EGL Ref: 9215 23 June 2022 Page 47 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. risk of instability and lining to prevent tailings seepage from entering the groundwater. A prefeasibility study of this option has been undertaken by GHD (Ref. 1). The Waihi North Project schedule has mining of Gladstone Open Pit commencing in 2024. This provides a source of fill for constructing a new TSF (Storage 3) which would satisfy the project scheduling requirements. Gladstone Open Pit could also be used for storage of overburden rock. Some overburden rock is required as partial backfill of the Gladstone Open Pit to form the proposed TSF. However, the option of utilising Gladstone Open Pit just for rock disposal would require expansion of other TSF options and would require stockpiling of large quantities while mining the pit. 8.10. Underground It is common practice to backfill the voids created by underground mining. Mine pit overburden rock can be used for backfilling of underground mines. This already happens with the current operation and will continue with the Waihi North Project. Tailings can also be used for filling underground mines. This requires dewatering of the tailings to create a pumpable paste and the addition of cement and other admixtures to improve its pumpability and strength. The resulting mix is commonly referred to as paste backfill. There are significant capital and operating costs associated with paste backfill compared to backfilling with overburden rock.
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