Paula Wallis, Reporter

Over-excavation on one of the Brightwater tunnel drives is being blamed for a 15ft deep x 30ft wide sinkhole (4.5m x 9m) in a residential neighborhood north of Seattle, Washington.

A woman discovered the sinkhole at about 6:30am Sunday morning (8 March) at the end of her driveway nearly 1.5 miles west of the North Kenmore access shaft.

The incident happened on the westward drive of the Central Tunnel contract held by the Vinci/Passons RCI/Frontier-Kemper JV for the 13-mile Brightwater conveyance tunnel in King County.

According to the owner, the BT3 machine was about 7,000ft into its 20,100ft (6.2km) drive towards the Ballinger Way reception shaft when the night shift experienced the trouble. The Machine is one of two Herrenknecht Mixshield TBMs working in opposite directions from the North Kenmore shaft (Fig. 1).

"We know definitely that the TBM excavated too much material," said Judy Cochran, Construction Manager for Brightwater. "Beginning Saturday night and into Sunday morning the graveyard shift over-mined about five rings. The why, is the part that we are still looking into."

Lionel Suquet, Project Manager for the JV did not refute the owner's assertion but would not elaborate on why the over excavation occurred.

Brightwater conveyance tunnel plan

"We are currently investigating the cause and it would be speculation at this point, until we know for sure what happened," said Suquet.

The BT3 machine had been making good progress in sandy conditions at about 150ft (46m) below the surface and following two weeks of downtime in January for cutter changes.

"They were successfully mining in sand so it's not a lack of capability to tunnel through this material," said Cochran. "The crew knew it had some issues with over-excavation at the time, but they have over-mined in other places along the alignment and were able to correct it from within the tunnel without incident."

Groundwater pressures of 7.5 bar are predicted in the contract's GBR and the TBM has encountered up to 5.2 bar to date, as well as high artesian water pressure. According to Cochran, the contractor had been operating at 4 bar and had "increased the pressure slightly on Saturday before the incident, possibly in an attempt to keep control of the situation."

Neither the owner nor the contractor would discuss the focus of their investigations, but these will likely look at several possible causes, including the experience of the slurry machine operator with the closed slurry system making it difficult to judge the amount of material being excavated during a shove. Another possible cause might be the presence of a large boulder in the face that stalled penetration without slowing extraction of material and caused over-excavation. A third possibility is the meeting of high artesian water pressure and its influence on the excavation cycle.

The Brightwater tunnel runs east west under hills that are north south trending. BT3 was progressing through one of the valleys when it started pulling in too much sand over a 20ft to 25ft stretch. At 150ft deep, the machine had less cover than it had been in recently, but it has operated under shallower cover without incident (Fig 2).

Geological section

This sinkhole comes days after the fatal excavation collapse in Cologne, Germany that brought down three buildings and claimed the lives of two residents in the two apartment units.

Martin Knights, President of the International Tunneling Association (ITA), said he was saddened by the news and expressed condolences to the families affected.

"The collapse in Cologne and the sinkhole in Seattle come at a critical time for the industry," said Knights. "There is so much tunneling work being proposed worldwide, but with governments struggling under the current economic constraints, the public, decision makers and insurers can be adversely influenced by any negative events."

The Brightwater sinkhole happened just as the Washington State Legislature is preparing to vote on a deep tunnel replacement for the Alaskan Way Viaduct highway under downtown Seattle. There is much public debate regarding the January 2009 decision in favor of the $4.2 billion tunnel replacement and this recent event will likely fuel opposition of the project.

Meantime truck loads of crushed rock and sand backfilled the sinkhole Sunday afternoon to stabilize the hole and cover an exposed gas line. The owners say in addition to the routine monitoring, the contractor has set up additional survey points on the road, adjacent to the homes, and on the buildings themselves and that surveyors are taking readings three times a day.

BT3 machine resumed excavation of the Central Tunnel's westbound drive on Monday, 9 March and had built more than seven rings without issue by Tuesday morning.

The second Herrenknecht machine (BT2), excavating the eastbound drive to the North Creek shaft and pump station, has been shut down since mid-December for extensive wear repairs to the cutterhead. With work being done under air pressure of up to 4.4 bar, compressed air maintenance crews work for 45 minutes only, followed by three hours of decompression. The machine is about 6,500ft (2,000m) into its 11,600ft (3.5km) drive.

A LOVAT EPBM for the Jay Dee/Coluccio/Taisei JV on the West Tunnel is progressing well after launch in September 2008 following a delay of roughly five months. The TBM stopped last week for inspection and maintenance, but has resumed excavation and is about 5,600ft into its 21,100ft (6.4km) drive.

A second LOVAT machine broke through on the East Tunnel contract for the Kenny/Shea/Traylor JV in November 2008 after taking about 14 months to complete its 14,050ft (4.2km) drive.

The $1.8 billion conveyance tunnel facility is scheduled to be complete in 2011.

References

Brightwater update - TunnelTalk, Jan 2009
Fatal collapse on Cologne’s new metro line - TunnelTalk, Mar 2009
Bored tunnel for downtown Seattle - TunnelTalk, Jan 2009

King County’s Brightwater Project website