One piece at a time, large stormwater pipe buried in Cottage Grove
Imagine trying to bury short cylinders end to end underground across a backyard.
Now try doing something similar — albeit on a much larger scale — with sections of concrete pipe at depths of roughly 40 feet across a full mile of sometimes rocky soil.
That is part of the work that’s creating the mess of dusty roads and traffic closures around Keats Avenue and 70th Street in Cottage Grove.
In conjunction with improvements this summer and fall to 70th and Keats and the addition of a roundabout to that intersection, crews are installing a 6-foot diameter pipe to move overflowing stormwater from the northern portion of the South Washington Watershed District — including parts of Lake Elmo, Woodbury and Oakdale — south through Cottage Grove’s East Ravine area along Keats to a drainage field near the Mississippi River.
From a basin northwest of 70th and Keats to a point southeast of that intersection, roughly 5,000 feet of pipe is being laid eight feet at a time. Another 550 feet of the pipe is being hydraulically pushed horizontally beneath the roadway and in areas where digging and placing the pipe from above would be too disruptive, such as along the northern edge of the Shoppes at Almar Village commercial development.
The watershed district-led project requires the heft of large construction equipment to move tons of sandy soil and buried rock but also the finesse of hand tools to move small amounts of dirt as pipe sections are jacked into place and sealed with just a large rubber seal.
Precision and patience are critical.
Once the pipe is buried, crews must carefully refill the hole that was dug to lay the pipe. That can only be done 8 inches at a time. One layer of fill is packed before another 8 inches of loose soil is dumped into the hole and packed.
“It’s a slow-moving project,” David Thompson, resident project representative of HDR Construction, told local officials during a recent tour of the project. “It takes days and days to put the dirt back in the hole.”
The project actually is being completed in multiple phases. Crews will finish the $3.1 million Phase 1 this construction season. The subsequent four phases will done in the coming years and completed by 2019 , watershed district Administrator Matt Moore said, extending the stormwater system south toward the Mississippi River. It will pass through Cottage Grove Ravine Regional Park as overland flow, beneath Highway 61 in a pipe and across 3M-Cottage Grove property as overland flow, Moore said.
The system is designed to serve a roughly 24-square-mile drainage area. Only 20 percent of that area is developed but both cities predict future housing growth in the area.
“As things are developed, this overflow will allow a way out for water,” said HDR Construction’s Matt Redington, who is the stormwater project’s engineer of record.
Drainage basins serving the area will hold stormwater that accumulates if 6.3 inches of rain falls in a 24-hour period. A more severe rainfall would trigger the need for the new overflow stormwater pipe.
“What we’re really building it for and planning it for is that rare event,” Moore said.
The section of pipe being installed this year will be temporarily capped while work continues on the next phases of the project.
The mile of pipe is expected to be completed by October, but Keats Avenue and 70th Street will not be fully reopened until November, said Cory Slagle, Washington County’s engineering and construction manager.
The road and stormwater projects were coordinated so that as the pipe and other underground utilities are buried, road crews can follow behind and pave the realigned 70th Street.