Cradle to grave
Drilling an African record
Drilling company Torque Africa recently drilled a
hole of 1 148m deep – a new African record.
Using diamond-impregnated bits
and a standard rotary percussion
rig, drilling company Torque Africa
Exploration recently drilled a hole
in the Springs area of Johannesburg,
South Africa, of more than 1 148m
deep. This is an African record and
one of the deepest rotary percussion
holes drilled worldwide. What makes
the feat more impressive is that it
was drilled in mixed and largely
unchartered geologies. According
to Nardus Bezuidenhout, director
at Torque Africa, more holes will
be drilled in future. “The company
is part-way through its contract to
bore several deep-level holes into
disused mine stopes for the treatment
of acid water mine drainage,” says
Bezuidenhout. These range from a few
hundred metres below the surface to
over 1 200m for the deepest hole.
According to Bezuidenhout, Torque
Africa took on the job in the face of
other experts who said the risks of
failure were too high. “We were never
in doubt, though, and in conjunction
with Pieter Coetzee and his team
from Sandvik Mining and Rock
Technology’s rock tools division, we
[36] MINING MIRROR JUNE 2018
planned the technical details of the
operation.
Sandvik’s product manager – DTH
Bits, Matthew Shoffner, also worked
with the local team of experts to
find the right technical solutions.
“Considering the unchartered nature
of the geology of the area, we chose to
use standard bits and then switched to
diamond-impregnated bits as soon as
formations became too hard or abrasive
for them,” says Shoffner.
“This is exactly what we
encountered after the initial depth
of about 300m of dolomite and shale
and from there on it was mostly hard
quartzite with some khaki shale,
black slate shale, mixed dolomite and
quartzite, diabase and more quartzite.
This made for some tough drilling, but
was made considerably easier with the
use of the diamond-impregnated bits,”
says Bezuidenhout.
Bezuidenhout adds that despite
the higher cost of diamond-
impregnated bits, the durability
was extended beyond 378m per bit.
This far outweighed the standard bit
alternative, which would have been
70m and required constant pullouts.
Bit changes can be complicated
and time-consuming as pull back
of the rods would have taken up to
24 hours to retrieve the 6m lengths
to replace the tooling. A shanked
bit at these depths would be even
more catastrophic and could require
drilling of a new hole if the bit could
not be retrieved.
“We had to deal with dolomite
wetlands, dykes, voids, and water ingress
at all levels, which made it tough. We were
also required to drill the hole telescopically
from its original diameter of 558.9mm at
the top through various layers, including
a 310m-deep grouted section, casing,
and eventual intersection of quartzite just
beyond 300m, at which point the drill
diameter was down to a 172mm bit.
“Thereafter we continued through
various phases until prematurely
intersecting the stope at 1 148m
instead of the predicted depth
of 1 200m. At this point, the bit
size was 124mm and the hole
ready to be used by our clients
to receive pumped products that
are a by-product of the treatment
of millions of litres of acid mine
drainage,” Bezuidenhout explains.