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i <br /> i SHANNON�WILSON,INC. <br /> As described previously, the slope consists of five different geologic units. The <br /> � chazacteristics of each unit and its relafive stability are described below. No groundwater <br /> scepage was noted on the slope. <br /> I 8.2.1 '�il� <br /> IT511 is relatively impermeable, except where sandy wnes are encounr��. <br /> Generelly, the till forms an impervious ]ayer below which surface water cannot penetrate. <br /> � In gene�al, till soils can stand near vertically for many years, as evidenced by bluffs at the <br /> site (see Figure 1), but the bluffs may retreat by block falls of till that are de8ned by joinu <br /> Iin the soil mass. Caving or block failure could also occur in the dll if it is underminr.d due <br /> to the retreat of the underlying outwash soils, which, in tum, may be caused by erosion or <br /> Iundercutdng fmm the soils below by the adjacent property owner. Besides block falls, dll is <br /> generally stable, such that rotational inshability or slumps are rarely seen. <br /> '- 8.2.2 Outwash <br /> ' Similar to dll, outwash soils can stand near-vertical for some time; however, the face <br /> will erode much faster due to exposure to the elements. Additionally, it is subject to erosion <br /> Ifrom averland water flow. These are the primary modes of failure of this material. <br /> Stumping or global instability is rarely seen in outwash. Outwash could condnue to erode <br /> � and retreat, which could cause the werlying till and till-like soils to slab off, or along the <br /> toe of the slope the recession of the outwash bluffs could trigger failures in the SIl and <br /> icolluvium and perhaps impact the proposed development. <br /> 8.2.3 Colluvium <br /> I <br /> The likely mode of failure within colluvium is a planar, shallaw (sur6cial) slide, and <br /> Inot rorational, deep-seated (global) instability. Failu�e would likely occur in periods of <br /> heavy reinfall or during situations where water is introduced into the slope from averland <br /> Iflow, downspouts, broken u61ity lines, and other means. Instability could also occur as a <br /> result of ground mwements associated with an earthquake. Failuce within colluvium would <br /> I not liloely impact the proposed development on the upper plateau, because the development <br /> will be founded on till-like soils. <br /> I W-6693-01 <br /> 11 <br />