Workers on the Highway 99 tunnel project are starting to cover Bertha with sand as they get ready to resume drilling the tunnel that will replace the Alaskan Way Viaduct (and that was originally supposed to open this month).

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Tunnel-boring machine Bertha is taking on a new nickname this week: Sandy.

Seattle Tunnel Partners (STP) began pouring sand into the stopped Highway 99 tunnel drill’s 120-foot-deep repair vault Sunday, surrounding the giant drill. The operation included a Sunday night shift that stretched past 12:30 a.m. Monday.

The giant filling effort offers tangible evidence the machine could restart by the new year, ending a 1½ -year succession of missed goals and deadlines.

Sand is being used instead of the original soil because it can be packed to a customized density, STP project manager Chris Dixon said last week. But original fill soil will be returned to the top 51 feet of the cylindrical access shaft.

The filling should take most of the week, says an update by the Washington State Department of Transportation.

Further tests are planned, to see how the cutterhead spins under a full load. That follows open-air tests to prove that Bertha could turn without internal scraping or friction.

Another sensitive job is to gradually shut off groundwater pumping wells so pressure returns to the vault area. Buildings and the Alaskan Way Viaduct are being monitored in case of upward soil movement.

Official schedules say the machine should be ready to dig by Dec. 23, though Dixon has said workers might take a holiday break first, after weeks of overtime.

Bertha overheated and stalled Dec. 6, 2013, as grit penetrated a seal casing that lubricates and protects the rotary drive parts. After repairs, STP plans to advance 400 feet, stop for inspections and adjustments, then dive under the viaduct — during which time the state expects to close the viaduct to traffic for up to two weeks as a precaution.

The four-lane, $2 billion tolled tunnel from Sodo to South Lake Union was originally supposed to open this month and is now projected to be done in April 2018. STP’s contract is worth $1.35 billion, but the partnership is attempting to bill its insurers and the state for cost overruns.