June 2022 G-14642.71-006-R-Rev0_FINAL 40 9.3.4 Rock mass characterisation Geotechnical characterisation of the rock mass was undertaken using the Barton’s Q’ system with the parameters RQD, Jn, Ja and Jr estimated from the logs and core photographs. Geological strength index (GSI) values were calculated from the Q’ values using an empirical relationship. A total of 516 values of Q’ and GSI were estimated from the geotechnical database for intervals within vein material, hanging wall or footwall zones. Statistical distribution of the Q’ and GSI values were then used to characterise the rock mass for use in the PFS mine design (Figure 24). Figure 24: Summary of Q' and GSI values calculated from the available geotechnical data (from OGC PFS study - Appendix D). 9.4 Assessment of proposed mining methodology The proposed mining methodology for the Wharekirauponga UG mine is Modified Avoca method. The potential dimensions of the stopes have been evaluated by OGNZL using empirical methods and numerical modelling. The empirical methods incorporate rock mass characteristics from the geotechnical assessment described in Section 9.3 and commonly adopted studies of the historical performance of a large number of underground mines. Conservative rock mass characteristics have been used in the assessment, which is typical of the current stage of mine design. The empirical methods have been used to evaluate the size of unsupported voids that can be adopted by the design. Numerical modelling of proposed mine was undertaken using MAP3 software to evaluate the likely behaviour of the surrounding rock mass in response to mining, particularly the potential damage zone above the mine and the interaction between closely separated voids. The modelling concludes that no severe stress damage will occur to the crown pillar (the zone between the mine and the ground surface) provided the stopes are tightly backfilled. The following summarises the results of WSP’s review of the PFS: 1) The geotechnical assessment follows current industry practice for a PFS based on limited available geotechnical information. Therefore, the interpreted geotechnical properties and the result of the geotechnical assessment are considered as preliminary and will require additional rock mass characterisation data and analysis as the design progresses. 2) The documented input parameters for the stope design are appropriate based on the available geotechnical information to date and interpretation of the data. The assessment is based on the method proposed by Matthew et al. (1981) and modified by Potvin et al. (1989) and
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