41 Tonkin & Taylor Ltd Hazardous Substances Technical Assessment – Waihi North Project - Waihi Processing Plant, Water Treatment Plant and Development Site Oceana Gold (New Zealand) Limited June 2022 Job No: 1015212.1000 vRev0 6 Overpressure offset distances assessment The HSW-HS regulations specify minimum offset distances around explosive magazine locations such that, in the event of an unintended initiation, specified activities are not subject to stated blast overpressures. The blast overpressure is the sudden onset of a pressure wave after an explosion. The areas within these overpressure radii are referred to as ‘controlled zones’ and under regulation 9.27 of the HSW-HS, certain activities must be excluded. The regulations specify these minimum offset distances “to ensure that any potential adverse effects are managed and minimised. This includes making sure that people and structures are well protected and are located at a suitable distance from the explosives.”18 The minimum offset distances that constitute the controlled zones for the proposed Class 1 storage facility were calculated in a feasibility study report prepared by Global Seismic Solutions (July 2021) and are summarised inTable 6.1 below for the largest magazine, HE magazine 4 (1,750 kg NEQ). The report used a TNT equivalent of 1.0 to calculate conservative offset distances. In reality, ANFO has a TNT equivalence of approximately 0.74, so these distances will be conservatively high for the mix of explosives proposed to be stored at this location. Table 6.1: Summary of minimum offset requirements for the proposed explosives storage facility (GSS, 2021) Ref Exclusion from zone Blast overpressure Minimum offset distance Comment r 9.27 (2)(a)(i) Public traffic routes of low density and places where people may occasionally be present in numbers up to 200 persons on average in any 24-hour period 13 kPa 133 m Area within OGNZL owned land with no nearby public access r 9.27 (2)(b)(i) Public traffic routes of medium density, places where people may occasionally be present in numbers up to 900 persons on average in any 24- hour period, and the interior of any proximate building within the boundary of the place where people not directly handling explosive substances 9 kPa 176 m r 9.27 (2)(c)(i) Public traffic routes of high density, areas of high intensity land use, or any area where a person may be legally present inside the boundary of the place where the hazardous substance location is located 5 kPa 281 m 18 Explosives (including pyrotechnics) - Blast Overpressure. https://www.worksafe.govt.nz/topic-and-industry/hazardoussubstances/guidance/substances/explosives/ Retrieved 20 May 2022
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