2019-06-28_REVISION - M1977004 (9)

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Homestake Mining Company May 10,2019 Regular(112d)Operation Reclamation Permit Application Package • CNI (1995) evaluated five alternative failure models for the East Wall of the North Pit, including: 1)deep seated rotational failure through bedrock and/or structures that extends from the East Wall main scarp to the floor of the North Pit, 2) two rotational failures, an upper rotational failure involving a pre-existing slide, and failure through rock mass below the 10,600 ft level, 3)failure above the 10,500 ft level along a pre-existing rotational failure surface, and failure along intermediate geologic structures below 10,600 ft level, 4) pre-existing slide above the 10,500 ft level and plastic deformation below the 10,600 ft level along faults within bedrock, and 5) both upper and lower slope failures, with progressive (retrogressive) failure in the upper slide. Six alternative strategies for reclamation of the East Wall were provided and include: 1. No Action -Allow pit slope to stabilize naturally. 2. Unload slide and backfill pit. 3. Backfill the pit without unloading. 4. Unload slide without backfilling of pit. 5. Depressurize slope and install permanent surface drainage. 6. Combination of unloading, backfilling and slope drainage. • Golder (1996b) constructed a geotechnical model, performed a back analysis, and a sensitivity analysis of the"Pinnacle slopes"on the south wall of the North Pit. The analysis considered a circular failure scenario within Paleozoic bedrock stratigraphy using XSTABL limiting equilibrium software. Golder (1996b) concludes that the slope is sensitive to a curvilinear groundwater surface reaches above the 10,400 ft level and that the slope is likely more stable than the two adjacent slopes which failed previously. • AAI (1999) performed the most recent slope stability analysis of the East Wall of the North Pit. This study simulated slope failure of a complex three-block geologic model using used Universal Distinct Element Code(UDEC). The AAI (1999)three-block model uses different block geometry than the previous CNI (1982). The simulation showed that rotational failure of the "lower" and "middle" blocks resulted in explaining slope deformation features observed at the site and that translational failure of the "upper" block occurs in response to failure of the"middle"block. Their results show a decrease in slope movement velocities over time that may increase periodically in response to unusual extreme climatic conditions. AAI (1999) makes the following conclusions about rates of slope movement: o Northwest Corner: The potential for slope failure in the Northwest Corner is perceived to be low due to backfilling and slope regrading. o South Wall: The potential for slope failure at the South Wall is perceived to be low due to observed low rates of slope movement. AAI (1999)suggests that significant changes in slope geometry or ambient conditions would be required to affect the rates of slope movement at the South Wall. o East Wall: Although the eastern extent of the East Wall is defined by a prominent set of tension cracks and scarps, AAI (1999) states that the East Wall has never exhibited large scale rapid slope failure. They state that decreasing slope movement rates between 1994 and 1999 are likely the result of slope regrading operations in 1996 and that rates should continue to decrease until a state of quasi- EXHIBIT E-RULE 6.4.5(AMENDED RECLAMATION PLAN) Page 41 HOMESTAKE MINING COMPANY