29 Tonkin & Taylor Ltd Hazardous Substances Technical Assessment – Waihi North Project - Storage and use of hazardous substances at Willows Road site and the Wharekirauponga Underground Mine Oceana Gold (New Zealand) Limited June 2022 Job No: 1015212.1000 vRev0 5 Overpressure offset distances assessment 5.1 Overview 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 (“PCBU to control adverse effects of unintended initiation”), certain activities must be excluded. The minimum offset distances for the proposed Class 1 storage areas were calculated in the two location assessment reports prepared by GSS (Error! Reference source not found. andAppendix B) and are summarised in the sections below. 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.”4 5.2 Surface magazine offsets The total aggregate quantity of stored H.E product (7,350 kg NEQ) is divided across six separate H.E magazine units at the surface. As illustrated inFigure 4.1, the largest of the magazines will be restricted to a maximum of 1,300 kg NEQ of mixed grade H.E. An additional 10 kg NEQ of I.E (detonators) is to be stored in an entirely separate magazine unit (I.E Magazine). To allow flexibility in the positioning of the I.E magazine, all controlled zone calculations assume the quantity may be up to 1,310 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. Table 5.1: Summary of calculated offset distances for the proposed explosives storage site at Willows Road (GSS, 2021) Ref Exclusion from zone Blast overpressure 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 24hour period 13 kPa 121 m Area within OGNZL owned land. Separation of over 300m to on-site worker facilities at the SFA. 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 24hour period, and the interior of any proximate building within the boundary of the place where people not directly handling explosive substances 9 kPa 160 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 255 m 4 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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