Sinkhole that closed lanes of 5th Avenue was caused by decades-old wooden septic system

Police relayed information from the Alaska Department of Transportation that the two right...
Police relayed information from the Alaska Department of Transportation that the two right lanes of 5th Avenue are closed between Concrete and Reeve as crews work to fix the roads.(KTUU)
Published: May. 27, 2021 at 9:46 AM AKDT|Updated: May. 28, 2021 at 7:29 PM AKDT
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ANCHORAGE, Alaska (KTUU) - A sinkhole that interrupted traffic for the better part of the day Thursday was caused by decades-old wooden structures used in the past as a septic system, which finally broke down.

Shannon McCarthy, a spokesperson the the Alaska Department of Transportation and Public Facilities, said the sinkhole on 5th Avenue near Merrill Field went down about 12 feet into the ground and took up about an entire lane. The two right lanes of the road were closed between Reeve Boulevard and Concrete Street from Thursday morning until about 5:30 p.m., she said.

“That’s what happens with sinkholes,” McCarthy said.

They’re different from potholes, she said, in that department staff will typically see a small hole at the surface coupled with sinking or sagging asphalt around it.

This particular sinkhole was caused by the breakdown of a much older septic system than what Anchorage residents enjoy today. Back in the 1930s and ‘40s, McCarthy explained, this area of 5th Avenue had homes in it. Those houses used wooden structures called cribs as a septic system deep in the ground.

So deep, that McCarthy said the department doesn’t always find those old structures when surveying to build a new road. That particular road out of Anchorage is quite old itself, McCarthy said, though she’s still working to pin down exactly when it was built.

“At that time the way you built a septic system was by building a ... wooden box,” she said.

Those old houses are gone now, but McCarthy said “the septic systems weren’t always discovered.”

“So, occasionally we’ll have this where there’ll be a sort of a wooden crib and it will be really far down,” she said. “Like 12 feet’s pretty far down.”

The old wooden cribs do eventually collapse. That’s what happened Thursday, with the collapsed crib taking road material with it. Falling material creates the void which leads to a sinkhole.

The department shored with the road with fabric, then filled the sinkhole with filling material and compacted it before covering it over with asphalt. McCarthy said the department will continue to monitor the area in case there is any more movement or shifting in the road material underneath.

Failing wooden septic systems are not the typical way a sinkhole forms in Alaska. More often, McCarthy said it’s something like a broken storm drain or water main. When those systems involving water break down, it creates opportunity for flowing water create voids under the pavement.

“What typically happens is that material is being washed away or is falling, you know if there’s a void under there, and then it creates a larger void under the pavement,” McCarthy said.

Department staff don’t want drivers going over those areas, she said.

“Because chances are that that asphalt is not structurally sound,” she said. “It’s not really able to take the weight of a car.”

If anyone sees a sinkhole, they are encouraged to call the department at 907-338-1466.

Editor’s note: This article has been updated with more information and quotes.

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