City staff scratching heads over U.S. 2 sinkhole
A sinkhole that appeared on the U.S. 2 strip in Columbia Falls near Meadow Lake Boulevard left city staff wondering who was going to pay for an expensive construction error.
The sinkhole appeared in the right-hand westbound lane about 7 p.m. on Feb. 25. Police chief Dave Perry said he responded and saw a deep void under the highway that extended north beneath the shoulder. He said the remaining six inches of pavement could not support the weight of a small vehicle, let alone a loaded logging truck.
Montana Department of Transportation road crews filled the void with gravel and had the lane open for a short while before closing it again on March 1.
Personnel from the Kalispell public works department brought a mobile video camera to the site, dropped it down a manhole about 100 feet west of the sinkhole, and then ran it east up a 36-inch storm drain to figure out what happened.
MDT installed the storm drain when U.S. 2 was rebuilt in 1983. From the railroad crossing, storm drains run east to a retention pond near the Flathead River and west to a retention pond below Tombrink Hill. The storm drain is about 6 feet below the highway.
Video footage shows a 6-inch steel conduit running straight through the storm drain, and 15 feet further a 12-inch PVC water main also running through the middle of the storm drain. Surrounding soil had entered the storm drain and been carried away, creating the large void beneath the highway.
The conduit and the water main were installed by horizontal boring to avoid tearing up the highway when utilities were run from the Super 1 Foods area north to the Timber Creek Assisted Living and Autozone area.
The steel conduit is owned by CenturyTel, but city staff were uncertain when it was installed.
The water main was installed in September 2011 by the developer of the land north of the highway and now belongs to the city of Columbia Falls.
While the 6-inch conduit may not need to be removed from the storm drain and rerouted, the 12-inch water main is blocking flow in the stormwater drain.
One solution is to dig up both westbound lanes of U.S. 2 and reroute the water main under and around the storm drain.
The Montana Department of Environmental Quality may want the water main relocated at least two feet below the storm drain. Who will pay for this expensive fix is not certain, but it will likely involve lawyers from the city and their insurers, the Montana Municipal Interlocal Authority.
In the meantime, the holes in the storm drain where the two pipes run through it have been grouted to prevent more soil from entering the storm drain and creating another sinkhole.
Where all that soil from the original sinkhole went is also a concern — the storm drain may be plugged up.