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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 increases significantly.

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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