WKP PFS – Geotechnical Section Approved by: Approval Date: TBA OceanaGold PEX-000-PLN-046-A Page 20 of 22 Figure 17: Stress damage with 20m horizontal pillar Figure 17 shows the affect of stress damage on a 20m horizontal pillar with a 15m long stope. The left side shows the situation when the hanging wall stope is mined first. The right side shows the situation when the footwall stope is mined first. When the horizontal pillar is at least 20m, it does not make any significant difference which stope is mined first. When the horizontal pillar is less than 20m, it is preferable to mine the footwall stope first and backfill it. If the mine planning team strongly prefer to mine the hanging wall stope first, then (for stability reasons) the length of the footwall stope may need to be reduced to about 7.5m to 10m prior to backfilling. Cable bolting will also assist with the stability of the internal pillar. Figure 18 shows a 5m horizontal pillar with a 7.5m long secondary stope, i.e. footwall stope is mine first, then backfilled, and then a 7.5m long stope is exposed in the hanging wall. Once the horizontal pillar is 5m or less, then it will be impossible to prevent the central pillar from failing into the hanging wall stope. For situations with horizontal pillars of 5m to 20m, it is preferable to mine the footwall first, backfill the stope, and then mine the hanging wall stope. Pillar stability can be managed with a combination of cable bolting, backfilling, and reducing the strike length on the secondary stope. Figure 18: 5m horizontal pillar with 7.5m long stope
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