ISSAQUAH, Wash. -- Westbound Interstate 90 east of Issaquah was fully reopened Thursday afternoon, some 10 hours after a landslide shut it down.
Thirteen vehicles were caught up in the slide that happened just before 5 a.m., but nobody was hurt.
Meteorologist Ben Dery says an intense cell of rain dumped in that area just before the slide came down.
Above the slide, water poured down from a small valley. Then that water was diverted by a clogged culvert onto an old railroad grade turned trail. It ran west along the grade for about a quarter mile, and crossed to the south along the grade, down the hillside, and onto I-90 west of Issaquah, triggering the worst of the damage.
Elsewhere, heavy streams of water poured down an access road leading to the interstate.
Washington State Department of Transportation geologist Chris Johnson said the situation was made worse, because the soil was so saturated by melted snow in recent weeks.
"Just too much water at one time," said Johnson. "And again, it's one of those crazy things with all of the snow that we had and the leaves that blocked up some of the drainages,"
"This one just happened to be the perfect storm, where it's come down and across the highway," said Johnson.
He says the mechanics for much of this slide are fairly typical for the region. Water saturates the top layers of soil down to a water resistant layer of clay. Then, with the help of gravity, the soil looses its strength and down it goes.
WSDOT maintenance crews were not only busy removing the mud, rocks, and downed trees off the highway, but armoring the damaged hillside with rocks of various sizes, up to about 18 to 24 inches in diameter, so the slide zone will be more resistant to further damage when the next big rain hits.
RAW VIDEO: Crews clear the I-90 landslide
WSDOT reopened one lane at 7:30 a.m. to relieve the backup, but closed it again 20 minutes later. That one lane was reopened at 8:30 a.m. All lanes were back open just before 3 p.m.
GALLERY: I-90 mudslide near Issaquah
Mudslide shuts down westbound I-90
Trooper Rick Johnson says there is a retention pond that was bleeding off onto the hillside.
"It's one-to-two feet in some places and we had upwards of 13 cars kind of stuck in the mud," said Johnson. "Thankfully, no injuries. Right now, we're going to make very sure the hillside is stable so we can get this roadway open."
"One thing we need is daylight. Daylight's going to allow our geotechnical experts as well as our maintenance folks to assess this hillslide. We want to make sure that once we reopen it, it's not going to slide again," said WSDOT spokesman Travis Phelps.
Drivers who were stuck behind the slide shut off their cars and turned the freeway into a parking lot as they waited it out. Some drivers were turned around and sent back to the Preston-Fall City exit.