EGL Ref: 9215 23 June 2022 Page 53 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. 9.1.4.4. Dry stack northeast of Storage 2 A dry stack could be constructed northeast of Storage 2.This land is not owned by OGNZL. For the reasons explained in section 9.1.4.3 this technology is not ideally suited to the Waihi Operation. 9.1.5. Mixed rock stacks Mixed waste stacks, sometimes referred to as co-disposal, is a newer concept where tailings (filter pressed) are mixed with mine open pit overburden rock fill to form a stack of tailings and rock that together has better geotechnical properties than tailings. There are few examples of this technology, especially in high rainfall environments. The same issues regarding the management of surface runoff for dry stacks exist for mixed waste stacks. This option is not recommended for use at the Waihi Operation at this stage. 9.1.6. Downstream and centreline embankment dams Downstream and centreline embankment dams constructed out of compacted earth and rockfill present robust options as the downstream stability of the embankment does not rely on the strength or behaviour of the tailings. The embankment can be zoned so water can also impound against the embankment without concern. The existing TSFs at the Waihi Operation are of downstream construction. This is the most conservative type of embankment that can be used for storing tailings. They have been proven as an effective option for the Waihi Operation. The large quantities of mine open pit overburden material have enabled construction of conservatively designed embankments to retain the tailings. They are a robust option in the event of an earthquake. Centreline embankments are more efficient tailings embankments than downstream embankments. In this option the embankment crest is maintained on the same centreline by placement of some fill upstream of the crest on tailings and some over the downstream shoulder of the existing embankment. The upstream stability during construction needs careful consideration where the embankment is required to be constructed a large height above the tailings to provide storage for tailings and to provide freeboard for extreme flood events. This is the case at Waihi. The practicality of a centreline embankment depends on the rate of rise, with faster rates of raise resulting in less consolidation of the tailings and weaker tailings. The locations of existing TSFs and potential new TSFs are shown in Figure A3. They are discussed in the following sections. 9.1.6.1. Raising of Storage 1A Storage 1A could be raised by downstream or centreline construction by steepening the existing embankment slopes as the embankment was conservatively designed and constructed predominately out of rockfill from the Martha Open Pit. This means
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