Laserfiche WebLink
April 21, 2020 Page 6 <br />in the first step, before the entire spoil pile was placed atop the current ground in one step. AAI <br />had initially planned to simulate the growth of the spoil pile in stages, but this was ruled out in <br />favor of modeling efficiency, considering the run times were exceeding 48 hours for the numerical <br />model in its present configuration. Additionally, the proposed spoil pile was assumed to be <br />constructed in a single step, which represents a more conservative scenario than staged <br />construction simulation. The N-Strike Pit was not included in this numerical model, as AAI <br />believes it is sufficiently far removed from the footprint of the proposed spoil pile to mutually <br />affect one another. Element sizes of 5 ft and smaller were selected for the numerical model. <br />Several monitoring wells in the vicinity of the N-Strike Pit have recorded static water levels <br />at 25 ft from the surface, which is the reason the water level was assumed to be approximately <br />25 ft below the bottom of the proposed spoil pile in the numerical model. The water table was <br />assumed to be static in order to develop pore pressures in the subsurface and no transient now <br />calculations were made. <br />The results of the analysis are presented in Figures 3a—c. The overall displacements of the <br />new spoil pile and old in -pit spoils are presented in Figure 3a, which shows total displacements of <br />approximately I inch at the spoil pile crest level, which is an insignificant level of deformation <br />considering the height of the proposed structure, relatively weak foundation, and water table 25 ft <br />below the top of the foundation level. The relatively higher displacements observed in the old <br />spoils in the I -Seam cuts (yellow to red hotspots) to the northwest of the new spoil pile may be <br />ignored, as they represent localized dips in the opposite direction of the ground surface and are not <br />related to the placement of the new spoils upslope. The material velocity plots for the model <br />elements are presented in Figure 3b, which are a metric for stability of a given structure. A velocity <br />of IE-5 or higher represents potential failure, while lesser values indicate stability. The peak <br />velocity is noted to be 2E-6, located around the northern tip of the southern portion of the new <br />spoil pile. This level of velocity indicates likely medium- to long-term stability for the spoil pile. <br />Lastly, Figure 3c presents the yield condition plot of the spoil pile, which shows very limited and <br />localized material yielding within the spoil matrix, predominantly along the northern perimeter of <br />the spoil pile. <br />AAI also utilized the two-dimensional (213) finite -element numerical modeling program <br />Phase 26 to evaluate the proposed spoil pile. The Phase 2 program uses a shear strength reduction <br />(SSR) approach to estimate the Stability Factor (SF) associated with a given slope geometry, where <br />the material shear strength parameters (cohesion and friction angle) of the constituent layers are <br />reduced in steps until failure (excessive yielding and/or displacement) is triggered. The SF value <br />is the ratio of the assigned strength parameters for a given slope model to the reduced strength <br />parameters at the onset of failure. The 2D numerical modeling results, performed along a vertical <br />section (Section A -A', Figure 1) through the spoil pile oriented along the true dip direction, <br />indicated that the proposed spoil pile is associated with an SF value of 1.6 against global instability <br />(Figure 4). A combined review of the numerical modeling results indicates that the proposed spoil <br />pile is likely to be stable over its projected life of 4-6 years. <br />6 Rocscience Inc. (2001), "Phase' User's Guide," available at https://www.rocscience.com/dowrdoads/phase2/ <br />Phase2 TutorialManual. <br />Agapito Associates, Inc. <br />