www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation 16 5. MODELLING ASSESSMENTS 5.1. PREVIOUS ASSESSMENTS AND MODELLING 5.1.1. CONCEPTUAL MODELLING A conceptual hydrogeological model (CHM) for the access tunnel was completed by GWS (ref 1) to assist in the assessment of groundwater effects (Section 5.4). In their conceptualisation, the tunnel length was split between two sections consistent with the recognised separate groundwater systems on either side of the Coromandel Ranges divide. The first tunnel section from the portal northwards to the divide lies within the Waihou groundwater catchment, and the second from the divide northwards to the WUG, within the Otahu groundwater catchment. 5.1.2. WAIHOU CATCHMENT CHM AND NUMERICAL ANALYSIS The Waihou CHM is shown in Figure 5-1 with the following features: • The initial section of the tunnel is assumed to be fully dewatered and depressurised due to the presence of an existing open pit and underground mines at Martha and Favona. • Fully saturated formation and ambient pore pressures develop progressively away from the portal at some undetermined point. • As the tunnel face advances groundwater is intercepted and depressurised. • Dewatering of the surface formations does not take place due to the absence of a hydraulic connection in the low permeability andesite bedrock between groundwater at depth and surficial aquifers. • Recharge to the surficial unconfined aquifer and consequent groundwater levels are maintained by rainfall. Figure 5-1: Access Tunnel CHM for Waihou Groundwater Catchment (after CWS) GWS completed a 2-D SEEP/W numerical analysis along a section cut perpendicular to the access tunnel alignment on the initial section from the portal to Willows Farm. In this section of the tunnel, the main surface water impacts would apply to the Mataura Stream which drains this part of the catchment. This analysis assumed preferential flow confined to an arbitrary set of three 5m wide fracture zones with an
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