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Ageing Mill Creek stormwater system causes sinkhole and emergency repairs

A failure in Mill Creek's ageing stormwater system caused sinkholes to develop in the Sweetwater Ranch neighborhood during December, January, and February. A proposed emergency project will hopefully repair underground piping and couplings between Sweetwater Ranch and Douglas Fir by April 2018.

By Richard Van Winkle, News of Mill Creek.

At their regular March 13, 2018, meeting the Mill Creek City Council will approve an emergency stormwater system repair project between the Sweetwater Ranch and Douglas Fir neighborhoods. The total cost is estimated to be less than $300,000.

According to the city council’s agenda summary, city staff first became aware of the stormwater system’s failure on Wednesday, December 20, 2017, when a sinkhole formed at 1328 144th Street SE in the Sweetwater Ranch neighborhood.

The agenda summary states, “The sinkhole extended from the south side of the road (adjacent to the curb), underneath the concrete sidewalk and into the resident’s front yard. The City took immediate action by dispatching Public Works staff and activating our Public Works Emergency Response Mutual Aid Agreement with Snohomish County. The site was secured overnight by installing two steel plates over the sinkhole and fencing off the area.”

Snohomish County crews mobilized under an emergency response mutual aid agreement with the city to conduct temporary repair work. They determined that the sinkhole was caused by a failed coupler and damaged 36-inch diameter corrugated metal stormwater piping 11 feet below the ground.

A temporary solution was installed, but another smaller sinkhole developed at the same place in January.

City staff conducted closed-circuit television inspections of the stormwater system, which showed a hole in the bottom of a pipe connecting the Sweetwater Ranch stormwater system to the stormwater system in the Douglas Fir neighborhood.

According to the city council agenda summary, “… the 36-inch diameter corrugated metal pipe was installed in 1981 as part of the Douglas Fir housing development and is approximately thirty-seven years old. The Sweetwater Ranch housing development was built approximately ten years later and has a separate stormwater drainage system.”

Because the City of Mill Creek has declared this to be an emergency project costing less than $300,000 a public bidding process is not required, and Shoreline Construction Company has been selected to perform the work.

It is estimated that the project should begin the week of March 12th and will take approximately 20 working days unless complications arise.

Map showing areas where closed-circuit television scans were made. Image courtesy of City of Mill Creek.

Map showing areas where closed-circuit television scans were made. Image courtesy of City of Mill Creek.

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