2025-05-05_PERMIT FILE - C1981010 (32)

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Tonia Perkins July 31, 2024 Page 33 Agapito Associates, Inc. Discussion Although the slope is expected to remain stable, the risk of rock fall from disturbed ground at or above the M Seam will present a hazard to the working area below when the pit progresses down to the Q Seam. Agapito recommends placement of a catch bench in the slope at the base of the M Seam to reduce the risk of falling objects from the free face that will exist above the Q Seam working level. The width of the catch bench should be calculated using the modified Ritchie criterion, which is a widely accepted method of calculating catch bench widths for open pit mine slopes. 4 Using the modified Ritchie criterion, catch bench width is calculated based on the height of the bench face above, in units of meters (m), according to the following equation: 𝑏𝑏𝑚𝑏� 𝑤�ℎ𝑏𝑡� (𝑚)=0.2 ∗𝑏𝑏𝑚𝑏� �𝑏�ℎ𝑏�𝑡+0.45 𝑚 For a free face 30 ft in height (9.1 m) between the floor of the M Seam and the next catch bench or roadway above, the recommended width of a catch bench located at the M Seam floor level would be calculated as: 𝑏𝑏𝑚𝑏� 𝑤�ℎ𝑏𝑡� (𝑚)=0.2 ∗9.1 +0.45 =2.3 𝑚= 7.6 𝑏𝑡 The width of the catch bench should be adjusted incrementally as the height of the free face above the M Seam changes. Conclusions and recommendations The web and barrier pillar designs provided in this report for the M and Q seams were derived using widely accepted empirical methods and validated using numerical modeling techniques. By using these design curves to determine the minimum pillar width for each panel as mining progresses and adjusting the width as conditions warrant, maximum resource recovery can be attained. With the safety factors specified in the design criteria and the confirmation of sufficient support capacity exhibited in the numerical modeling results, it is anticipated that the M Seam will remain stable during mining of the Q Seam in the top-down mining approach. Results of the FLAC3D model suggest that the global stability of the N-Pit slope is not impacted by mining of the M and Q seams under the guidance of the given design specifications. Although the slope is expected to remain stable, Agapito recommends placement of a catch bench in the slope at the base of the M Seam to reduce the risk of falling objects from the free face that will exist above the Q Seam working level. 4 Ryan, Thomas M., and Paul R. Pryor. "Designing Catch Benches and Interramp Slopes." Slope Stability in Surface Mining (2000): 27-38