June 2022 G-01483.84-017-R-Rev0 Because the 51 ▪ Foundation soil conditions and shape of the natural depression that allows for geotechnical stability, a thicker layer of weak clay founding soils in Tributary 1 was a key reason for selecting Tributary 2. ▪ Size of natural depression was large enough to accept the required rock storage volume. ▪ Cost of locating elsewhere. A single WRS option was preferred over having to build two facilities to manage volume requirements, mainly due to additional disturbance footprint required for rehabilitation, reduced surface water effects. The WRS is located adjacent to the Willows Access Tunnel portal, with the elevation of the portal (210 mRL) between the highest (265 mRL bench) and lowest elevation (204 mRL bench) of the WRS. The haulage gradient and distance from the portal has a twofold effect on cost first for initial placement and secondly when reclaimed as backfill material for the WUG. 9.0 WRS CLOSURE PLAN The WRS is designed as a temporary structure, given that the material is planned to used for backfill of the underground mine. The WRS material movement schedule was provided by OGNZL (reference schedule WAI-985-000-SCH-0001.0.IFU) showing reclaiming started in approximately year 6 (Q23) with the WRS full volume reclaimed by year 10. Once the rock is reclaimed, the WRS footprint topsoil can be replaced, harrowed (where gradients allow machinery) and seeded to return to an end use of grazing pasture. Monitoring and maintenance will be required for a period until deemed a self-sustaining good pasture cover is established. It is recommended due to steepness of the natural terrain that grazing in the first 2 winters is avoided, and only light weight animals allowed in the other months (weaners or sheep) to prevent soil damage from pugging and soil compaction. The original drainage channel will be reinstated and the collection sumps removed when the water quality meets regulatory requirements. 10.0 CONCLUSION Stability analysis results show that with the weak surficial soils (firm clay) removed from beneath the WRS toe, the FoS is above 1.3 for the proposed design. A single WRS design, based on the geotechnical assessment results, has capacity for approximately 875,000 lcm. The WRS is located in a relatively steep gully, and construction is expected to take careful management until a wide working platform (bench) is established. The WRS design incorporated access and water management elements. The management of clean and contaminated surface water flows will require detail design of channel linings and erosion control structures at the next study level.
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