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co fuii!IFJU;~Jr ~ Causes and Types of Landslides What is a Landslide? The term "landslide" is used to describe a wide variety 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 in areas of generally low relief. In these areas landslides occur as cut-and-fill failures (high- way and building excavations), river bluff fail- ures, lateral spreading landslides, the collapse of mine-waste piles (especially coal), and a wide variety of slope failures associated with quar- ries and open-pit mines. Underwater landslides on the floors of lakes or reservoirs, or in offshore marine settings, also usually involve areas of low relief and small slope gradients. Why Do Landslides Occur? Landslides can be triggered by both natural and man-induced changes in the environment. The geologic history of an area, as well as activities associated with human occupation, directly determines, or contributes to the con- ditions that lead to slope failure. The basic causes of slope instability are fairly well known. They can be inherent, such as weaknesses in the composition or structure of the rock or soil; variable, such as heavy rain, snowmelt, and changes in ground-water level; transient, such as seismic or volcanic activity; or due to new environmental conditions, such as those imposed by construction activity (Varnes and the International Association of Engineering Geology, 1984). Human Activities Human activities triggering landslides are mainly associated with construction and invol- ve changes in slope and in surface-water and ground-water regimes. Changes in slope result from terracing for agriculture, cut-and-fill construction for highways, the construction of buildings and railroads, and mining operations. If these activities and facilities are ill-conceiv- ed, or improperly designed or constructed, they can increase slope angle, decrease toe or lateral support, or load the head of an existing or pot- entiallandslide. Changes in irrigation or sur- face runoff can cause changes in surface drain- age and can increase erosion or contribute to loading a slope or raising the ground-water table (Figure 6). The ground-water table can also be raised by lawn watering, waste-water effluent from leach fields or cesspools, leaking water pipes, swimming pools or ponds, and application or conveyance of irrigation water. A high ground-water level results in increased pore-water pressure and decreased shear strength, thus facilitating slope failure. Con- versely, the lowering of the ground-water table as a result of rapid drawdown by water supply wells, or the lowering of a lake or reservoir, can also cause slope failure as the buoyancy pro- vided by the water decreases and seepage gradients steepen. Natural Factors There are a number of natural factors that can cause slope failure. Some of these, such as long-term or cyclic climate changes, are not dis- cernible without instrumentation and/or long-term record-keeping. Climate Long-term climate changes can have a signifi- cant impact on slope stability. An overall de- crease in precipitation results in a lowering of the water table, as well as a decrease in the weight of the soil mass, decreased solution of materials, and less intense freeze-thaw activity. An increase in precipitation or ground satura- tion will raise the level of the ground-water 9