2019-06-28_REVISION - M1977004 (9)

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Homestake Mining Company May 10,2019 Regular(112d)Operation Reclamation Permit Application Package Two areas of slope failure were reported at the IRD. The 10,300 ft slip" measured approximately 200 ft wide and 150 ft long, and vertical displacement was"a few feet". Slope movement occurred in a saturated zone caused by groundwater seepage. The "10,200 ft slip" is thought to have been caused by elevated pore water pressure. No dimensions are reported, but the mapped extent (Figure 3-15) is smaller than that of the 10,300 ft slip. These TCRD and IRD slopes were remediated and no additional slope movements were noted. Three inclinometers were installed in 2017 to monitor the impacts of the phosphate injection program on the overall slope movement, as discussed in Section 3.4.2.2. 3.4.2. Slope Movement Monitoring Program Slope stability has been monitored since the open pit mine development began in 1979. In early 1983 the slope monitoring system included survey stations, slope monitors, tension crack monitors, established lines of sight through the forest, piezometers, and inclinometers (HMC, 1983). In 1994, the monitoring program was updated and included mapping of tension cracks and scarps, new installation of inclinometers, a semi-annual photo station documentation program, and installation of a new network of survey points. 3.4.2.1. Surficial Mapping of Landslide Features Detailed mapping of tension cracks and scarps on the east walls of the North Pit and South Pit was first completed in 1994 by Golder, as reported by AAI (1999). The original map was updated by AAI (1999) (Drawing 02) and later by SRK (2007) based on field reconnaissance. Mapping methods, areal extent of mapped area, and accuracy of mapped features are not reported in either AAI (1999) or SRK (2007). The map shows the North Pit's East Wall main scarp, relative sense of motion, and measured vertical displacements over a topographic base map. Since being created in 1994, the map has been used as a basis for monitoring and assessing the extent of surface deformation caused by slope failure at the Mine. 3.4.2.2. Inclinometers A total of six inclinometers are installed and monitored annually at the Mine (Drawing 03). In the fall of 1998 three inclinometers(198-1, 198-2, and 198-3)were installed to monitor slope movement in the North Pit (AAI, 2000b). Two inclinometers are located on the South Wall of the North Pit and one is installed above the main scarp on the East Wall of the North Pit. The installation report (AAI, 2000b) discusses and shows the location of two additional inclinometers/piezometers (I/P96-8 and I/P97-10); however, no information such as readings of these inclinometers were encountered during the review of Mine documents. Inclinometers IN-1, IN-2, and IN-3 were installed on the downstream face of the IRD in August 2017 (BGC Engineering USA Inc. (BGC), 2019a); these inclinometers were installed midway between groundwater extraction and reinjection wells to monitor rock dump stability during phosphate reagent injections. There are currently no inclinometers located west of the East Wall main scarp within the area of previous slope failure. EXHIBIT E-RULE 6.4.5(AMENDED RECLAMATION PLAN) Page 43 HOMESTAKE MINING COMPANY