Landsliding in Seattle is caused by a combination of geologic conditions, steep topography, concentration of rainfall in the winter months, and the influences of an urban environment.  The geologic conditions are primarily a legacy of repeated glacial incursions during the past 2 million years.  The topography is the result of mass wasting in the past 13,000 years, since the disappearance of the last glacial ice.  Although Seattle does not receive a large volume of precipitation, concentrations of rainfall in the winter months can be significant enough to saturate the glacial and colluvial soils.  Overlying this natural setting is the human pattern of residential, commercial, and industrial development, and the infrastructure that binds it together.

Seattle is unique in that it has a rich record of landslides that dates back as far as 1890.  A database was created with 1,326 landslides.  Information in the database includes the location, date, type of landslide, geologic conditions, and possible contributing factors.  The landslides were also plotted on maps using Arcview Geographic Information System (GIS) technology, and then the locations were field checked to reduce the error.  In spite of this verification process, some of the locations are still approximate.  The database and GIS maps are useful tools for City of Seattle (City) departments.

Four landslide types were recognized from the data amassed in the study:

High Bluff Peeloff - blockfalls of soil from the high bluffs that are found primarily along the cliffs of Puget Sound.

Groundwater Blowout - catastrophic groundwater/soil bursts caused by the buildup of groundwater pressures along the contact of pervious/impervious soil units.

Deep-Seated Landslides - deep, rotational or translational sliding and slumping caused by groundwater pressures within a hillside.

Shallow Colluvial (Skin Slide) - shallow rapid sliding of the outer rind of a hillside slope, sometimes also resulting in a debris flow.

The most common type of slide is the shallow colluvial slide, particularly in response to an intense, short-duration storm.  The largest and commonly most destructive are deep-seated landslides; however, they are not activated as frequently as the other types of slides.  The preponderance of landslides occur in January after the water table has risen in the previous months, although destructive landsliding can sometimes last until March.  The landsliding occurs in only about 1 percent of the City, around the edges of the steep, mostly linear hills.  Although all of the steep slopes on the hill margins are susceptible to sliding, the GIS maps clearly show that certain areas are highly susceptible to slope instability.  Contributing causes of landsliding may be myriad, but water is involved in nearly all of the cases.  Consistent with other studies in the City and the region, 84 percent of the reported landslides may have had some factor of human influence associated with them.

Of the total number of landslides in the database, 58 percent were within existing potential slide areas and 76 percent were within the steep slope areas, as defined by the Department of Design Construction and Land Use (DCLU).  The percentage of landslides within either a steep slope or existing potential slide area was 88.  Several dense clusters of slides were clearly outside of existing mapped potential slide areas, so studies were performed to remap the potential slide areas throughout the City using the historical record as the primary factor.

Typical improvements to slope instability in Seattle are presented for each of the types of landslides.  They include surface water and groundwater improvements, retaining structures, soil reinforcement, grading, and catchment or diversion structures.  Unit cost estimates were prepared for each of the landslide improvement features.  The role of vegetation to maintain  stable slope conditions and reduce erosion is discussed.  The role of utilities and roads in landslides and how to reduce landsliding through the design of utilities are also presented.

Forty-three stability improvement areas were defined throughout the City.  They are areas that share somewhat similar geologic and groundwater conditions, and are geographically contiguous.  For each of these stability improvement areas, engineering solutions were tabulated, so rough cost estimates could be made by the City; however, no site-specific subsurface explorations were performed.

SHANNON & WILSON, INC.