OceanaGold Waihi North Project Vibration Performance Assessment Page 26 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 development blasting, it may be possible to initiate the blastholes across the back on several different delays and therefore reduce the size of the sheet of the rock that is moving simultaneously. 13.2. Overpressure Levels from Tunnel Blasting A review of the overpressure data collected from the Favona decline, plus other data in the literature can be used to estimate the level of overpressure at either of the WUG portal. The data collected from the Favona Portal blasting suggests that blasting within the initial section of the decline can produce high levels of overpressure. The level of overpressure at sensitive receivers around the portal will depend upon their orientation relative to the portal, the effect of topography and how these overpressure pulses attenuate and reflect with distance as they propagate away from the portal. The measured data from the Favona blasting are shown in Appendix F and provide an indication of overpressure levels. Plate 1 in Appendix G shows the modelling results based upon overpressure levels measured from the Favona blasting. The orientation of the portal will necessarily promote the higher overpressure levels directly in front of the portal with reduced values behind and only those locations in front of the portal are considered in the preliminary assessment. The preliminary modelling has been based upon an overpressure level of 138dBL at a location 30 metres in front of the portal with an attenuation of 6BL for every doubling of distance. This is consistent with the Favona portal measurements for blasting the initial section from the portal face. The results in Plate 1 show that the separation distance between the blast and the nearest residence is sufficient for the overpressure to attenuate to levels less than 120dBL. 13.3. Other Tunnel Overpressure Controls Should the measured overpressure level exceed the permissible value at the nearest properties, modelling indicates that if the extent of the exceedance is less than 6dBL, it may be possible to adjust the blast pattern to reduce the value. If the exceedance is by more than 6dBL, other physical deflectors within the tunnel may be required or some other form of mitigation necessary. Blasting for the Favona portal assessed options of implementing external elements, such as shields, rubber curtains, insulation, or blockages in the decline. Deflectors Portal shields are sometimes used as a technique to reduce overpressure levels and may include steel deflectors (large steel plates designed to deflect the overpressure upwards) positioned across the portal, similar to those used at airports for controlling engine noise. The plates are a heavy mass, moved into place prior to blasting with a loader (or similar), and removed again immediately after the blast to permit normal decline traffic. The deflector reduces the overpressure level by dispersing the wavefront to a greater extent, and thereby reducing the level at the portal, but the dispersion may have limited effect at the residents who live further away. Simultaneous monitoring at multiple sites however has not confirmed the effectiveness of deflectors in the far field. Tunnel Blockages Like deflectors, blockages obstruct the passage of the overpressure pulse have been used at some sites to control overpressure, including trials at the Favona portal. Thesemay include concrete blocks, or trucks parked across the decline. The effectiveness of the barrier will depend upon the ratio of the area occupied by the barrier to the area of the decline as well as the mass of the object. Where the ratio is low (less than 50%), the effectiveness in controlling overpressure will also be low. The application is often limited and restricted to a barrier that is easy to use as it must be placed prior to blasting and removed again to allow trucks into the tunnel.
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