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ENG <br /> INCORPORATED <br /> IV. Slope Instability <br /> Earth stability is the GHAD's prime geotechnical concern at this site. This is not unique to this <br /> project, but is of importance for hillside projects in the San Francisco Bay Area. This section <br /> describes several types of slope instability which are within the GHAD's responsibility, subject <br /> to the provisions of Section VI. <br /> Landslides are a common geologic phenomenon and are part of the process of mass wasting. <br /> Weathered or fractured bedrock and soil are transported downslope over geologic time as a result <br /> of gravitational and hydrostatic forces. Landslides and earth movement in this bedrock formation <br /> are typically rotational slumps and earthflows. Depth of movement is typically about 5 to <br /> 25 feet below the ground surface. Earthflows are confined to the upper 3- to 5- foot -thick clayey <br /> soil mantle. In the winter rainy season, these earthflows can move at a rate of several feet per <br /> day. <br /> A landslide is a deposit of soil and /or bedrock moving downward under the influence of gravity. <br /> Landslides include a variety of morphologies and are further defined by type of materials, <br /> wetness, and mode of movement. They can consist of mass movements of earth materials that <br /> are primarily intact, and occur along discrete shear surfaces. These surfaces (shear or slip <br /> planes) can be rotational (conchoidal or concave), such as for earth slumps, or planar, as for <br /> translational earth slide or bedrock block glides. Most landslides are truly "complex landslides ", <br /> sliding, falling and flowing with more than one type of movement and/or material. • <br /> Falls are an abrupt free -fall of earth materials off cliffs, steep cuts, or steep stream banks while <br /> earthflows are mass movements of earth materials in which the type of movement is one of <br /> flowing. When composed of soil finer than gravel size, the flowing material is commonly called <br /> a mudflow. A debris flow /debris avalanche is composed of natural earth materials, artificial fill, <br /> 4867.1.002.01 <br /> October 10, 2000 5 <br />