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<br />0028Z4 <br /> <br />have environmental and socioeconomic impacts. Although physical techniques <br />may be the only means for protecting existing land uses in hazard areas, sole <br />reliance on them has significant drawbacks. Such reliance may create a false <br />sense of security on the part of hazard-zone residents and communities, and <br />may encourage more intense development of the land than if structural controls <br />were absent. Also, the probability always exists for the occurrence of an <br />event of greater severity than that for which the project was designed. In <br />addition, failure of structures as a consequence of aging, changing <br />conditions, or inadequate design is possible, and could be catastrophic if the <br />hazard zone has been developed intensively. <br /> <br />Design considerations: When designing corrective measures, it is essential to <br />look well beyond the slide mass itself. A translational slide may propagate <br />indefinitely if the surface on which it rests is sufficiently inclined and the <br />shear resistance along the surface remains lower than the more or less <br />constant driving force. Debris flows can frequently be better controlled if <br />mitigation efforts concentrate on stabilizing the source area. An <br />understanding of the surface- and ground-water regimes, under both natural and <br />man-imposed conditions, is essential to any mitigation planning. <br /> <br />Some factors that determine the choice of physical mitigation include: <br />1) type of movement (e.g., fall, slide, avalanche, flow) <br />2) kinds of materials involved (rock, soil, debris) <br />3) size and location of failure <br />4) process that initiated movement <br />5) place or thing(s) affected by failure <br />6) potentta1 for enlargement (certain types of failures [e.g., debris <br />flows, translational slides] will enlarge during excavation) <br />7) available resources (money, labor force, materials) <br />8) accessibility and space available for physical mitigation <br /> <br />The physical mitigation of landslides usually consists of a combination of <br />methods. Drainage is used most often, with slope modification by cut and fill <br />the second most often used method. These are also the least expensive methods. <br /> <br />In considering which physical methods of mitigation to use, a two- or <br />three-tiered approach may be advantageous; combine vegetative with structural; <br />combine short-term with long-term, e.g., use structural methods to allow time <br /> <br />- 39 - <br />