The fall occurred at the New Zealand mine near the bottom of the new 108m shaft, which was successfully raise-bored last month.
No-one was hurt in the incident.
At the time a contractor had been working from the surface end of the shaft to bolt and mesh the walls heading towards the coal seam. The top 66 metres of the shaft had been successfully supported and stabilised.
The rock fall has blocked ventilation to the mine, delaying mining operations underground until ventilation is re-established.
“Whilst very frustrating and unexpected by all parties including the contractor and its technical experts, the problem will be remedied and all steps are being taken to do so in the quickest time possible,” Pike River chief Executive Gordon Ward said.
“The area identified as possibly problematic before raise boring was the top 35 metres where we had injected cement and this area has stood up well.”
Pike River is currently investigating alternative methods to fix the problem and reinstate ventilation.
The mine has already protected the top 66m of the shaft by flying concrete in by helicopter and piping it down the shaft to fill a cavity at the top of the rock fall.
It is expected that the bottom part of the shaft will be remedied by raising an angled drive from the tunnel using the “Alimak” method to intersect the main shaft at about 40 metres above its base.
Sub-contractor to McConnell Dowell, AVKO Mining, is being mobilised from Australia to undertake the driveage.
In the meantime, there is only enough ventilation for limited pit bottom development operations, although alternatives for getting more air into the mine are being investigated.
Pike River was trading unchanged today at 73 cents.