Decades-old wooden septic structure caused sinkhole on Fifth Avenue, officials say

A large sinkhole that snarled eastbound traffic on Fifth Avenue near Merrill Field Airport on Thursday was caused by an old wooden septic system associated with homes that once existed in the area, an Alaska transportation official said Friday.

“There were houses in this area in the 1940s and 1950s, and the old septic systems were made out of wooden cribs,” said Shannon McCarthy, a spokeswoman with the Department of Transportation and Public Facilities.

“It’s really deep, about 12 feet underground, and when we built the road, we didn’t realize the old wooden crib was down there,” she said.

Bore holes drilled before the road was built, to analyze underground conditions, did not locate the structure, she said.

Handmade underground wooden structures, called cribs, were sometimes part of old household wastewater systems in Alaska’s territorial days, said Bill Rieth, an engineer with the Alaska Department of Environmental Conservation. Some were phased out as late as the 1940s or so.

The structures inevitably fail as the wood rots, causing problems like sinkholes, he said. Today they are illegal under state law. Buried septic tanks are used now.

A driver noticed the sinkhole early Thursday and reported it to transportation officials, McCarthy said.

[Spenard sinkhole signals a looming crisis over Anchorage’s aging drainpipes]

The hole was in the middle of three eastbound lanes, near the Wendy’s restaurant at Reeve Boulevard.

The two right lanes were closed for investigation and repairs, from about 10:30 a.m. until about 5:30 p.m. she said. An online traffic alert allowed many commuters to find an alternate route as they headed out of Anchorage, she said.

The road has been reinforced and repaired with asphalt, and the state transportation agency will closely monitor the area, she said. The crib remains underground, McCarthy said.

“We run into these on occasion on our older roads,” she said.

The state repairs about two to three sinkholes in Anchorage a year, she said.

They’re often associated with roads that are 50 years old or so. One problem are old underground pipes for road drainage that fail, eroding the earth behind a street and causing pavement to fail.

“Over a period of time, water starts to move material and create a void,” she said.

Drivers noticing pavement subsidence or small holes should call the transportation department, she said. Reports in Anchorage can be made to (907) 338-1466.

After regular business hours, drivers can also call the 311 non-emergency line, and the Anchorage Police Department will relay the message to road officials, McCarthy said.