EGL Ref: 9215 23 June 2022 Page 21 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. FIGURE 14: PARTICLE SIZE DISTRIBUTION FOR TAILING SAMPLES FROM STORAGE 2 The normalised soil behaviour plots based on CPTs and Seismic Cone Penetration Tests (SCPTs) using the method of Robertson (1990, Ref. 10) indicate that the tailings have behavioural characteristics typical of clay and silty clay with occasional thin lenses of silty sand. This is consistent with the laboratory classification testing. The fine-grained cohesive nature of the tailings at Waihi results in a low permeability profile. Also, with time and consolidation tailings typically further reduce in permeability. Oedometer testing gives an indication of the permeability of the tailings with consolidation. Oedometer testing has been undertaken on the Storage 2 tailings in 1992 (Langbein, 1993, Ref. 11). The results from the oedometer testing with effective stress are summarised in Figure 15. The grading of the tailings tested indicated that the sampled Zones 1 and 2 were clayey silt and Zone 3 was silty clay. The gradings of the samples are included in Figure 14, labelled 1992 Sample 1, Sample 2 and Sample 3. The depth of tailings in Storage 2 is up to approximately 45 m. With an estimated bulk density (dry tails and water) of 1.75 t/m3 (17.2 kN/m3) and assuming porewater pressures equal to a hydrostatic profile the stress at the base of the impoundment would be at least 330 kPa and the tailings permeabilities are estimated to reduce to less than a value of 1E-8 m/s which is the specified minimum permeability for the earthfill liners for the TSFs.
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