of 96-inch (243.8cm) reinforced pipe is a large undertaking. He worked closely with Swidrak and Independent Pipe, and even visited the plant to personally witness dosing, mixing and pipe production.

Independent Pipe made test cylinders from each day’s production. These cylinders can be tested at independent labs to ensure all bacteria are killed, thus providing effective verification that the anti-microbial agent has been properly integrated. The fact that ConmicShield is mixed into the concrete, and isn’t just a coating applied to the pipe’s surface, is what makes it effective in long-term applications.

As a final quality assurance measure, Tolcou asked that the inside crown of the pipe be painted with a proprietary green paint. This will help identify the treated pipe in future CCTV inspections, and will also provide a quick visual check of surface integrity.

After phase three is completed, Coldwater Creek will resume flows. Tolcou is cautiously optimistic that this concrete pipe “will stand the test of time.”
 

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