Drill Bits
The pit rim walkway to the west of the open pit from Martha Street through to No.3 shaft close to the mine truck display will be closed for about four weeks.
The temporary closure to is to allow safe access for drill rigs and associated service vehicles, which will be operating on the western perimeter of the pit as detailed in the last Update of June 24.
While this section of the walkway is closed to the public we will take the opportunity to complete some minor upgrades to the pathway and immediate area.
Access to the Western Viewing Platform and the walkway east of Martha Street to the Pumphouse will not be affected.
Our apologies for any inconvenience this temporary closure may cause.
The Newmont Waihi Gold freephone is attended 24 hours a day, seven days a week.
0800 NEWMONT
(0800 639 6668)
Typical blast hole at martha mine
A message from the manager
Since Martha Mine reopened we have fired over one million holes. This year alone we have fired 36,000 holes. On Friday June 29 we fired 292. We got two wrong.
You might think that getting four wrong out of one million over a twenty-year period - having a 99.9996% success rate - would be acceptable.
It's not acceptable in our business and we know it's not acceptable to our community.
As I write this on Friday July 6 we are continuing our comprehensive investigation into the cause of the two misfires. This investigation is wide ranging, detailed, and involves specialists, regulators and Newmont and Macmahon staff.
You have my assurance that we will not recommence blasting in the open pit until we have completed all aspects of the current investigation and we are confident that we can safely resume operations.
Glen Grindlay
General Manager
'This year alone we have fired 36,000 holes. On Friday June 29 we fired 292. We got two wrong.'
Misfire at Martha Mine: 29 June
Two blast holes misfired during the 2.40pm blast in the Martha open pit on Friday 29 June. Fly rock from the two holes was scattered in an area of approximately 30 to 40 metres outside the blast exclusion zone.
Some of the rock fragments landed on the roof of Newmont’s office in Moresby Avenue and also on the roof of the Video Ezy store in Seddon Street. Other fragments were found in Newmont’s carpark at the top of Martha Street.
Blasting in the open pit was suspended pending a full investigation into the incident.
Newmont Waihi Gold worked with mining contractor Macmahon, explosives suppliers Orica and Prime and a range of advisers to investigate the incident, determine its cause, and reduce the likelihood of the event repeating.
One similar event has been recorded at Martha Mine in the almost twenty years since the site reopened as an open pit in 1987.
Flyrock is reasonably common at many mines, but is totally unacceptable at Martha as we operate so close to the community. We take this incident particularly seriously.
How is a typical blast prepared?
A drilling machine bores a probe hole 10.6 metres in depth. Drilling to this depth lets the blast crew know about the existence of any historic underground workings. The hole is backfilled to 5.7 metres from the top (called the collar). A detonator and primer are lowered into the hole. A carefully measured amount of explosive is then pumped into the hole. The waterproof explosive emulsion used at Martha Mine looks like a thick grey slurry. Between .3kg and .5kg of explosive emulsion is used for every cubic metre of rock.
Next a polypropylene bag is carefully pushed down the hole to within two metres of the collar. The bag sits above the explosive and creates an air gap, which the gasses will expand into when the explosive is detonated.
The remainder of the hole is filled with stemming material (crushed aggregate from a local quarry). A wire from each detonator is connected to a computer controlled firing box. Each hole is scanned into the computer using a bar code reader similar to those used in shops. The blast manager allocates a blast sequence and pattern.
The speed of the sequence and the pattern are modified to suit each blast. These variations are made after checking the data from the previous blast. At Martha Mine the blast holes are short and each hole receives a low charge rate.
Short holes and a low charge rate mean that lots of holes must be drilled to effectively blast the rock, and sequencing is vital. Between three and seven hundred holes may be fired at any one time.
Five drilling machines at work on the Southern Stability Cutback in May of this year. The pattern of drill holes can be clearly seen in the top centre of this aerial photo.