Landslide

Landslide Classification and Causes

Many systems for classifying mass movements have been created in the past century. The most widely used and accepted classification system in the United States was presented in 1958 by David Varnes in Landslides and Engineering Practice: Special Report 29, with revisions in Landslides: Analysis and Control, Special Report 176 in 1978. Six types of movements are defined in this system, and each type is further divided based on the type of material in which the movement occurs. Slides (landslides) are one of the six types of movements defined (Table 1).

The likelihood of a landslide occurring depends on the relationship between shear stress (forces driving materials downslope, namely, gravity forces) and shear strength (forces resisting the movement of material). More specifically, a slope remains stable if the shear stress exerted on the slope is less than or equal to the shear strength of the slope material (a function of normal stress-resisting movement, cohesion, and the internal angle of friction). When shear stress on a slope is greater than shear strength, mass movement occurs. Factors that either increase shear stress or decrease shear strength, therefore, increase the likelihood of a landslide occurring.

Factors that can increase shear stress include the removal of slope material or the oversteepening of slopes, as can occur with stream erosion or road building; heavy precipitation or snowmelt events that increase pore fluid pressure and reduce resistance to movement; overloading of slopes; and earthquakes. Compositional, textural, and structural properties of rocks and soils and the influence of weathering on these properties are the dominant factors determining the shear strength of a slope. Slope material properties strongly influence cohesion and the normal forces acting on slope material. Cohesion-less slope material, soils with high clay content, and soils [Page 1721][Page 1722]and rocks composed of inherently weak materials or that become weak with change in water content and jointed rock formations can all decrease the shear strength of slope material, increasing the likelihood of landslide occurrence.

Table 1 Classification of mass movements

Gros Ventre Slide, Grand Teton National Park, Wyoming

Source: U.S. Geological Survey.

The Loma Prieta, California, earthquake on October 17, 1989, near the San Francisco and San Mateo County coast produced this landslide north of Fort Funston. The slide mass is approximately 2,830 m3 of material and is 30 m high.

Source: U.S. Geological Survey.

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