2020-09-16_GENERAL DOCUMENTS - C1981010

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Mr. Steve Hinkemeyer September 16, 2020 Page 4 the deformation moduli for the deeper benches were estimated using an empirical approach provided by Seed and Idriss,6 which positively correlates the deformation modulus to confining stresses within the spoils matrix. The cohesion values for all new spoil benches were kept constant at 475 psf. Cohesion values of end-dumped spoils vary with depth, but those of constructed piles may be assumed to be constant. The foundation soil (approximately 30 ft thick) and rock mass underneath were assigned strength properties based on a previous subsurface investigation performed in the area and associated laboratory testing results.7 A summary of material strength parameters used in the numerical modeling analysis is presented in Table 1. Table 1. Summary Numerical Model Input Parameters Unit Average Depth Deformation Friction Weight of Confinement Modulus Cohesion Angle Poisson's Material (pcf) (ft) (psf) (psf) (0) Ratio New Spoils-A 110 10 1,000,000 475 34.0 0.35 New Spoils-B 110 30 1,732,051 475 32.5 0.35 New Spoils-C 110 50 2,236,068 475 31.3 0.35 New Spoils-D 110 70 2,645,751 475 30.9 0.35 Foundation Soil 102 - 7,200,000 418 25.0 0.35 Mudstone 140 - 51,500,000 3,888 47.0 0.30 Several monitoring wells in the vicinity of the proposed spoil pile have recorded static water levels at 19.5 ft to 77.5 ft from the surface, which is the reason the phreatic surface was assumed to be approximately 20 ft below the ground surface in the numerical models, for conservatism. The water table was assumed to be static in order to develop pore pressures in the subsurface and no transient groundwater flow calculations were made. Slope Stability Results The results of the analysis are presented in Figures 2 through 5. The results indicate that all four analyzed sections are associated with SF values appreciably greater than 1.5, which is AAI's threshold for long-term stability of slopes. Even though the hypothetical failure surfaces extend below the phreatic surface in the foundation soil, the relatively high SF values (>1.75) observed in the analysis results for all the analyzed sections can be attributed to the lower overall height of the spoil pile (<80 ft) combined with the relatively flat gradient of the ground surface (<5°)• FOUNDATION STABILITY ANALYSIS In addition to the numerical modeling, AAI performed a foundation bearing-capacity analysis to test if the natural ground foundation could withstand the load of the new spoil pile. The proposed spoil pile was assumed to exert a constant strip load over a 700-ft-wide space,which was 6 Seed,H.Bolton and I.M.Idriss(1970),"Soil moduli and damping factors for dynamic response analyses,"Rep.No. EERC-70/10,Earthquake Eng.Research Center,Univ. of California at Berkeley,Berkeley,California. 7 Agapito Associates, Inc. (2013), "AE-Pit, I-Pit, and J-Pit Highwall Slope Evaluations," report to Trapper Mining Inc.,534-28,January 3,57 pp. Agapito Associates, Inc.