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Geologic/Geotechnical/Hydrogeologic (Geology and Groundwater) <br />The geologic conditions, and engineering (geotechnical) and groundwater (hydrogeologic) <br />characteristics of the geologic units that compose the slope greatly influence its stability. <br />Generally, the upper portions of the slopes along the corridor are underlain by a sequence of <br />glacial sediments deposited in advance, beneath and <br />during the last continental glaciation (Vashon Stade). <br />Fine-grained lake sediments that formed in front of and <br />then compacted by the advancing ice sheet typically <br />underlie the coarse -grained Vashon advance deposits, and <br />have been referred to as transitional beds (Minard, 1982, <br />1983, 1985; Yount et al., 1993). These transitional beds <br />are underlain by a variable sequence of very compact <br />interglacial deposits (called the Olympia beds and <br />Whidbey Formation) and older glacial deposits (known as Possession and Double Bluff Drifts), <br />which typically outcrop in the middle to lower portions of the slope. Of all the geologic units <br />within the corridor, several are recognized as "bad actors" — over 60 percent of the landslides <br />reported between 1914 and 2001 originated within the transitional beds or the Whidbey <br />Formation (Shannon & Wilson, 2001). <br />Glaciation <br />Alteration of any part of <br />the earth's surface by <br />passage of a glacier, such <br />as erosion or deposition. <br />Landslides also commonly recur in the same areas. Remobilized landslide debris from previous <br />landslides was another geologic unit significantly contributing (approximately 13 percent) to <br />landsliding (Shannon & Wilson, 2001). Baum et al. (2000) noted that roughly two-thirds of the <br />landslides generated during the winter storms of 1995-96 and 1996-97 initiated within the <br />bounds of mapped landslide events. <br />Human Activity <br />Human activities have repeatedly been observed to be a substantial contributor to landslides <br />within the corridor. These adverse and widespread activities primarily involve the discharge of <br />stormwater onto or above slide -prone slopes; the cutting and re -grading of slopes; and the <br />disposal of yard, construction, and earthen or other debris onto the upper portion of the slope <br />(Shannon & Wilson, 2001). In addition to these adverse <br />practices by adjacent landowners, the density of upslope <br />development, even hundreds of feet behind the top of the <br />slope, has the potential to significantly contribute to <br />groundwater recharge through more concentrated discharge <br />of storm water runoff. This in turn has the potential to <br />adversely impact stability of the slopes along the rail <br />corridor. <br />Transpiration <br />The evaporation of <br />water from leaves. <br />More complex in its relationship to slope stability is the effect of removing vegetation. Rooting <br />depth and the interception and transpiration potential offered by mature conifers during the <br />winter wet season can be important contributors to stability. Conversely, the effect of wind on <br />mature conifers, referred to as windthrow, can disturb the substrate in which they root, resulting <br />in localized slope instability. For these reasons, the presence and type of vegetation and its <br />14 <br />