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Causes and Types of Landslides

What is a Landslide?The term "landslide" is used to describe a widevariety of processes that result in the percept-ible downward and outward movement of soil,rock, and vegetation under gravitational influ-ence. The materials may move by: falling, top-pling, sliding, spreading, or flowing.

Although landslides are primarily associ-ated with steep slopes, they also can occur inareas of generally low relief. In these areaslandslides occur as cut-and-fill failures (high-way and building excavations), river bluff fail-ures, lateral spreading landslides, the collapseof mine-waste piles (especially coal), and a widevariety of slope failures associated with quar-ries and open-pit mines. Underwater landslideson the floors of lakes or reservoirs, or inoffshore marine settings, also usually involveareas of low relief and small slope gradients.

Why Do Landslides Occur?Landslides can be triggered by both naturaland man-induced changes in the environment.The geologic history of an area, as well asactivities associated with human occupation,directly determines, or contributes to the con-ditions that lead to slope failure. The basiccauses of slope instability are fairly well known.They can be inherent, such as weaknesses inthe composition or structure of the rock or soil;variable, such as heavy rain, snowmelt, andchanges in ground-water level; transient, suchas seismic or volcanic activity; or due to newenvironmental conditions, such as thoseimposed by construction activity (Varnes andthe International Association of EngineeringGeology, 1984).

Human ActivitiesHuman activities triggering landslides aremainly associated with construction and invol-ve changes in slope and in surface-water and

ground-water regimes. Changes in slope resultfrom terracing for agriculture, cut-and-fillconstruction for highways, the construction -ofbuildings and railroads, and mining operations.If these activities and facilities are ill-conceiv-ed, or improperly designed or constructed, theycan increase slope angle, decrease toe or lateralsupport, or load the head of an existing or pot-ential landslide. Changes in irrigation or sur-face runoff can cause changes in surface drain-age and can increase erosion or contribute toloading a slope or raising the ground-watertable (Figure 6). The ground-water table canalso be raised by lawn watering, waste-watereffluent from leach fields or cesspools, leakingwater pipes, swimming pools or ponds, andapplication or conveyance of irrigation water. Ahigh ground-water level results in increasedpore-water pressure and decreased shearstrength, thus facilitating slope failure. Con-versely, the lowering of the ground-water tableas a result of rapid drawdown by water supplywells, or the lowering -of a lake or reservoir, canalso cause slope failure as the buoyancy pro-vided by the water decreases and seepagegradients steepen.

Natural FactorsThere are a number of natural factors that cancause slope failure. Some of these, such aslong-term or cyclic climate changes, are not dis-cernible without instrumentation and/orlong-term record-keeping.

ClimateLong-term climate changes can have a signifi-cant impact on slope stability. An overall de-crease in precipitation results in a lowering ofthe water table, as well as a decrease in theweight of the soil mass, decreased solution ofmaterials, and less intense freeze-thaw activity.An increase in precipitation or ground satura-tion will raise the level of the ground-water

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table, reduce shear strength, increase the weight of the soil mass, and may increase erosion and freeze -thaw activity. Periodic high-intensity precipitation and rapid snow- melt can signifcantly increase slope instability temporarily (Figure 7).

Erosion Erosion by intermittent running water (gully

Figure 7. The remains of a

house where three children died in a

mudflow in Kanawha City, West Virginia. The movement was triggered by heavy

rainfall from a cloud- burst on July 9, 1973 (Lessing et al., 1976).

Figure 6. Aerial view of the Savage Island land- slide on the east shore of the Columbia River, Washington, 1981. This landslide was caused by irrigation water (photograph by Robert L. Schuster, U.S. Geological Survey).

ing), streams, rivers, waves or currents, wind, and ice removes toe and lateral slope support of potential landslides.

Weathering Weathering is the natural process of rock deter- ioration which produces weak, landslide-prone materials. I t is caused by the chemical action of air, water, plants, and bacteria and the physical

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