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April 21, 2020 Page 3
<br />clay -sized particles in the spoils. An average cohesion value of 475 pounds per square foot (psf)
<br />and friction angle of 34' were selected to be representative of the N-Strike Pit spoils, which were
<br />the lower of the peak and residual (post -peak) shear strength parameters obtained from the LSDS
<br />tests. It needs be pointed out that the LSDS test apparatus, even with its relatively large 12-inch X
<br />12-inch X 6-inch (length -width -height) mold compared to conventional direct shear testing
<br />machines, excludes particles larger than 2 inches. The implication is that the shear strength is likely
<br />to be greater with the inclusion of the coarser and boulder -sized particles that make up typical spoil
<br />materials. However, the above -mentioned cohesion (475 psf) and friction angle (34°) were
<br />assumed to be representative of the new as well as old spoils. This lends another level of
<br />conservatism to this study, given the new spoils in the proposed pile are likely to possess slightly
<br />greater shear strength compared to the old spoils that have been characterized, given the level of
<br />weathering in the latter.
<br />Following laboratory characterization of the spoils, AAI utilized a newly developed
<br />approach to assign strength parameters to zones within the spoil pile depending on their relative
<br />depths of confinement. This approach has been developed based on several recent studies on
<br />spatial variability of spoil shear strength parameters based on constituent rocks, particle size, and
<br />even the deposition location within a spoil pile.1,2 Since the proposed spoil pile will be built in
<br />relatively smaller benches 25 ft thick, it is expected that little density- or size -based segregation
<br />will occur within each bench. Therefore, each bench of spoils was assumed to have the same
<br />strength properties, while the bench -to -bench strength parameters were varied. Barton and
<br />Kj aernsli have demonstrated that the effective friction angle of rockfill will decrease with greater
<br />confining stress and have provided a methodology for estimating variable friction angles within a
<br />spoil pile, based on laboratory determined friction angle values.' This counter -intuitive
<br />phenomenon is explained by the fact that the failure behavior of rockfill/spoils is guided by coarse -
<br />grained particles in the matrix at low confining stresses and fine-grained particles at higher
<br />confining stresses. Therefore, the friction angle values for the deeper benches, with higher
<br />confining stresses, were estimated by reducing the base friction angle value (34°) assigned to the
<br />uppermost bench of new spoils.
<br />Additionally, the deformation moduli for the deeper benches were estimated using an
<br />empirical approach provided by Seed and Idriss,' which positively correlates the deformation
<br />modulus to confining stresses in the spoils matrix. The cohesion values for all new spoil benches
<br />were kept constant at 475 psf. Cohesion values of end -dumped spoils vary with depth, but those
<br />of constructed piles may be assumed to be constant. Cohesion, friction angle, and deformation
<br />moduli for the old spoils currently deposited in the old I -Seam cuts were varied using the
<br />methodologies described above, as in -pit spoils tend to segregate based on particle size and density
<br />differences. The rock mass underneath and surrounding both the old and new spoils were assigned
<br />one set of ubiquitous jointed material properties, for efficiency of numerical modeling. A
<br />summary of material strength parameters used in the numerical modeling analysis is presented in
<br />Table 1.
<br />' Moffitt, Karen (2000), "Mine Waste Dump Instability," Masters Thesis, University of Toronto, April, 106 pp.
<br />Bradfield, Leonie (2018), "Reliable Shear Strength Estimation for Very -High Spoil Dumps," PhD Thesis, The
<br />University of Newcastle, Australia, 153 pp.
<br />s Barton, Nick and Bjorn Kjaernsli (1981), "Shear Strength of Rockfill," Journal of the Geotechnical Engineering
<br />Division, Proceedings of the American Society of Civil Engineers, 107(GT7):873-891.
<br />a Seed, H. Bolton and L M. Idriss (1970), "Soil moduli and damping factors for dynamic response analyses," Rep. No.
<br />EERC-70/10, Earthquake Eng. Research Center, Univ. of California at Berkeley, Berkeley, California.
<br />Agapito Associates, Inc.
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