Small sinkhole closes 2 lanes of Hwy. 204 in Lk. Stevens

As reported June 24, 2011 on KOMOnews.com

A state Department of Transportation official examines the small sinkhole, which has closed two lanes of Highway 204 in Lake Stevens.

LAKE STEVENS, Wash. - A small sinkhole has closed the eastbound lanes of Highway 204 in Lake Stevens, state transportation officials report. The sinkhole, which appeared Friday at the Highway 204 intersection with 81st Avenue SE, apparently was caused by an inactive sewer line under the roadway, officials said. A state Department of Transportation spokesperson said both eastbound lanes of the highway are closed. Eastbound traffic is using one of the westbound lanes, with one lane of traffic in each direction. Officials weren't able to say when the eastbound lanes might reopen. The sewer line was installed for the new plant in Lake Stevens, but hasn't yet been used.

That's no DOT official in the above photo - it’s none other than Puget Sound’s celebrity geotechnical engineer and founder of HWA, Sa Hong!

HWA determined the cause of the Lake Stevens sinkhole was a void in the soil surrounding the steel sewer pipe, which was horizontally bored under the roadway some three years earlier. The pipe itself was undamaged; the 13 foot deep void was an artifact of the pipe boring process. Voids at depth will eventually migrate to the surface as soils slowly collapse into them from above, until the hole breaches the surface and causes the sinkhole.

HWA was called to immediately investigate other areas over the pipeline with rapid, nondestructive testing using our Falling Weight Deflectometer (FWD) to see if other voids were lurking underground. HWA found one other hidden void, and both voids were then filled with cement to prevent future sinkholes.