interior of bolted steel reservoir

Extending the Lifespan of a Bolted Steel Reservoir

The City of Duncan, British Columbia, Applies Innovative, Cost-Effective Technology to Interim Safety Repair

When it comes to infrastructure challenges, size doesn’t seem to matter. Like many cities in British Columbia, both large and small, Duncan has been forced to cope with a failing reservoir that serves several critical water supply functions.

Duncan, on British Columbia’s Vancouver Island, is technically Canada’s smallest city. At 2.07 sq km (less than a square mile), it boasts the tiniest municipal footprint in the nation.

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Small City, Big Problem

“There was a massive infrastructure boom in this region in the 1970s, and the mandate was to build out necessary facilities quickly, with more attention to cost-effectiveness than quality,” explains Glenn Votkin, owner of Infrastruct. “At the time, bolted steel reservoirs were common practice, because they were quicker and cheaper to install than concrete reservoirs. Forty to 50 years later, almost all of those reservoirs are near the end of their expected lifecycles, and cities are starting to pay the price.”

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Duncan’s Eagle Heights Reservoir is a good example. Built in 1979, with steel walls and floors, it’s held together with bolts, rather than welds. It’s a 12.19-m-high, 11.89-m diameter structure with a volume of 1,359.21 cubic meters. It serves three purposes in Duncan’s water network:

  1. Potable water supply for nearby residents
  2. Water supply and pressure maintenance for firefighting
  3. Surge tank for the entire system

With little redundancy in Duncan’s water network, reservoir failure could be catastrophic, city officials paid close attention when problems with the reservoir began to reveal themselves.

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“The first leak we noticed was a very small hole in the shell,” says Len Thew, City of Duncan operations manager. “It wasn’t a major leak, maybe a liter per minute; but since the exterior epoxy appeared sound, we suspected a little corrosion inside the tank.”

After draining the tank to perform a thorough inspection, those suspicions proved correct. Well, partially correct.

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“We did discover corrosion, but it was way worse than we thought,” Thew reveals. “There are thousands of bolts holding this reservoir together. Nearly all of them were corroded, with heavy tuberculation on the bolt heads and along lap joints.”

Still, this inspection didn’t conclusively prove that the tank was close to structural failure, and didn’t reveal the total number of leaks.

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Initial Optimism

“After the initial inspection, we were fairly optimistic about the reservoir’s remaining lifespan, and weren’t thinking about immediate replacement,” Thew explains. “Despite the obvious corrosion, we assumed that we could apply a coating of some sort — doing the work ourselves — and get several more years of service.”

Preparing to perform this non-structural rehabilitation, Duncan commissioned an engineering firm to conduct a more thorough inspection of the reservoir — and the results were alarming. Basically, corrosion was so serious that the reservoir was already at the end of its useful life and — rather than one small leak in the shell — the reservoir in fact suffered from 240 leaks, mostly at bolt placements. Rather than repairing the existing reservoir, engineers recommended immediate replacement with a concrete structure.

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This put Duncan’s public works department in a difficult position. Funds had been made available for the small-scale repair they’d envisioned. But actually replacing the tank was a much larger proposition — about $700,000 larger. Funds for replacement wouldn’t be available for two to three years.

What Duncan really needed was something between a quick repair and a full rehabilitation. Put another way, they needed a cost-effective, lifespan-extending solution that would keep the reservoir safely in service for the two to three years needed to raise replacement funds.

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The sealant they considered (and even tried) wouldn’t work; a simple coating wouldn’t address the reservoir’s structural issues. Also, the theoretical solution had to be identified and applied quickly —Eagle Heights Reservoir really appeared to be on the verge of failure.

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Thew began to explore options and made many calls to colleagues. One possibility was mentioned several times: Infrastruct had a reputation for innovative infrastructure solutions in the water, wastewater and stormwater fields, and for fast response. Thew gave the local contractor a call.

Innovating on Demand

“It was an interesting phone call,” says Votkin. “As a contractor responding to a failing reservoir, our usual challenge is to fix the leaks and address structural issues as permanently as possible. But that wasn’t what Duncan was asking for. In this case, what they really needed was a repair, within their budget, that would keep the reservoir going safely for a few years. It wasn’t what I was used to… but after thinking about it a bit, I did come up with something that might work.”

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The Infrastruct team proposed a cost-effective solution that incorporated two technology innovations.

1) Avoid Sandblasting – “Normally, for a bond directly to steel we would abrasion blast to bare metal, to remove corrosions and ensure the best possible bonding,” Votkin says. “Here, we realized that might be overkill, for a couple of reasons. First, we could do targeted mechanical cleaning around the worst areas, to achieve bonding sufficient for the two-to-three-year extension needed. And second, sandblasting would itself cause a great deal of damage to the hundreds of very fragile, corroded bolts in this tank—it might do more harm than good. We opted for a water blast cleaning with greater emphasis on the wall bond then floor while still achieving the quality of repair needed.”

