On the morning of June 22, the bottom of a 36-inch water main in the heart of Robbinsdale burst, peeling back several feet of concrete-coated steel pipe like a can of sardines.

Over the next 40 minutes, an estimated 600,000 gallons of water blasted downward, creating a hole 20 feet deep at the city's busiest intersection.

"This is your worst nightmare," said Crystal City Engineer Tom Mathisen, who supervised the repairs. "It's always kind of hair-pulling, but yet, because we do this kind of stuff all of the time, there's a process to do it."

The complete repair and reconstruction of the giant Robbinsdale Sinkhole was finished in three weeks, an impressive feat considering the magnitude and complexity of the damage. Water gushing from the broken water main bored down 10 feet and destroyed a sanitary sewer line, which filled with sand and dirt. Then water, dirt and debris churned upward, taking out a storm sewer pipe that sat less than a foot above the water main.

The water continued to drive toward the surface, and eventually popped off several manhole covers, flooding the intersection of 42nd Avenue (County Rd. 9) and Bottineau Boulevard (Hwy. 81) around 10 a.m. In a stroke of luck, a nearby gas line was unscathed and no one was injured.

"I certainly have to commend the [various public works departments] for how quickly they turned the water off ... and then repaired it," said Robbinsdale Mayor Regan Murphy. "It was an amazing response — I mean, it was a 20-foot hole, and they had [Hwy. 81] open in two weeks,"

The repairs were especially tricky because the water main takes two slight turns near the break, one at a 45-degree angle and one at a 12-degree angle. The bends had to be replaced with custom piping, which was trucked in overnight from Dayton, Ohio.

"Thirty-six-inch ductile iron pipe is not something you just keep on your shelf," said Mathisen.

There was speculation that the blowout was related to the severe weather that ripped through the Twin Cities June 21-22, but city officials say that the break was probably a result of a leak that slowly built for years. Mathison pointed out clusters of pinholes around the spot where the 50-year-old pipe burst as evidence of it weakening over time.

A community curiosity

Packs of onlookers gathered to marvel at the massive hole as workers began the repair process.

Mathisen insists that the Robbinsdale Sinkhole was not actually a sinkhole at all but rather a "blowhole" that resulted when the water pressure from the break blew the road apart. A sinkhole occurs when water weakens the ground below the surface, causing a collapse.

People cracked jokes and shared funny Photoshopped pictures on Facebook, and crowds spent their lunches sitting in the grass on the side of the formerly busy road watching the repairs.

"It became like a peanut gallery. People were gathered at the parking lot at McDonald's and over at Pilgrim Cleaners, cheering," said Mathisen.

"On Facebook, someone posted a picture of a Tyrannosaurus rex coming out" of the hole, added Murphy. "There was talk of making T-shirts, and a lot of people drove down to come look at it."

A Twitter account, @Rdale Sinkhole, even sprang up to make light of the giant crater.

Mayor Murphy said that while all of the wisecracking was fun, most people he talked to were just relieved that no one was hurt. And some nearby residents had minor flooding due to the sanitary sewer getting clogged, which wasn't funny at all for those homeowners.

Tangled teamwork

The burst water main is owned by the Joint Water Commission, a coalition of three cities — Crystal, Golden Valley and New Hope — that buy their water from Minneapolis. The Robbinsdale Police Department provided security for the site — it was, after all, the Robbinsdale Sinkhole.

Hennepin County got involved because the break occurred on County Road 9, and private contracting companies Northern Dewatering (water and sewage excavation), Valley-Rich (pipe repair) and C.S. McCrossan (repaving) worked on the repairs, in addition to a team of municipal workers headed by Mathisen.

In all, Mathisen estimates that there were 60 workers on-site initially, but that subsided to a daily number closer to 20 once repairs commenced. At this point, it's still unclear who will be paying for what.

"How much reimbursement is due to us for our costs handling our emergency response and having to get the sand out of our sanitary main and storm sewer main has yet to be worked out," said Marcia Glickman, Robbinsdale city manager. Glickman expects that the specific payments will be worked out through insurance companies. Total costs were estimated by Mathisen to be between $300,000 and $500,000.

Over the next two summers, a large section of County Road 9 east of Hwy. 81 is scheduled to be rebuilt, and the Water Commission already planned to replace the water pipes below that section of County Road 9 as part of the project.

Now, in addition to replacing the pipes, Mathisen hopes to purchase Smart Balls from Pure Technologies to help monitor leaks in the system.

Smart Balls are periodically inserted into pipes and record audio as they roll along. They can detect, record and pinpoint the location of minor leaks within 10 feet, according to the Pure Technologies' website.

Mathisen hopes that aggressive maintenance measures like this will extend the life of the pipes and prevent another Robbinsdale Sinkhole from happening.

Ben Johnson • 612-673-4499