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March 1998 Slnmp Invettigakon and Mitigation Report ~ Page 13 <br />the sliding surface. The driving and resisting forces appear to be roughly balanced and sliding is <br />likely to occur only during wet periods when enough precipitation occurs to lubricate the <br />sliding surface and initiate movement. <br />Options for mitigating the slump include increasing the resisting forces (soil nailing or <br />construction of a buttress or a retaining wall below the toe of the slump), reducing the driving <br />forces (preventing the accumulation of water within the slump mass and/or regrading), or a <br />combination of the two. Soil nailing or construction of a buttress or a retaining wall below the <br />toe are not anticipated to he effective without also being done in combination with additional <br />drainage and soil regrading. Additional French drains, surface drainage ditches, and regrading <br />of the slump area are likely to be effective mitigative measures without soil nailing or <br />buttressing. Soil nailing and/or buttressing could be implemented in the future if it becomes <br />necessary although they present some concerns and limitations. The effectiveness of soil nails <br />would be a direct function of the density of the material within which they are placed, and <br />given the type of soils present in the slump area, a high density of nails would be needed. A <br />buttress or retaining wall would increase soil loading immediately below the toe of the slump <br />and possibly transfer existing stresses down slope creating the opportunity for movemen[ <br />below [he existing slump. In any case, an accura[e means of monitoring slump movement is <br />necessary to assess the effectiveness of the chosen mitigative measures. <br />Montgomery Watson Americas ° PO. Bax 774018 "Stean:~oat Springs, Colorado " 80477 (970J 879-6160 <br />Y'"A!)]~sbmp Mnryainnls6mp ft•pm <br />snirel,m <br />