Civil/Structural Engineering, Vashon, Washington, USA 206-463-5311

(Almost) Everything You Ever Wanted to Know About Landslides.

A Landslide Primer
As in many areas of the United States, landslides are a fact of life in the Northwest, and especially in the communities around Puget Sound.  They can occur without warning, but most often happen during periods of heavy rains or immediately after an earthquake.  Areas of concern include:
 • Steep slopes
 • Waterfront lots
 • Slopes with active springs day lighting on the hill (horsetails  are a common indicator).
 • Soil cracks
 • Cracks in road pavement
 • Leaning trees (non-vertical) on slopes
 • Leaning retaining walls

 

If you have a slide or notice any of these "warning" signs, contact us immediately.  We can help, either through our engineering expertise, or through our extensive referral system to geotechnical engineers and reputable contractors.

 

Types of Slides

• Shallow Slides - Shallow slides, sometimes called "small slides", may seem anything but small when they happen to you.  Shallow slides usually occur on steep slopes (over-steepened slopes) after a long period of heavy rains.  The soil becomes saturated with water and can no longer support its weight plus the additional water weight.  Often, they may be precipitated by erosion problems or spring conditions on the hill.  These slides may occur on the same slope every 30-50 years, often linked with the aging of alder trees.  The resulting debris flow can carry a wall of mud, trees, and other materials at 20mph or more down the slope.

 Large Slides - Large slides involve much more than the upper few feet of soil.  They can cut back the top of the several feet or more, often threatening homes built at the top of the bluff.  The distinguishing mark of this slide is the huge amount of material that moves, and the catastrophic results that may occur. 
• Deep-Seated / Rotational Slides - Deep-seated, or Rotational, slides are difficult to predict, difficult to control, and the results can be devastating.  They involve large sections of hill, often a mile or more long.  Mitigating efforts may help, but geologically-speaking, the slide will likely progress over many years.  Generally, these are ancient landslides that have moved in the past, and will continue to move.

Causes of Landslides
Many homeowners deal with shallow slides.  For the typical shallow slide, there are three main causes:


1. Disturbing or cutting into the toe of the slope
2. Uncontrolled water saturating the soil or eroding the slope
3. Disturbing the slope vegetation, or inadequate plant cover on the slope

 

Obviously, if you can avoid excavating at the toe of the slope (if you must, install an engineered retaining wall), collect and control all water running toward the slope, and maintain a healthy plant cover on the hill, your landslide risk will be significantly mitigated.  With landslides, there are no guarantees, but there are things you can do to tilt the odds in your favor.  Most steep hills are in a tenuous state of equilibrium, and implementing mitigation measures may make a difference.  Always remember that with landslides, we need to shift our thinking from human lifetime to geologic time (100's of years).