GHD | Oceana Gold (New Zealand) Limited | 12537997 | Gladstone Pit TSF 12 respectively1. In addition, where core specimens showed strength values exceeding R2, point load index tests were conducted. Table 2 presents the recommended geotechnical parameters for the GOP by PSM (2021). These parameters have also been adopted for the GOP TSF design. Table 2 Geotechnical parameters for the GOP recommended by PSM (2021) Material Unit Weight (kN/m3) Strength parameters UCS (kPa) GSI Mi D Friction Angle (°) Cohesion (kPa) Ignimbrite 18 25 40 - - - - Dacite 18 - - 8,000 40 25 0 Volcaniclastic 19 15 40 - - - - Andesite Class 4 22.5 24 65 - - - - Andesite Class 2 22.5 - - 25,000 40 25 0 Andesite Class 3 22.5 - - 12,000 29 25 0 Note: there is no Andesite Class 1 intersected by the pit design. 2.4 Hydrogeology and groundwater 2.4.1 Background Site investigations have previously been carried out by GHD (2019). The site investigations focused on assessing the hydrogeology of new mine elements, including GOP, and considered the installation of several nested monitoring wells. This provided information for assessment of groundwater levels, hydraulic conductivities and groundwater interactions. The main outcomes of the previous investigations are summarised in the following subsections. Gladstone monitoring bores and locations are shown in Figure 6. 2.4.2 Shallow groundwater system The shallow groundwater system in the Waihi area comprises groundwater flow through shallow soils, reworked young volcanics and weathered rock. The weathering of the historic surface andesite and breccia, prior to deposition of the younger volcanics, provides the hydraulic separation for shallow and deep groundwater systems. Ongoing weathering, erosion and fracturing of andesite where it outcrops on Gladstone Hill has provided a material with notably greater water storage than deeper, less weathered andesite. The weathered material, which is more prevalent towards the east and south-east of Gladstone Hill, also provides a medium through which rainwater can infiltrate. The difference in water storage properties and desaturation of deeper andesite limits the volume of recharge which can percolate deeper. This promotes flow of infiltrating water radially away from Gladstone Hill into the younger volcanics of the shallow groundwater system. Hydraulic conductivity testing of the dacite indicates very low permeabilities, which result in high groundwater levels, consequently supporting the streams and wetland in this area. In contrast, the higher permeability measured for the rhyolitic tuff and sandy ignimbrite on the north and west extent provide greater drainage, generally lower groundwater levels and promote more rapid flow of groundwater. 1 Refers to quality grade according to Brown (1981). R1 corresponds to very weak rock (typical UCS= 1-5 MPa) and R2 corresponds to weak rock (typical UCS = 5-25 MPa).
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