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<br />0J2733 <br /> <br />movements where there is a distinct surface of rupture or zone of weakness <br />which separates the slide material from more stable underlying materia]." <br />(Rogers et al, 1974). The two major types of landslides are rotational slides <br />and translational slides. <br /> <br />Rotational slide: These are slides in which the surface of rupture is curved <br />concavely upward and the slide movement is more or less rotational about an <br />axis that is parallel to the slope (figure ). The scarp formed at the head <br />of the slide may be almost vertical since the movement at that point is almost <br />wholly downward. The toe usually bulges upward, but sometimes flows outward. <br />A "slump" is an example of a small rotational slide (Varnes, 1978; USGS, 1981, <br />1982). <br /> <br />Translational slide: In translational sliding, the mass moves out, or down <br />and out along a more or less planar surface and has little rotational movement <br />or backward tilting (figure __I. The mass commonly slides out on the original <br />ground surface. Such a slide may progress indefinitely if the surface on <br />which it rests is sufficiently inclined and as long as the shear resistance <br />along. this surface remains lower than the more or less constant driving <br />force. The movement of translational slides is commonly controlled by <br />surfaces of weakness such as faults, bedding planes, and variations in shear <br />strength between layers of bedded deposits, or by the contact between firm <br />bedrock and overlying loose soils (Varnes, 1978). Slide material may range <br />from loose unconsolidated soils to extensive slabs of rock. <br /> <br />Block slide: A .translational slide in which the moving mass consists of a <br />single unit, or a few Closely related units, that move downslope as a single <br />block and remain relatively undeformed is a block slide (Varnes, 1978). If <br />the slide material is a solid block of bedrock, it is a "rock block slide" <br />(figure __I. If, however, the rock material is broken, but still slides along <br />a flat plane, it is a "rockslide" (figure ). <br /> <br />Lateral spreads <br /> <br />This type of ground failure is distinctive because it usually occurs on very <br />gentle slopes (commonly between 0.5 and 5 percent). According to Varnes <br />(1978), "...the dominant mode of movement is lateral extension accommodated by <br />shear or tensile fractures." The failure is caused by liquefaction, the <br /> <br />- 7 - <br />