EGL Ref: 9215 23 June 2022 Page 22 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. FIGURE 15: OEDOMETER TESTING CALCULATED PERMEABILITIES WITH EFFECTIVE STRESS FROM 1992 TESTING ON STORAGE 2 TAILS The vane shear strengths were generally greater than 30 kPa around the perimeter of the impoundment and increase with depth to greater than 90 kPa at 17 m. This is equivalent to a firm-stiff cohesive soil. The tailings pore water is in a sub-hydrostatic state due to the low permeability of the consolidated tailings and some underdrainage. However, as the tailings are still saturated or partially saturated, they can liquefy or cyclically soften in an earthquake where the shaking is equal or greater than that expected every 150 years on average. The embankments themselves are not liquefiable and are designed to hold back a full profile of liquefied tailings. 4.3. Existing zoned embankment design Both Storage 1A and 2 embankments are designed with a series of different fill zones. The purpose of zoning the embankments is to control seepage of leachate and achieve a stable profile to mitigate any effects possibly caused by natural hazards, like earthquakes and floods. It is this careful attention to zoning in design and construction of the embankments that has set the facilities up to have good performance. This has been demonstrated through regular monitoring, maintenance, and rehabilitation, over thirty years since the commissioning of the first facility, Storage 2, followed by the construction of Storage 1A and its continued operation. Water seepage through the facility will encounter the tailings and waste rock and therefore may pick up contaminants and heavy metals. This contaminated water seepage is called leachate. The existing embankments have been specifically designed to manage this leachate, by providing for low permeability zones, base and capping layers, subsurface drains and leachate collection drains. These work together by first limiting the generation of acid which can occur when both oxygen and water reach PAF material, secondly limiting their movement through and from the embankment and thirdly collecting leachate at points where it can be monitored and treated (if necessary) before returning clean water to the environment.
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