EGL Ref: 9049 23 June 2022 Page 20 File: WAI-985-000-REP-LC-0050-Rev0.docx This report shall only be read in its entirety. The area of greater tilts over Favona Underground appear to be limited, potentially by the Favona Fault on the west side of Favona Underground. 5.2. Project Martha settlement effects The further dewatering to RL500m of the Project Martha underground will have minimal effect on any ongoing settlement in the upper andesite or younger volcanics. However, it will result in additional depressurisation and settlement of the deeper andesite rock mass. These settlement effects can be expected in the general areas of previously experienced settlement. New settlement trigger levels for each settlement zone were set for Project Martha and are shown in Figure 51. These were determined by projecting observed settlements using the settlement trends determined using numerical models. These settlement trigger levels are also plotted on Figure 53 to Figure 61. The settlement trigger level for Zone 7 for Project Martha is 540 mm immediately east of Martha Open Pit. Around Favona and Gladstone, Zones 4 and 5 are specified with a settlement trigger level of 160 mm and 260 mm respectively. 5.3. Settlement exceptions to predictions Generally, settlements have been in-line with expectations. Where there have been exceptions they have been investigated and in all cases there have been explanations. These are discussed in the following sections. 5.3.1. Gladstone Road Settlement occurred in 2012 causing damage to some dwellings in Gladstone Road to the east of the Martha Open Pit. Differential ground settlements (tilts) were the cause of the damage, and investigations confirmed the settlement was a result of dewatering of the younger volcanic deposits in this area caused by a deep geotechnical drillhole (CGD008) not being properly grouted, which allowed groundwater to leak into the lower strata. Following re-grouting of the drillhole the ground water was restored to the original levels and the land rebounded quickly close to original levels. 5.3.2. Slevin Street Since May 2015 the tilt between marks 20C and 20D (near Slevin Street) (Figure 49) has at times been greater than 1:1,000. OGNZL understands the settlement to be unrelated to dewatering associated with the current project. It is considered to be due to the weak nature of the ground and the presence of unfilled stopes in the old underground Martha Mine that underlie the area. The area is near a high hazard zone identified by GNS (Ref. 42). Slevin Park is a swampy area, historically infilled with weak material where slumping/subsidence has occurred previously. No damage to any dwellings has occurred. The closest dwelling to the affected area is 95 m away.
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