The cause of the recent slide that pushed a Bellevue home off its foundation this week is not yet known. But unquestioned is the regularity with which homes and lives are lost to landslides in Washington.

It is the nature of the state’s geology and topography to slip, slide and slump during the wet months of the year. Take our slopes, just add water — and gravity.

Landslides are one of the most common and devastating natural hazards in the Pacific Northwest — and the damage they cause is almost never covered by insurance, warns the state Department of Natural Resources in its Homeowner’s Guide to Landslides, at st.news/slide.

That there even is such a guide should be taken as fair warning: We all live in and around landslide country.

Now is the time when the risk is accentuated, as abundant rains saturate soils. What happens next, explains Washington State Geologist Casey Hanell of the DNR, is predictable — and in some cases, preventable.

The principles at work are familiar to anyone who has ever built a sand castle. Those grains of sand that stick together making vertical walls with just the right amount of moisture? They dissolve to liquid as more water is added.

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So it goes with many slopes, given the right combination of soils, water, slope and gravity.

There are two major types of landslides, Hanell explains.

One is the shallow, or rapid landslide — basically a debris flow that happens quickly. In this case, water soaks through soil and rock, and ponds on impermeable bedrock. The layer of saturated soil and debris eventually will slide in a quick sloughing of material.

Any commuter on the Sounder trains traveling at the base of bluffs along the shore of Puget Sound is all too familiar with these sorts of landslides, which disrupt service on the mud-piled tracks.

An aerial look shows the hillside where a Bellevue home slid off its foundation early Monday. Forest Ridge School’s parking lot is above the home. (Steve Ringman / The Seattle Times)

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The other type of landslide is called a deep-seated slide. These occur much more deeply in the subsurface of the earth. These slides typically move slowly — over decades, even centuries.

There are hundreds of such slides across Washington, Hanell notes, that moved under different climate conditions and are presently dormant. But they can be set off again by human activities, such as excavating at the base of a slope, or adding water to it, Hanell said.

Water is not only heavy, increasing the driving force of a slope, but it pushes soil grains apart, insinuating its way between the tiniest particles. If enough water saturates the soil, it will not hold together on a slope. It will slide.

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Washington has seen many large and lethal landslides.

The Oso landslide in Snohomish County on March 22, 2014, killed 43 people and destroyed 49 homes and other structures when an unstable hill collapsed, sending mud and debris south across the North Fork of the Stillaguamish River and Highway 530.

Over the past year, scientists have acquired new insight into the circumstances surrounding the Oso slide. Research by the U.S. Geological Survey indicates that the landslide’s average speed was about 40 mph, with maximum speeds likely even higher. The area overrun by the landslide was about half a square mile, and the landslide moved about 18 million tons of sand, till and clay. That amount of material would cover about 600 football fields 10 feet deep.

Precipitation in the area during February and March 2014 was 150% to 200% of the long-term average, and likely contributed to slide’s initiation and mobility, the USGS analysis found.

The hillside’s history of slides dates back more than 60 years and some experts said they were shocked when homebuilding was permitted after a big slide in 2006. A 1999 report filed with the U.S. Army Corps of Engineers warned of the potential for catastrophic failure. Two creeks in the area are known as Slide Creek and Mud Flow Creek. Experts also warned against logging on a plateau above the slope, because of risk of a catastrophic slide.

The Aldercrest-Banyon landslide was a major slow-moving landslide in east Kelso, Cowlitz County, that began in 1998 and continued for nine months into 1999. The disaster caused $110 million in damage and destroyed 138 homes.

On Oct. 11, 2009, a massive landslide in the Nile Valley of Yakima County demolished a half mile of state Route 410 and redirected the flow of the Naches River.

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A 1997 slide on Perkins Lane in the Magnolia neighborhood sent houses tumbling to the beach.

Another massive slide in 2013 on Whidbey Island sent a 1,000-foot-wide swath off a hillside cliff into the Ledgewood Beach development on the west side of the island. The powerful slide shoved one home at the bottom of the cliff about 200 feet into the water, and took out nearly 400 feet of Driftwood Way, the road to the shoreline.

Incredibly, there were no injuries.

A Bainbridge Isand family of four was killed in 1997 by an avalanche of mud that hit their home as they slept on Rolling Bay Walk. The crushing slide knocked the third floor of the home off, sending it into Puget Sound, and buried the rest of the house under tons of mud. Five other houses in the neighborhood were declared unsafe due to slide risk.

The cause of the slide was determined to be rainfall and melted snow, which saturated the slope. The neighborhood had historically been a slide area, with winter months routinely sending mud pouring onto beaches.

There are steps that can be taken to live more securely with our state’s landslide risk, DNR advises.

Don’t add water to steep slopes. Avoid placing fill soil, yard waste or debris on steep slopes, or excavating beneath them. Always drain surface runoff away from slopes, fix plumbing leaks that could saturate soil, and plant and protect native ground cover and trees. Consult professionals if you suspect you are on a landslide.

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DNR’s Geologic Information Portal, searchable by address, is one place to start.

For Seattle residents living in a house in hilly terrain or thinking of buying one, the city maintains an online page of resources as well as survey maps showing areas having a 40% or greater slope. Make no mistake, landslides are us, in a city that has recorded more than 1,300 slides.

But it’s up to the homeowner to research and, if needed, take preventive measures, including hiring a geotechnical engineer for a site survey. 

As the Northwest’s rainy season takes its usual toll, the Northwest Insurance Council, a trade group, once again is posting the warning on its website that standard homeowners, renters and business insurance policies do not include coverage for landslides.

Special coverage called a “Difference of Condition” policy is available for stand-alone purchase but is expensive and usually obtained from a broker in specialty lines of insurance.

Relentless rain once again this fall and winter — at times breaking records — is a good reminder that water is heavy, soils get saturated, and gravity wins. Don’t ignore the sign of soil movement, Hanell said.

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Cracks in walls, bulging walls or cracks in floors. Tilting retaining walls, cracks in foundations and bent trees. These are clues that the ground is shifting.

“Be aware of your surroundings,” Hanell said. And always, evacuate any structure that starts to slide.

Material from The Seattle Times archives was used in this story.