WKP PFS – Geotechnical Section Approved by: Approval Date: TBA OceanaGold PEX-000-PLN-046-A Page 13 of 22 1.7 The Stability number N’ The stability number, N’, is used with the empirical stability graph chart to determine the critical hydraulic radius (HR) for stope stability. The equation for N’ is as follows: N’ = Q’ x A x B x C (Equation 6) where, Q’ is from Barton’s equation, i.e. Q’ = (RQD/Jn) x (Jr/Ja) A = Rock Stress Factor (ranging from 0.1 to 1.0) B = Joint Orientation Factor (ranging from 0.2 to 1) C = Gravity Adjustment Factor (ranging from 2 to 8) Typically, the A-parameter is simply a function of the ratio of the UCS to the induced stress parallel to the surface that is being evaluated. Due to the complexity of stoping on multiple ore bodies at WKP, the stress damage is more likely to be a tensile failure and each stoping area must be evaluated with numerical modelling to determine the extent of stress damage. For the base case, the Aparameter at WKP will be set to a value of 1, i.e. no stress damage. The A-parameter used in the Scoping Study (SRK, 2019) ranged from 0.3 to 1.0, and the main source of variability in the Scoping Study A-parameter was the due to the depth of mining, i.e. the in-situ stress. The B-parameter is evaluated by determining the angle between the critical joint or shear and the surface being evaluated, i.e. hanging wall, footwall, or backs. The B-parameter used in the Scoping Study (SRK, 2019) was 0.4. Based on the orientation of the ore body and the typical orientation of the major structures, this value is equivalent to a 40o angle between the poles of the stope surface and the major structures. The analysis in this report has also assumed a value of 0.4 for the Bparameter. The C-parameter is an adjustment factor that accounts for the relative dip of the stope surface. It can range from 2 to 8, i.e. a value of 2 is assigned to flat backs; a value of 8 is assigned to the hanging wall and footwall of a vertical stope. The C-parameter was assigned a value of 5 in the Scoping Study, and this was also used to calculate the N’ values in this report, except that a value of 2 was assigned for evaluating the N’ in the backs. A value of 5 corresponds to a 60o dipping hanging wall and footwall. A value of 2 corresponds to the flat-dipping stope backs. Table 10 below provides a summary of critical values for Q’, N’, and maximum HR at WKP assuming that unsupported backs and walls, i.e. no cablebolts or other ground support. For the stope design for a PFS should assume the most conservative design parameters, i.e. 1st quartile values (denoted as “q1” in the table). Figure 10:Summary of Q’, N’, and maximum HR for unsupported stopes at WKP
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