Civil/Structural Engineering, Vashon, Washington, USA 206-463-5311
Ellisport Engineering Blog
One of the Best Seminars I've Ever Attended

In the late 1990's, the Puget Sound Region experienced heavy rains, followed by a heavy wet snow, and then a fast melt.  The result:  Many Landslides.

After things calmed down, the local geotechnical society held an all-day seminar to talk about landslide types and causes.  Hills will often shed their top layer (surface slough slide) every 40 - 50 years, roughly correlated to the lifetime of alder trees, which are part of the slide debris and cause much damage.

Regarding landslide mitigation, the lessons were clear:
  1. A slope is in a tenuous state of equilibrium.  Protect your slope and especially the toe (base) of the hill.  Do not cut into the toe, unless you provide an alternate and equal means of support, such as a retaining wall.
  2. Collect and control all water draining toward the slope.  A saturated slope is heavier and more susceptible to sliding.
  3. Protect all vegetation on the slope.  Plant native deep-rooted evergreen shrubs and trees.  These will mitigate erosion, and deep roots will help to anchor the hill against sliding.

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