Martha East/West Layback Proposals
The dotted lines on the photograph give an indication of the possible layback areas to the east and the west. These lines are not yet confirmed and show the likely greatest extent of the layback should the decision be made to proceed with one or both of the proposals. When more information is known, the area may be smaller, but is unlikely to be larger.
Pre-Split Blasting
In Waihi gold and silver are found in quartz reefs that lie within the host rock comprising andesite and other volcanic rock types. Quartz and andesite are extremely hard compared to other types of rock in the open pit. Rock type and hardness are taken into account when drilling boreholes, planning and designing blast patterns and excavating the loosened material.
In the Martha open pit's south wall cutback the final walls are being contoured. In areas where rock is extremely hard the best way to achieve a smooth and stable final wall surface is to use a blasting technique known as pre-splitting.
In pre-splitting a row of holes is drilled along the plane of the required final excavation. A string of cartridges containing a blasting agent is lowered into each hole. Electronic detonators are linked to a programme entered into a computer and the holes are fired.
Normal production blasting in the open pit uses stemmed holes. Stemming means packing gravel into each blast hole above the blasting agent to ensure that the energy created by the blast goes through the ground to fracture the rock.
Pre-split holes are not stemmed, allowing explosive gases to escape and minimising damage to the final wall behind the line of holes. The resulting cracking along the plane of the pre-split holes reflects the stress waves from the subsequent, stemmed production blasts so that there is little effect on the rock behind the pre-split plane. The light explosive charges cause cracks to spread between the holes, 'joining the dots'. In effect the action could be likened to tearing a piece of paper along a perforated line.
Loud Cracking Sound Likely
The absence of stemming increases the amount of overpressure (noise or airblast) produced. It is likely that pre-split blasts will be heard as a rather loud cracking sound.
Pre-split blasts are no larger than other blasts, will not cause any structural or cosmetic damage to surrounding properties, and ground vibration limits will not be exceeded.
The pre-split technique will be used in the open pit within the next two weeks and again around the middle of next year in addition to the standard production blasting programme.
All blasts will be monitored.
If you are concerned or wish for further information please phone our free phone - 0800 NEWMONT (639 6668).
