2019-06-28_REVISION - M1977004 (9)

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Homestake Mining Company May 10,2019 Regular(112d)Operation Reclamation Permit Application Package (SRK) (1983); DPI (1993); Golder (1996a); Golder (1996b); and AAI (2000b). A summary of the slope stability is presented below in the following sections. 3.4.1.1. North Pit The highest concentration of significant slope failures at the Mine have occurred at the North Pit. Observations of significant failure features began in August 1979 when tension cracks and scarps began to develop 500 ft back from (east of)the crest of the East Wall of the North Pit(PAH, 1980). By comparison to all other significant landslides in the North Pit, the 1980 East Wall Slope Failure was a relatively small event. It occurred at the 10,500 ft level and measured approximately 200 to 250 ft wide and experienced approximately 30 ft of vertical displacement at the headscarp (AAI, 1999). Most major slope failures occurred in 1983 during an abnormally wet year attributed to an El Nino weather cycle(AAI, 1999). The relatively high levels of precipitation and rapid snow melt attributed to increased levels of pore water pressure, leading to slope instability (AAI, 1999). Major slope failure events within the North Pit in 1983 included the Northwest Corner failure, South Wall failure, two failures of the Northeast Corner, and development and slope movement along the prominent East Wall headscarp and tension crack system. Partial backfilling and slope regrading in the North Pit have made most major slope failure features indistinguishable on the landscape; However, the East Wall headscarp and tension crack system remains prominent. These features formed in March and October 1983 during major slope failure events, and minor slope movement continued into the early 1990's (EAI, 2014). The surface expression of the East Wall main scarp area is characterized by zone of open cracks, voids, tilted trees, and both uphill and downhill facing scarps. Up to about 120 ft of vertical displacement has been measured across the center of the main scarp, and movement diminishes to the north and south. Since the major events of 1983 and the development of the East Wall main scarp, other slope failure events have occurred. These include the development of tension cracks and scarps on the South Wall in 1996 (AAI, 1999; AAI, 2000a) and smaller scale slope movement within the east wall of the North Pit such as the 2015 "sluff' of the east wall (HMC, 2016). Previous slope stability models include: • CNI (1983) evaluated slope stability of the East Wall using a three-block geologic model and large rotational failure extending from the East Wall main scarp to the base of the North Pit. CNI (1983) concluded that equilibrium will be reached within 10 years if no backfill is placed in the pit and within 5 years of backfill is emplaced. The headwall scarp will continue to slough to the angle of repose, and that it is possible that the East Wall overlies a pre-existing landslide. CNI (1983) does not report a specific angle of repose. • DPI (1993) evaluated four configurations of rotational failure for the North Pit and concluded that stabilization of the pit slopes can be stabilized for Mine reclamation through backfilling, regrading, and revegetation. EXHIBIT E-RULE 6.4.5(AMENDED RECLAMATION PLAN) Page 40 HOMESTAKE MINING COMPANY