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Mr. Dan Hernandez September 25, 1909 Page Three d. Sedimentation pond design should exceed a 25 year storm event, end be designed for a 100 year storm event. This comment also a~~plies to the pond in the amendment III application. There are several reasons for this more conservative design approach. Storms which occur at that elevation ere intense, with the exact rainfall amounts unknown. There is not a corresponding historical base for comparisons. the previously referenced letter within item c speaks to the intense rainfall amounts. The mine has operated in the past and is projected to operate in the future for time spans excF:eding 25 years. When dealing with design features like this, the possibility for error needs to be minimized; therefore, a 100 year storm event appears to be the only rational choice. e. What will be the resultant stability or factors of safety for the engineered fills recommended in the geotechnical study? Does the design allow for natural water seepage to occur within the embankment? Is there provisions for an interceptor drain to control seepage into the fills? The factor of safety should exceed a minimal standard of 1.5, which would be used for ideal soi'. conditions. For non-homogeneous soils end unknown seepage conditions, larger uncertainties exist, and more caution is necessary; therefore, a factor of safety of 2 or 2.5 would be: judicious at this site. What testing and quality control will be done by the geotechnical engineer during remedial efforts" The August 16, 1909, letter by CTL/Thompson suggests stabili~:ing the slopes by fill at the base. At one case this is activit~~ beyond the permit boundary, and at a second location is on bind owned by others. f. With regards to Mr. Ifeifner's corrective action plan dated August 14, 1909, we would expect significant modifications once the engineering studies are complete. Also this plan does not address other areas along the access road which have been harmed by erosion and sedimentation. The report repeatedly addresses north facing slopes with associated growing characteristics. We would expect after geotechnical modifications, the majority of the slopes would be facing easterly, with the access road at the top or west side of the remedial slope work. Immediate retention of the slopes is beneficial. Work was originally requested to be corrected by October 20, 1909, it is now obvious that date cannot be met.