Construction workers cautiously resumed drilling a tunnel for King County's Brightwater sewage-treatment plant Monday afternoon after seeing no sign of settling in or around a sinkhole that swallowed the driveway of a Kenmore home a day earlier.

Construction workers cautiously resumed drilling a sewage tunnel Monday afternoon after seeing no sign of settling in or around a sinkhole that swallowed the driveway of a Kenmore home a day earlier.

Observers were watching the sinkhole and using survey devices to detect even slight movements in the soil 150 feet above the tunnel that will carry treated sewage from King County’s Brightwater sewage-treatment plant to Puget Sound.

“We’re standing by to watch for it and backfill if we need to,” Judy Cochran, a Brightwater construction manager, said after a 16-½-foot-wide <137,2009/3/9/17/13/26,kervin>CQ<137> tunnel-boring machine drilled another 5 feet of tunnel.

Kenmore resident Pauline Chihara discovered a 30-foot-wide, 15-foot-deep sinkhole early Sunday in front of her house at 61st Avenue Northeast and Northeast 195th Street.

Cochran said the sinkhole apparently was caused by the boring machine pulling in too much sand over a 20- to 25-foot stretch, leaving a void that allowed the soft soil to collapse.

“They need to balance the pressure at the face of the tunnel-boring machine against the groundwater and the ground pressure that’s in front of them, so they can excavate exactly that size of hole, and not bigger and not smaller,” Cochran said.

The sinkhole opened up nearly 1-½ miles west of the North Kenmore tunnel portal, where a construction worker died in November 2006 after he was struck by a steel beam while a noise barrier was being built.

Vinci/Parsons/Frontier-Kemper, a joint venture, is digging the Kenmore portion of the 13-mile tunnel that will carry treated sewage from the $1.8 billion Brightwater plant north of Woodinville to a Puget Sound outfall at Point Wells.

In December the state Department of Ecology (DOE) issued a notice of violation to King County after a series of tunnel-related mishaps in August, September and October stirred up more mud and caustic sediment than allowed in Bothell’s Horse Creek and downstream in the Sammamish River.

The DOE said pressurized air shot into the creek, stirring up sediment after escaping from a tunnel-boring machine 160 feet below the creek. The air was pressurized to prevent an area in front of the machine from collapsing while workers did routine maintenance to the face of the boring machine, Cochran said.

King County’s wastewater director, Christie True, responded to the DOE that the county and the contractor had taken steps intended to prevent damage to other creeks and to repair damage that might have harmed fish and other animal life in Horse Creek.

True contended the accidents didn’t violate water-quality laws because they were “caused by an upset condition, outside of the immediate control of the county and contractor.”

The DOE has not issued a final order.

Keith Ervin: 206-464-2105 or kervin@seattletimes.com