1997-07-28_GENERAL DOCUMENTS - M1973021 (9)

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After the repair, 26 new surface monitor points (2001 through 2026) were established to monitor the effectiveness of the repair. Through December 1996, all 26 post-repair monitor points remained stable with maximum displacements of less than 0.1 ft. Based on these data, the existing'reconfigured benches and toe buttress appear stable. 7. Results ' Haley & Aldrich analyzed the failure through geometric analyses of the subsurface data and by performing numerical slope stability analyses using XSTABL. Numerous repair options consistent with t the existing permit constraints were considered and the preferred "fix" was to 1) construct a temporary, unreinforced buttress to immediately support the toe of the slide, 2) unload the top of the slide ' mass via excavation, and 3) replace the temporary buttress with a narrower reinforced earth buttress that could only be constructed safely after unloading at the top was complete. Key to the repair ' strategy was the geometry of the slide feature which was at a shallow angle near the toe and steeper near the head. The plan and profile sheets included in Appendix E labeled "Stage I" through "Stage III" ' illustrate these three repair steps as originally proposed. A hypothetical, simplified, slide plane geometry assumed based on the above-described data for the purpose of affecting a repair design is shown on the sketches. It was decided to perform some additional excavation and probe hole ' drilling on the fourth bench to better define the topography of the older, underlying, rock bench to see if it might be possible to leave the temporary buttress as the permanent buttress without reinforcement. In so doing, it was discovered that the older underlying bench was partly missing further east (into the hillside) than previously believed in front of the slide area and buttress limiting available room to leave the temporary buttress as permanent. ' Again, viable options were reassessed, but this time including options that allowed more room to maneuver by moving the mining limits to ' the east at the top of the hill. The solution that was agreed upon was to remove the temporary unreinforced buttress and then replace it later, move the mining limits east to the Limits of Disturbance within ' the existing permit to gain additional room to work via a Technical Revision, and adjust the orientation of the highwalls more southeasterly south from the slide area. The temporary condition of no buttress had a lower factor of safety (FS = 1.2 to 1.3) than exists with the permanent buttress, but was acceptable for a short term situation given the nature of the facility. The key was having some ' additional room at the top from which to further flatten the slope, if necessary, if additional movement occurred after the temporary ' 14 G:\PROJECTS\20200\002\96ANNRPT.WPD •® w