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Morrison Knudsen Corporation <br />• January 7, 1997 <br />Page 4 <br />and 2.3. The results of the stability analysis indicated a factor of safety of 1.5 for topsoil pile <br />Section 1 and a factor of safety of 1.6 for topsoil pile Section 2. The results of all of the <br />stability analyses are shown on each cross-section, Figs. 1 through 3 and Figs. 5 and 6. As <br />indicated above, the strength values used in these analyses are generally at the low end of the <br />range of values determined by the laboratory testing and would indicate that the results are <br />conservative. However, these results assume relatively dry conditions and some precautions will <br />be necessary if ground water is encountered in cuts. <br />Construction Recommendations: The recommendations presented in our previous studies <br />regarding the construction of cut slopes should be followed. Our recommendations include <br />diverting surface drainage uphill of both cut and fill slope faces. The grading plan for the site <br />indicates that culverts and drainage ditches along the portal road will adequately transport water <br />from the road and fill slopes to the collection system. For many of the cut and fill slopes <br />located along switchbacks the uphill road, if properly maintained, will help to divert water from <br />cut slopes located on the corresponding lower portion of the switchback. Without suitable <br />surface drainage above cuts slopes, oversteepening caused by erosion could create localized <br />• instabilities and introduce surface water to the cut slopes. The promotion of vegetation growth <br />on the face of cut slopes would help minimize erosion. <br />As stated in our previous studies, surface and ground water was observed only sporadically <br />throughout the site. However, it should be expected that with the amount of cut planned along <br />the portal road and upper benches, some ground water seeps may be encountered. It is also <br />likely that after grading operations are completed the reconfigured topography will alter the <br />existing surface and ground water conditions causing seeps to develop near cuts or fills after an <br />extended time period. Seasonal ground water fluctuations will also contribute to varying <br />conditions throughout the site. <br />If, during the construction of the proposed cut slopes, conditions are encountered that appear <br />detrimental to the stability of a slope (ground water, soft or loose soils, or conditions not <br />encountered in our borings) we recommend that a qualified person observe and assess the <br />conditions and determine an appropriate method of remediation. This should also be done in <br />the event that ground water or instabilities occur during the life of the project. For situations <br />where ground water is encountered it is important that cut slopes are properly drained. For <br />isolated cases, the use of a face drain should adequately transport water off of the cut. If the <br />area of seepage increases or if signs of instability occur, it may be necessary to provide <br />additional drainage of the slope. This could be done with horizontal drains into the cut. For <br />worst case scenarios where excessive seepage occurs over a large area, the use of a drainage <br />blanket in conjunction with a mechanically stabilized slope may be necessary. Gravity retaining <br />• walls, Gunite reinforced walls or tieback walls could be used if stabilization of the slope is <br />necessary. Benches could be used to encourage vegetation growth and also to transport water <br />