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ENGEO iNCORPORATED suddenly during an earthquake or slowly in the form of fault creep. Fault creep is the stow rupture of the earth's crust. 5. Slope Instability: Unstable slope conditions as a result of landsliding, oversteepened slopes, and soil creep and/or slope erosion. Landslide: A deposit of soil and/or bedrock moving downward from its original position under the influence of gravity. Landslides include a variety of morphologies and are further defined by types of material, wetness and mode of movement. For example: Falls are abrupt free-falls of earth materials from cliffs, steep cuts, or steep stream banks. Earthflows are mass movements of earth materials in which the type of movement is one of flowing. When composed of soil finer than gravel size, the flowing material is commonly called a mudflow. Debris flow/debris avalanches are composed of natural earth materials, artificial fill, and/or organic debris that flow downslope with speed. Most of the material is transported away from the area of initial ground failure. Soil creep is the slow, often imperceptible, deformation of slope materials under low stress levels. It normally affects the shallow portion of the slopes, but can be deep seated when a weak zone of soil or bedrock exists. It results from gravitational and seepage forces and may be indicative of conditions favorable for other types of landsliding. Creep can be caused by wetting and drying of clays, by solution and crystallization of salts, by the growth of roots, by burrowing animals and by down-slope movement of saturated ground. Colluvium refers to the mantle of loose soil and bedrock debris that progress down hillsides by creep. Landslides can consist of mass movements of earth materials that are primarily intact, and occur along discrete shear surfaces. These surfaces (shear or slip planes) can be rotational (conchoidal or concave), such as for earth slumps, or planar, as for translational earth slide or 4992.5.001.01 February28, 2003 8 DRAFT2