WAI-985-000-REP-LC-0056 Revision 0 Page 19 of 21 Waihi North Project – Tailings and Rock Storage – Functional Need Assessment 6 WILLOWS ROCK STACK Development of the access tunnels to the Wharekirauponga orebody will produce approximately 850,000 m3 of rock that will require temporary storage during operation before it can be repurposed as backfill in the underground mine. The proposed WRS is a temporary rock stack covering approximately 5 ha and having a total storage capacity of up to 900,000 m³ of rock, to a maximum height of 20 m above natural ground level. A single WRS option was preferred over having to build two facilities to manage volume requirements, mainly due to the additional disturbance footprint required for rehabilitation, and the additional disturbance of surface water bodies a second facility would involve. 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. The proposed WRS is sited in a narrow gully (referred by others as Tributary 2 (GHD 2021) approximately 200 m upstream from the confluence with Mataura Stream. Key factors that led to WSP confirming the site as the preferred option include: • Close proximity to the planned WUG portal. • The size of natural depression was large enough to accept the required rock storage volume. • The gradient and shape of the natural depression that allows effects to be effectively and efficiently managed. • The location was furthest from the public roads and properties, with the WRS sited in a natural depression that would lead to least visual amenity and landscape effects. • The natural depression creates a noise barrier for much of the rock stack development reducing impact on neighbouring properties. • The low value of the tributary; the WRS was located above an existing farm access track, in an active grazing area, avoiding the higher ecological value of the lower reach and Mataura Stream. • Avoids surveyed wetland areas and areas of high value flora and fauna. • 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. The only potential alternative that would avoid the need for the Willows Rock Stack would be transport of the development rock to the existing stockpiles at Waihi incurring significant additional rehandle cost and energy. Such an option would introduce its own adverse environmental effects and would render the project unviable.
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