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“It allows us to expedite construction and get in the ground sooner,” Laughlin said. But it also comes with risk. In the Mill Woods case, crews ran into pockets of wet sand that they couldn’t drill through. They also had more complex connections at the manholes than anticipated.

“The project would have been done anyway. It’s the backbone for flood prevention,” said Laughlin, calling it one of the largest drainage projects the city has undertaken. “But we should have spent more time at the front end.”

Conceptual design was done in 2006, after severe flooding hit Mill Woods in 2004. The 3.4 kilometres of pipe run from Gateway Boulevard and 30 Avenue to 85 Street and Knottwood Road.

The report says, the city knew the area was risky — the glacial till underlying that area is known to contain random water-bearing layers and large rocks — but the bore holes didn’t pick up the pockets similar to quicksand, and crews hadn’t run into problems previously tunnelling in that area.

But a detailed chronology reads like a series of tunnelling nightmares. They ran into the first patch of wet sand in November 2012. Crews first tried to freeze the sand with a chemical mixture, only to see it shifting around the drop shaft as they drilled. They try to reinforce the shaft but eventually abandoned it and realigned the pipes in the middle of the project.

They hit a new patch of wet sand in March. After it drained, they filled the hole with grout and continued tunnelling. A new patch was found that September, when a drilling rig brought in to investigate had to be quickly pulled from unstable ground. Then in December, the drill bit was damaged by a boulder and flooded with wet sand.