MINE EXCURSION
Perfect mining conditions
As our vehicle descends the decline one cannot but notice the
perfect ground conditions. According to Walter Shamu, chief
operating officer at Orion, and a qualified rock mechanic, the old
timers didn’t use a lot of ground support. “The gneiss rock in the
hanging wall and footwall is extremely competent at 300MPA,
and even the ore measures at about 200MPA. Nonetheless,
despite these above-average mining conditions, we will still
respect the integrity of the mine and will use a patterned
support system and paste-filling as we develop the ore body
underlying the existing infrastructure further,” says Shamu.
He adds that there are more than 37km of primary access
development in place and even more lateral ore development.
All the underground infrastructure is perfectly intact, and this is
Orion’s great advantage – they are developing a new footprint
underneath an old one, and more than 70% of the work has
already been done.
When Orion broke through the concrete caps plugging the
shafts and removed backfill material placed to seal the network
of underground tunnels thirty years ago, they found a gem.
Despite fears that the mine had caved in on itself, pre-empted
by the appearance of several sinkholes on the surface, all the
infrastructure was still neatly in place, albeit flooded to within
300m of the surface. “The fact that the underground network
of tunnels and steelwork in the vertical shaft were still in
such good condition, will save Orion more than R5-billion of
development costs in the long run. To sink a similar vertical shaft
today will cost at least R1.2-million per metre of development,
in addition to the time wasted and the technical and safety risks
involved,” says Shamu.
project underway when African Mining visited Prieska, was
to do a detailed inspection of the steel used in the original
construction of the shaft. A critical part of the mine re-entry will
be dewatering the historic workings, which have rising water
levels, and have risen about 40m in three years. During our visit,
the water level was 300m below surface, which means almost
700m of the shaft was now submerged.
Shaft remains intact
Normally this would be of major concern for a project of this
nature, but there are a number of reasons why Prieska is unique:
the water is PH neutral and not acidic – which would have
weakened the steel structure and damaged the concrete walls;
"The vertical shaft is an
absolute key part of the
Prieska project.
The vertical shaft is an absolute key part of the Prieska project,
and it is critical that the integrity of the headgear and shaft
is maintained and that it can be used again. Once up and
running, the vertical shaft will be used to transport men and
material as well as for ventilation. Therefore, the most important
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African Mining April 2020
11