GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 95 Table I.10 Summary of steady transient model scenarios for Cross-section 2 (construction dewatering) Scenario Stage of work Days Description 1 – Steady state (Parent for 2B and 3) Base model 0 Material properties as per Cross-Section 1 calibration. Boundary conditions provided in Table I.8. 2A - Transient (Parent for 2B) Partial excavation (15 m) 0 - 30 Removal of surficial alluvium down to sensitive tuff, and includes excavations for the cut off and toe drains, and shear key. 2B - Transient (Parent for 2C) Full excavation (15 m) 30 - 75 Removal of sensitive tuff to maximum depth present in model (15 m). Assumes no inflow mitigation (such as piling). 2C - Transient Partial backfilled (15 m) 75 - 120 Backfill to original sensitive tuff surface, using NAF structural backfill (to be subsequently overlain by the Zone A soil liner). 3 - Transient Full excavation (20 m) 0 – 75 As per above full excavation to 15 m, but with additional region removed into underlying rhyolite tuff to achieve 20 m excavation depth. Analysis undertaken for drawdown only. * As discussed in the Rationale (Section 5.4.2) of the main report, additional analyses were run to provide a range of potential effects. Refer to appendix sections I.2.2 and I.2.3. Table I.11 Cross-Section 2 model set-up Boundary type Scenario Adopted boundary conditions Justification Rainfall recharge All Water flux - potential seepage face review: 3 x 10-5 m/d (Lower catchment rate) As per Cross-section 1. Upgradient groundwater boundary (NW) All Constant Head – 1,108 m RL To achieve groundwater flow direction toward the SE, and a high water table near ground level at the area of the excavation, as per existing site conditions. Downgradient groundwater boundary (SE) All Constant Head – 1,113.5 m RL Ruahorehore Stream 2 and 3 Constant Head – 1,109.1 m RL Set to nominal stream depth of 1 m assuming discharge of dewatering groundwater will occur to the stream. Dewatering 2 and 3 Water rate – potential seepage face review: 0 m3/day Removal of water from undercut. Table I.12 Adopted material parameters for Cross-Section 2 (where additional to Table I.2) Material (1) (2) Scenario/s applied to KH (m/s) KV (m/s) VWC and K function(3) Hydraulic conductivity justification Tailings All 8 x 10-9 8 x 10-10 Sandy silt (coarse tailings) Long-term consolidation as per Figure I.6 Operational – closure tailings All 1 x 10-6 1 x 10-6 Silty sand As per Zone D2 of Crosssection 1. 1. Geological units are colour coded as per the SEEP/W cross section 3. VWC = Volumetric water content function (SEEP/W 2012 database), selected based on EGL descriptions of materials provided by email (EGL 2019b and 2019c).
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