2020-09-16_GENERAL DOCUMENTS - C1981010

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Mr. Steve Hinkemeyer September 16, 2020 Page 3 GLOBAL SLOPE STABILITY ANALYSIS Investigation Approach The two-dimensional(2D)finite-element modeling program Phase 21 was used to develop the numerical models representative of the proposed spoil pile for the analysis. The Phase 2 program uses a shear strength reduction (SSR) approach to estimate the static Safety Factor (SF) value associated with a given slope geometry, where the material shear strength parameters (cohesion and friction angle) of the constituent layers are reduced in steps until failure (excessive yielding and/or displacement) is triggered. The SF value is the ratio of the assigned strength parameters for a given slope model to the reduced strength parameters at the onset of failure. Three-dimensional numerical modeling was deemed unnecessary given the relatively simple geometry of the spoil pile and the ground surface. The variations in surface topography within the spoil pile footprint were addressed by developing four numerical models along vertical sections of varying orientations through the proposed spoil pile, as shown in Figure 1. The shear strength parameters for the spoils were developed based on recently performed large-scale direct shear(LSDS) testing performed on Trapper spoils samples and a unique depth- based strength determination approach developed by AAI.2 The LSDS tests were performed on decades-old and weathered spoil samples obtained from Trapper and a relatively large 12-inch X 12-inch X 6-inch(length-width-height)mold, which better represents the long-term shear strength of the spoils. The average cohesion and friction angle obtained from the LSDS tests were 475 pounds per square foot (psf) and 34°, respectively. AAI utilized a newly developed approach to assign strength parameters to zones within the spoil pile depending on their relative depths of confinement. This approach has been developed based on several recent studies on spatial variability of spoil shear strength parameters based on constituent rocks, particle size, and even the deposition location within a spoil pile.3'4 Since the proposed spoil pile will be built in relatively smaller benches 20- to 40-ft thickness, it is expected that little density- or size-based segregation will occur within each bench. Therefore, each bench of spoils was assumed to have the same strength properties, while the bench-to-bench strength parameters were varied. AAI's approach for determining depth-dependent spoil strength parameters is rooted in the observations made by other researchers that the effective friction angle of spoils decreases with greater confining stress, given spoil failure is dominated by coarse-grained particles at low confining stresses and fine-grained particles at higher confining stresses.5 Therefore, the friction angle values for the deeper benches, with higher confining stresses, were estimated by reducing the base friction angle value (34°) assigned to the uppermost bench of new spoils. Additionally, 1Rocscience Inc. (2001), "Phase2 User's Guide," available at https://www.rocscience.com/downloads/ phase2/Phase2—TutorialManual. 2 Agapito Associates,Inc. (2020),"Nighthawk Strike Pit Spoils Pile Geotechnical Evaluation,Trapper Mine,"report to Trapper Mining,Inc.,534-46,April,38 pp. 3 Moffitt,Karen(2000),"Mine Waste Dump Instability,"Master's Thesis,University of Toronto,April, 106 pp. 4 Bradfield, Leonie (2018), "Reliable Shear Strength Estimation for Very-High Spoil Dumps," PhD Thesis, The University of Newcastle,Australia, 153 pp. 5 Barton, Nick and Bjorn Kjaernsli (1981), "Shear Strength of Rockfill,"Journal of the Geotechnical Engineering Division,Proceedings of the American Society of Civil Engineers, 107(GT7):873-891. Agapito Associates, Inc.