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<br /> <br />Figure 6. A rotational landslide (modified from <br />Varnes. 1978). <br /> <br /> <br />ORIGINAL <br />GROUND SURFACE <br /> <br />Figure 7. A translational landslide. <br /> <br />bedrock, it is a "rock block slide" (Figure 8). If, <br />however, the rock material is broken, but still slides <br />along a distinct surface of failure, it is a "rockslide." <br /> <br />Lateral Spreads <br /> <br />Lateral spreads (Figure 9) are distinctive because they <br />usually occur on very gentle slopes (between 0.5 and <br />5.0%). According to Varnes (1978, p. 14), "the dominant <br />mode of movement is lateral extension accommodated <br />by shear or tensile fractures." The failure is caused by <br />liquefaction, the process whereby saturated, loose, <br />cohesionless sediments (usually sands and silts) are <br />transformed from a solid into a liquefied state. Failure is <br /> <br />SUP SlIlFACE <br /> <br /> <br />Figure 8. A block slide. <br /> <br /> <br />Figure 9. A lateral spread. <br /> <br />usually triggered by rapid ground motion such as that <br />experienced during an earthquake, but can also be ar- <br />tificially induced. <br />When coherent material, either bedrock or soil, rests <br />on materials that liquefy, the upper units may undergo <br />fracturing and extension and may then subside, <br />translate, rotate, disintegrate, or liquefy and flow. Lateral <br />spreading in fine-grained materials on shallow slopes is <br />usually progressive. The failure starts suddenly in a <br />small area and spreads rapidly. Often the initial failure is <br />a slump, but in some materials movement occurs for no <br />apparent reason (v.unes, 1978). <br /> <br /> <br />Flows <br /> <br />Creep <br />Creep is the imperceptibly slow, steady, downward <br />movement of slope-forming soil or rock. Movement is <br />caused by shear stress sufficient to produce permanent <br />deformation, but too small to produce shear failure <br />(American Geological Institute, 1974). Hansen 0984) <br />distinguishes three types of creep: 1) seasonal, where <br />movement is within the depth of soil affected by <br />seasonal changes in soil moisture and soil temperature, <br />2) continuous, where shear stress exceeds the strength <br />of the material, and 3) progressive, where slopes are <br />reaching the point of failure by other mass movements. <br />Creep is indicated by curved tree trunks, bent fences <br /> <br /> <br />13 <br />