A sinkhole developing in the Marine Hills West neighborhood has been declared an emergency by interim city manager Brian Wilson.

The hole, approximately 7 feet deep, is located in Andy McDonnell’s side yard and was reported Oct. 19. The city’s public works department investigated and found that a corroded 18-inch corrugated metal storm drain pipe is to blame. City staff is proceeding with an option known as cured-in-place pipe, or insituform.

The existing pipe is cleaned and lined with a resin-filled, flexible sock. The sock is inflated with high-pressure steam, causing the material to permanently adhere to the failing pipe.

“Essentially, what it is, is a liner,” surface water manager William Appleton said.

It will add structural integrity and stop up any existing areas prone to future seepage. Grout will then be used to fill voids surrounding the pipe.

The work is expected to only minimally impact the pipe’s surrounding environment, which includes McDonnell’s garage, located about 15 feet away. The old pipe is located 17 feet below the ground’s surface and drains 23 acres of mostly residential land. There will be no excavation, Appleton said. A crew will pull the liner through one end of the pipe until it arrives in the area needing repair, he said. Access is located nearby at a storm drain.

“When you use this type of repair material, you’re not having to dig up the old pipe,” Appleton said.

City staff investigated the pipe upstream and downstream of where the corrosion has caused a sinkhole, and found no other weak areas, Appleton said. The liner will only be used in about 166 lineal feet of the pipe, he said. Staff estimates the pipe was installed by the county in the 1960s. There were no signs that the pipe was corroded and failing prior to the sinkhole, he said. McDonnell discovered it when he was in his yard mulling over mole holes.

“It wasn’t there one day and the next day it was there,” he said.

Repairs are four weeks out and will take about a day to complete, Appleton said. A temporary fix is in place. Costs are estimated, not including tax, between $24,000 to $28,000. The money will come from the city’s street water management capital improvement projects funding. McDonnell said the hole has not caused him much inconvenience and the city has worked diligently to eliminate the problem.

The city is currently working on formalizing an asset management program to track Federal Way’s infrastructure and better estimate when repairs are needed prior to emergency situations, Appleton said. Staff currently tracks larger structures, but does not have the resources to inspect the many smaller pipelines throughout the city, he said. Instead, repairs and replacements are done on prioritization based on the pipe’s size, capacity and the risks to residents and properties, Appleton said.