For the second time during construction of the Brightwater sewage-treatment plant, a large sinkhole has opened up above the 13-mile tunnel being mined to carry treated waste to Puget Sound — this time in Bothell.
For the second time during construction of the Brightwater sewage-treatment plant, a large sinkhole has opened up above the 13-mile tunnel being mined to carry treated waste to Puget Sound.
King County wastewater-treatment officials don’t know when the new hole, up to 30 feet wide and more than 15 feet deep, opened up in a wooded area in Bothell. It was discovered this month after a neighbor reported to the city that a tree was leaning against a cable-TV line.
“It’s so close to the Brightwater alignment that it’s pretty reasonable to assume that it’s related to Brightwater,” said Annie Kolb-Nelson, spokeswoman for the county’s Wastewater Treatment Division, which is building the $1.8 billion plant and related pipelines.
Contractors have fenced off the sinkhole for safety and will fill it after they receive a city permit, Kolb-Nelson said.
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The sinkhole is the latest problem in a deep-bore tunneling project that has been complicated by rapidly changing, sometimes unstable soils and the difficulty of maintaining and repairing machines under high air pressures.
The treatment plant is almost complete, and all but 1.5 miles of the 13-mile tunnel are finished. The sewage-treatment plant and pipelines are supposed to be fully operational by mid-2012.
A 30-foot-wide, 15-foot-deep sinkhole obliterated the driveway of a Kenmore resident after a Brightwater tunnel-boring machine passed below the house in March 2009. Officials said the machine excavated too much sandy soil.
Tunnel construction manager Judy Cochran said the Bothell sinkhole apparently was caused by an August 2008 construction mishap that also stirred up sediment in Horse Creek and the Sammamish River and temporarily altered the chemistry of the two waterways over a two-month period.
The state Department of Ecology issued King County a notice of violation and ordered tighter supervision of underground repairs on tunneling machines.
The problems occurred while compressed air was being used to hold back the soil during routine maintenance on the cutterhead of a boring machine 150 feet below ground. As workers were entering the excavation chamber, they discovered sand had poured into it and they retreated, closing the hatch behind them.
“They kind of left in a hurry,” Cochran said. Repairs were postponed until the machine could reach a safer area.
The collapse of soil into the machine’s excavation chamber apparently sent compressed air into Horse Creek several hundred feet away and created the sinkhole directly above the machine, Cochran said.
Because the sinkhole is in an area of brush and trees separated by a fence from a nearby condominium complex on 93rd and 94th avenues Northeast just west of Highway 527, it may have gone unnoticed for months, Cochran said.
“They’re doing their best to correct it, I’m sure,” said Jorge Landa, who owns the property with the sinkhole and the Delanda Dog Inn kennels. His main concern, he said, is that the well serving his kennel not be damaged again, as it was when the tunnel was bored beneath his land.
Residents along the tunnel route have complained of noise, a flood of sewage, cracked foundations and other problems.
Cochran said the county asked geotechnical engineers a couple of months ago to take a close look at excavation records for any indication there may be other “voids” that could lead to surface settling or sinkholes.
Problems on the Brightwater tunnel have led some critics to predict more serious problems in the planned replacement of the Alaskan Way Viaduct with a world-record 58-foot-diameter tunnel.
The state’s tunnel contractors will try several tactics to prevent soil settlement that can cause sinkholes. These include grout injections in the soil around buildings, especially in the soft soils near Pioneer Square; laser-guided measuring devices at the rear of the tunnel machine to ensure soil isn’t being removed too quickly; grout injections from the machine; and a $20 million incentive payment to contractors if they succeed in avoiding settlement of buildings, known as “deformation.
The two boring machines blamed for the Bothell and Kenmore sinkholes were idled for most of 2009 and part of 2010 as their operator struggled to make major repairs without subjecting workers to extremely high air pressure. The contractor completed the Kenmore-to-Bothell tunnel segment last June and was replaced on the Kenmore-to-Lake Forest Park portion.
Seattle Times staff reporter Mike Lindblom contributed to this report. Keith Ervin: 206-464-2105 or firstname.lastname@example.org