A smaller but more frequently occurring landslide category is the surficial slump. This is a form of slope instability in which a relatively thin zone of soil, generally parallel to the slope face, slides down the slope. The surficial slump is the most common form of landsliding, but is not typically referred to as landsliding.
The failure area in a slump generally takes the form of an oval. The depth of the affected zone is usually less than about four feet, but occurrences to six feet and greater are possible. The slump debris generally winds up a few feet below its original location, leaving a near vertical escarpment above and a bulged toe area below.
In natural slopes, this form of instability is generally limited to
topsoil profiles and occasionally a weathered portion of the underlying
formational material. In graded slopes, the problem is typically
manifested in the weathered soil zone near the surface. Accordingly, the
problem in graded slopes is most prevalent in compacted fill slopes
composed of expansive clayey soil. When the outer face of an expansive
fill material swells even slightly, soil particles in the swelling zone
move apart ever so slightly. Nonetheless, as the soil porosity increases
even slightly, the permeability of the soil parallel to the slope face
Once permeability differences develop between
the outer zone of the slope and the compacted inner core, seepage occurs
parallel to the slope face in response to prolonged heavy rainfall.
When this seepage does occur, the buoyant effect is triggered, the soil
strength is roughly cut in half, and the slumping begins. Occasionally,
granular fill slopes and bedrock slopes may be impacted by surficial
slumping. The failure mechanism is controlled by the steepness of the
slope, permeability differences, and the soil or rock strength
characteristics. In order to initiate a failure, sufficient water is
required to saturate the soil and cause seepage to develop essentially
parallel to the slope surface.
Information & illustration courtesy of American Geotechnical, Inc
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