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2) Address Structural Issues with a Foam-and-Polyurea “Sandwich” – The urgent structural issues identified by engineers were mainly due to the reservoir’s failing floor. With more than 240 leaks identified in the bottom, and significant corrosion affecting nearly all bolts and joints, the steel floor was about as sturdy as wet cardboard and could give way at any time. Simply coating the floor with epoxy or polyurea wouldn’t work, since the fragile steel substrate provided minimal structural support.

To address this, Infrastruct proposed a three-layer “sandwich” comprised of primer, structural foam, and polyurea to provide a higher strain capacity. “Since this was the bottom surface, and water pressure would be holding everything in place, a simple primer with minimal surface prep would be enough to ensure good bonding,” he explains. “Then, on top of the primer, a 76.2-mm layer of 6-8 lb structural foam would provide a sturdy floor that would fill valleys, cover all the bolts, and inhibit further corrosion. Finally, a top layer of Armour Polyurea (NSF 61 Certified) product would seal everything very tightly and be flexible and strong enough to cope with any settling after water loading.” This combination of materials was not in itself an innovation it has been used in rooftop restorations and OBIC utilizes this approach for manhole and wet well restoration.

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An Efficient Project

After Infrastruct’s concepts had been vetted and approved, the first order of business — after implementing safe confined space entry procedures — was a thorough visual inspection, to identify problem areas on vertical walls. These areas of potential leaks would require polyurea coating. Because they were on a vertical surface, extra care would be needed to achieve a good bond and avoid the applied material sliding off.

With sandblasting ruled out, Infrastruct instead used mechanical grinding with coarse sandpaper and a wire wheel to apply an anchor pattern. “It basically makes the metal rougher, but if you do it right, it more than doubles the surface area, and that makes for a very good bond,” Votkin says.

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After grinding, the crew vacuumed and swept up dust, then primed. Within the two-hour window after the primer application — when bonding is most reliable — the polyurea coating was applied to all vertical surfaces.

That was day one.

On day two, Infrastruct cleaned and dried the floor, without doing extensive grinding. On the horizontal surface, with water holding things in place, bonding would not be as critical a factor. Primer was applied, then structural foam.

“The primer and foam worked together to cut off oxygen and inhibit corrosion,” Votkin says. “Once rust is established we can’t stop it entirely, but we can certainly slow it down and minimize further damage.”

The foam used was a two-part composite, pumped at 2,500 psi and mixed at the nozzle. The mixed material heats up to about 140 degrees and expands quickly, but it also sets quickly. “It actually sets up immediately, within 15 seconds,” Votkin says, “and can be walked on without doing any damage.”

The foam layer is stiff, crush-resistant, and can easily support the weight of the water in the filled tank. But the reservoir is on a gravel bed, and the floor was likely to shift and settle a bit when being filled. The polyurea layer, in addition to providing a waterproof seal, is also strong and flexible enough to maintain structural integrity during water loading. The end result after the application of three layers (in a single day) was a smooth, waterproof, perfectly solid floor.

Infrastruct’s application equipment is computer-monitored and will shut down automatically if anything gets out of whack with material ratios or temperatures.

“That’s our main method for assuring a good quality project,” Votkin says. “We also do visual inspections mid-application, and can patch the foam with a polyurethane caulk if needed. It wasn’t, on this project.”

bolted steel reservoir

Impressive Results and Lessons Learned

The City of Duncan was certainly happy with the results of this rush project.

“In all, we had the reservoir shut down for a little more than three weeks,” says Thew. “But most of that was for the initial inspection, and for our own attempt to apply a sealant. Infrastruct responded within a couple of days, and was only onsite for two days. After that, we took a couple of days to inspect and fill the tank, and check for leaks. I’m happy to say we didn’t find any.”

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Overall costs of the project — including the engineer’s report, Duncan’s repair attempts, and contractor costs — came to just under $50,000.

“Honestly, we’re impressed,” Thew allows. “Costs were reasonable, and this seems like a repair that could hold up for a very long time. In hindsight, we learned from this that we should have been inspecting more frequently, so the reservoir’s corrosion wouldn’t have been such a surprise. Thankfully, we found a good, smart contractor, who really listened to what we needed, and came up with an innovative solution that solved our exact problem.”

That was in 2018, and four years later, the repair is holding fast. The city is coming up on collecting enough funds to cover the permanent reservoir replacement. Duncan authorities have kept their eye on the status of the repair with close monitoring. Residents have been able to sleep soundly in the interim, knowing they could rely on the tank to maintain its structural integrity and keep the water where it belonged.

A Timely Reminder

In addition to the engineer’s inspection report, there was another factor that made Duncan’s Eagle Heights Reservoir repair especially urgent: Within a few days of the initial damning inspection of the failing structure—when the reservoir was still out of service—the city’s fire department responded to a building fire.

All went well in this case, as the fire was in an area not served by the reservoir. But the incident was a timely, stark reminder of all that was at stake, and of the urgency required in finding a fast, safe solution.
When Votkin offered to send a proposal based on his ideas, Duncan officials instead asked, “Can you be here on Monday?”

He could.

“I agreed to the accelerated schedule, got my crew and equipment together, and we caught the first ferry Monday morning,” he says. “We were onsite by 10 a.m.”

Angus W. Stocking, L.S., is a licensed land surveyor who has been writing about infrastructure since 2002.

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