OceanaGold Waihi North Project Vibration Performance Assessment Page 6 of 41 Project Number: HP2006-2 Heilig & Partners Save Date: 21/06/2022 5:55:00 PM ABN 56 082 976 714 File Name: WAI-985-000-REP-LC-0018_Rev 0 blasting according to the distance to the nearest sensitive receiver, the levels of ground vibration and overpressure can be controlled to acceptable values. Flyrock can be considered to refer to the movement of rock beyond a small working area around the blast pattern, commonly of the order of 20 to 50 metres. Like ground vibration and air overpressure, flyrock can also be controlled, as demonstrated by the few isolated instances of flyrock that have occurred from the many thousands of blasts that have occurred at the Martha open pit mine. All nitrate-based blasting explosives produce large volumes of gas in very short time frames. The action of these gases facilitates the rock breakage process, and they therefore remain an integral component of an effective blasting program. Under ideal detonation conditions, the gases produced are nitrogen, carbon dioxide and water (vapour), all of which are colourless gases. Under non-ideal detonation conditions, undesirable gases are also produced and includes the oxides of nitrogen (NO and NO2) and carbon monoxide. NO2 is a red coloured gas and it is the observation of this gas after a blast which is commonly described as “post blast fume”. Whilst there are multiple causes that can contribute to non-ideal detonation of nitrate-based blasting explosive used in practical mining situations, bulk explosives are formulated and manufactured to minimise post blast fume when used in accordance with the technical data sheet recommendations. Dynamic effects when blasting under certain conditions are believed to influence some of the fundamental explosive design properties. Controls, decisions and practices at the time of blast hole charging can have an impact on the potential for post blast fumes. It is known that combinations of highly confined blasts, soft and weathered ground types, water saturated ground types, highly fractured ground types and blasts with high powder factors present as conditions that have a higher risk of producing post blast fumes. These are considered in the blast design and scheduling process. As part of the OceanaGold Blast Safety Management System, several control measures for addressing for post blast fumes are assessed. The same successfully applied procedures for Martha would continue to be implemented for the GOP blasting. Because of the very low likelihood of a post blast fume event occurring when blasting at GOP, the recommendation is to continue with the existing Management Plan practices. 3.1. Factors Influencing the Level of Vibration The vibration produced by blasting occurs as a function of both controllable and uncontrollable aspects of the design and ground conditions. Blast design parameters, including the amount of explosives per blasthole used, the design and sequencing of the blastholes and to a lesser degree the size of the blast, affect to varying degrees the level of vibration but can be adjusted in the mine design. Other factors, such as the rock mass, the degree of water saturation, and the presence of any geological discontinuities or previously mined areas, also influence the measured level of vibration but cannot be controlled. Where the influence of these uncontrollable aspects is known, the impact can be managed through adjustments to the blast design (e.g. use of smaller explosive weights). In those cases where the presence of all uncontrollable factors is not known in advance of the blast, such as the possible presence of old workings near to the blast location, elevated or reduced levels of vibration may occur. OceanaGold’s practice in these instances is to conservatively design blasts to a lower level of vibration, such as around half the permissible value, which allows for increase in vibration because of unknown factors without exceeding the consented vibration limits. 3.2. Perceived Effects of Vibration Although people may recognise sounds and vibrations, it is difficult to quantify perceptions and sensitivities. Inside a structure, people may feel the building vibration and hear the objects around them rattle, such as windows and trinkets on tables and walls. When a vibration is perceived in this way, some people will say that they felt very strong vibrations. When outside of a building, these potential secondary effects are absent, and the perceived effect is much less for the same level of vibration.
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