Worst-case scenario: backups for 30 miles

There is no chance all four lanes of Interstate 90 at Snoqualmie Pass will be open by Thanksgiving — and what a mess that could make.

The state Department of Transportation says if some drivers don't choose other routes across the mountains Thanksgiving weekend, there could be an additional 20,000 vehicles trying to cross Snoqualmie Pass on Sunday.

Traffic on the pass has been limited to just one lane in each direction after a Nov. 6 rockslide east of the summit spilled boulders onto the westbound lanes. Some weighed up to 5 tons.

Ordinarily, 27,000 vehicles cross the pass each day. The DOT estimates that the extra traffic could create as much as a 30-mile backup on the westbound side of the pass at the end of next week's four-day weekend.

I-90 is the main artery connecting Eastern and Western Washington. With one lane each way, only about 800 vehicles can get through each hour.

The day after the highway reopened with just half of its capacity, traffic across the pass dropped 26 percent from the November 2004 level, while traffic across Stevens Pass on Highway 2 increased 73 percent, and traffic across White Pass on Highway 12 rose 42 percent, according to the DOT.

Slow, tedious work

Rockzilla (hydraulic demolition hammer)

Reach: Eight stories

Weight: 220,000 pounds

Owner: R.W. Rhine of Tacoma

The business end: A hydraulic jackhammer

Purpose: Release loose rock from the rock wall

Previous project: Demolition of the old Seattle Public Library

Source: R.W. Rhine

Meanwhile, work to repair the crumbling rock walls looming over the pass has been slow, tedious and difficult. The weather has been unpredictable, and the equipment that holds the drill bits has been breaking.

Inside a platform that stretches 80 feet into the air and is covered with a blue tarp, workers drill holes into the loose rock wall and hope the equipment doesn't break.

When the holes are done, the workers fill them with an epoxy mixture and insert 20-foot-long steel rods to stabilize the slope.

Nearby, another crew high on the wall pries rocks off the face and lets them fall to the shoulder of the interstate. Some are the size of softballs; others weigh up to 2 tons.

Not only is DOT trying to fix the latest slide area, the department is under orders from Gov. Christine Gregoire to prepare a full report on slope hazards on I-90 over Snoqualmie Pass. Officials hope to have an early version done by next week.

"This is some of the worst rock on Snoqualmie Pass," said Tom Badger, DOT's assistant chief engineering geologist. He said the state, through contractor Wilder Construction, plans to install 10 to 15 anchor rods into the rock wall. Normally, he said, contractors can drill holes for five to eight rods a day, but because the rock is so unstable they're only able to do two to three each day.

Badger said the casings that hold the drill bits are breaking because the rock wall is inconsistent, ranging from hard rock to cracks to dirt.

On Tuesday, work was slowed because two drill casings broke and needed to be replaced. "It's crumbling like cookie crumbs," said Jamie Holter, a spokeswoman for DOT.

When geologists determine that the rock is too broken and unstable, crews try to remove it, rather than bolt it.

On Tuesday, the DOT brought a new machine to the site to release loose rock, an excavator that stretches eight stories high. Christened "Rockzilla" by the DOT, it's owned by R.W. Rhine Inc. of Tacoma and was used in the demolition of the Seattle Public Library.

The state also plans to bring in another contractor this week that will use a different technology to drill the holes.

3 killed in September

When a slide in September killed three women in a car about eight miles west of the latest slide, DOT was able to bolt the rocks easily because it was granodiorite, a hard, granite rock that has few seams and is easily drilled, Badger said.

But the latest slide is made up of rock called "tuff," volcanic rock that is difficult to drill through.

Badger is now looking at all the Snoqualmie slopes to fulfill the orders from the governor.

"We want to see if there's anything imminent. We all need to recognize and accept that we have a lot of problems with the slopes up there, especially along Lake Keechelus, where the latest slide occurred."

He wouldn't say if he found any immediate threats, but said it's hard to predict when a slope might fall.

The state has had its eye on the Snoqualmie Pass slide area since March, when some smaller rocks came down. The state had scheduled work next spring to keep things safe up there.

As part of the 9.5-cent gas tax approved by the Legislature this year, $388 million is set aside for a new six-lane I-90 highway from Hyak to Keechelus Dam that would bypass the rockslide area. But construction wouldn't start until 2011, and Badger said he's not sure the state can wait that long.

"You can't have an extremely hazardous situation and face these potential problems for another 10 years," he said.

Susan Gilmore: 206-464-2054 or sgilmore@seattletimes.com