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herent in the composition or structure of the rock or soil; variable, such as changes in ground-water level; transient, such as seismic activity; or due to new en- vironmental conditions, such as those imposed by con- struction activity (\3mes and the IAEG, 1984). Human Activities Human activities triggering landslides are main1y associated with construction and involve changes in slope and in the surface-water and ground-water regimes. Changes in slope result from terracing for agriculture, cut -and-fill for highways, the construction of buildings and railroads, and mining operations. If these activities are ill-conceived, or improperly designed or constructed, they can increase slope angle, decrease toe or lateral support, or load the head of a landslide. Changes in irrigation or surface runoff can cause changes in surface drainage and can increase erosion or contribute to loading a slope or raising the ground-water table. The ground-water table can also be raised by waste water effluent from leach fields or cesspools, leaking swimming pools or ponds, and irrigation or con- veyance of irrigation water. A high ground-water level results in increased pore pressure and decreased shear strength, thus facilitating slope failure. Conversely, 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 provided by the water decreases and seepage gradients are steepened. Vibrations from manufacturing or construction machinery, blasting, and traffic can also trigger land- slides in rare instances. Natural Factors There are a number of natural factors that can cause slope failure. Some of these, such as long-term or cyc1ic climate changes, are indiscernible without instrumenta- tion and/or long-term record-keeping. Oimate Long-term climate changes can have a significant impact on slope stability. An overall decrease in precipitation results in a lowering of the water table, as well as a decrease in the weight of the soil mass, solution of materials, and freeze-thaw activity. An increase in precipitation or ground saturation will raise the level of the ground-water table, reduce shear strength, increase the weight of the soil mass, and may increase erosion and freeze-thaw activity. Periodic high intensity precipitation can also significantly impact slope stability. Erosion Erosion by intermittent running water (gullying), streams, rivers, waves or currents, wind, and ice removes toe and 1ateral slope support. Weathering Weathering is the natural process of rock deterioration which produces weak, landslide-prone materials. It is caused by the chemical action of air, water, plants, and bacteria, the physical action caused by changes in temperature (expansion and shrinkage), the freeze-thaw cycle, and the burrowing activity of animals. " Earthquakes Earthquakes not only trigger landslides but, over time, the fault activity associated with them can create steep and potentially unstable slopes. 15