OceanaGold Waihi North Project Vibration Performance Assessment Page 25 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 13. TUNNEL OVERPRESSURE ASSESSMENT The project seeks to develop a tunnel between the processing plant and WUG as well as a decline from the mine surface facilities to the underground stoping blocks. This assessment addresses the level of overpressure which would be expected around the portal area of the WUG tunnel, including the initial 100 metres before the heading of the tunnel turns towards the north-west. Topographical features, portal geometry, weather patterns, orientation and the location of residences will all impact on the attenuation of these levels from the portal and the preliminary modelling results will be verified after site specific blasting data are available. This section provides a preliminary assessment of overpressure levels as well as providing comments on measures that could be implemented to control overpressure from the tunnel blasting. Experience from the Favona portal blasting has shown that the greatest effect on overpressure levels is the distance between the blasting face and the portal. At chainages very near to the portal and for properties very near to the portal (<200 metres), almost irrespective of the design and the additional measures implemented for controlling overpressure, compliance with restrictive overpressure criterion may not be possible. Compliance with 120dBL is only expected to be achieved through strategic placement of the portal, effective shielding of properties through topographical and vegetation barriers, potential barricades in the decline, well implemented blast designs and practices and an adequate separation distance to the sensitive receivers. 13.1. Sources of Overpressure There may be several sources of overpressure from an underground development blast, including the venting of explosion gases either through the blasthole collar or through the free face, the vibration of the rock mass, and the movement of rock at the face. The largest overpressure peaks will be produced by the venting of explosion gases with the next major contributor occurring as a result of the rock movement at the development face. Generally, the peak level of overpressure occurs from one of two areas within the blast: • The initial cut holes and therefore the peak occurs at the start of the blast • The perimeter blastholes, including the penultimate breasting row, associated with the firing of the backs and therefore at the end of the blast. On few occasions is the peak overpressure expected to occur from elsewhere in the blast, and on those occasions when this occurs, the peak level is expected to be lower than on the other occasions identified above. The overpressure levels at the portal are expected to show a peak level that occurs from the cut holes. This may occur as a result of the near simultaneous detonation of the cut holes and the high velocity ejection of the broken rock. This phenomenon is considered analogous to stemming ejection in open pit environments which generally produces the highest overpressure level. Given that many blastholes around the cut region contribute to the ejection of the blasted rock, increasing the delay interval between successive detonations, or limiting the number of blastholes that share a common delay in the cut, is expected to slow down the velocity at which the broken rock is ejected, and thereby reducing the peak overpressure level. In addition, and perhaps only a small effect, the broken cut rock is ejected up the decline directly towards the portal whereas a portion of the movement of the broken rock with stripping holes is towards the void created in the centre of the blast. The second dominant source of the high overpressure levels is expected to occur from the perimeter blastholes around the back of the drift. In this area, there are multiple blastholes sharing a common delay and it is expected that the peak levels occur because of the simultaneous movement of the sheet of rock across the back. Although all blastholes sharing a high number long period delay do not detonate at precisely the same time from a vibration perspective, they are sufficiently accurate to cause the sheet of rock to movement simultaneously. As with blasting in open pit operations, as the rock moves it pushes the air in front and creates a low frequency overpressure pulse. In an open pit operation, an option to reduce overpressure levels is to slow the movement of the rock down. Whilst this is not an option for
RkJQdWJsaXNoZXIy MjE2NDg3