Workers first had to finish evaluating the area for further rock falls and crews needed to build a 160-foot long, 12-foot high steel mesh fence being bolted to the pavement on the left shoulder of the westbound lanes, said Ryan Bianchi, a Transportation spokesman.
"Anything's possible," department spokesman Michael P. Westbay said concerning how long the interstate would be closed. "They might get up there and say we have to pry 150 rocks off. We don't know how much blasting we have to do. It could be tomorrow. It could be later."
Meanwhile, crews studied how much more rock would have to be removed from the slope to make it safe for drivers below. About 50 cubic yards of rocks and debris had fallen off the slope and officials estimated at least twice that much could still come down.
Boulders big as refrigerators hit the westbound lanes shortly after 3:15 a.m. Sunday, blocking the freeway about 6½ miles east of the 3,022-foot pass.
No one was injured, but the freeway was closed to westbound traffic at Easton, about 70 miles southeast of Seattle, and to eastbound traffic at the pass summit.
Transportation has a list of more than 3,000 areas they watch for rockslides. Geologists visit the sites and try to predict when rockslides will occur, but Sunday's slide was near the bottom of the priority list until this past summer when geologists decided to move it up, said Linda Mullen, a spokeswoman for the agency.
"There's a lot of science, some art and mother nature involved," she said. "It's sort of like predicting when earthquakes will occur. You can make guesses on what might happen but I don't think you can pinpoint it."
Two major rockslides this fall on I-90 near Snoqualmie Pass - including the Sept. 11 slide that killed three women - were the most dramatic in the state so far this year.
There also have been a series of rockslides on U.S. 101 south of Forks, Mullen said. And a day after the Sept. 11 rockslide, a large boulder fell at the same site. A rockslide in late September over the Burlington Northern tracks near Edmonds closed the tracks for one night.
Mullen said U.S. Highway 2, which includes Stevens Pass, has more areas listed on the "unstable slopes" list than Snoqualmie Pass, but has not seen any significant slides yet this fall.
Other recent trouble spots, Mullen said, include State Route 20 in the North Cascades, State Route 410 at the intersection with State Route 123 on the southeast side of Mount Rainier National Park, U.S. Highway 101 and U.S. Highway 2.
A $357 million project to rebuild a 15-mile stretch of I-90, including the areas of the past two landslides, was scheduled to begin next spring.
"Obviously nature has its own schedule and dropped those rocks before we could repair it," Mullen said.
She said the pass is closed regularly each winter for avalanche control and in response to avalanches. This fall's two rockslides have closed the pass for more than two days. The winter average is about 120 hours, with 80 of those hours because of avalanches or avalanche control.
The department asked motorists to avoid unnecessary traffic through the Cascade Range. About 28,000 motorists cross Snoqualmie Pass on an average weekday.
The immediate cause of the slide was unclear, although Westbay cited wet weather and an already unstable slope where trees and plants were growing up between the rocks.
Westbay said state Transportation Secretary Doug MacDonald has ordered department workers on Monday to immediately reevaluate all the rock fall faces on I-90.
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