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Bolstering
bolting
for Barrick
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| Vital
operation: In the fissured and fractured rock, mesh and bolts are
placed simultaneously. |
Faced
with the difficult task of mining in a complex network of tectonically
disturbed faults, efficient and cost-effective rock reinforcement is of
major importance for Barrick Gold's Goldstrike property in Nevada.
Located north of the small town of Carlin, the mine produced more than
2.26 million ounces of gold in 2001 with a 24/7 working week.
There are two major underground operations in the area - the Meikle and
Rodeo deposits - and out of a total workforce of 1,700 approximately 200
people are involved underground.
The development of first Meikle and then Rodeo has required an increasing
amount of ground support due to the difficult geology in the area and
this, coupled with the need to contain costs, has led Barrick to continue
refining its rock reinforcement practices.
Network
of faults
The compact Meikle orebody is mainly comprised of high-grade ore surrounded
by competent limestone, in addition to fissured and fractured rock.
The Rodeo deposit, which extends about 800 ft vertically across an area
of approximately 2,500 x 1,000 ft, is expected to mine 600,000 t in 2002.
This will contribute 250,000 ounces to a total output from the company's
underground division of about 700,000 ounces of gold using longhole stoping
(slot and slash) with backfilling as the preferred mining method.
Scott Herr, Superintendent Underground Operations, explains that the predominantly
competent rock at Meikle allows for most of the development to be supported
using friction bolts, with resin grouted rebars used in some permanent
excavations.
Tectonic events have created random joints, further reducing stability.
Excavations must therefore be supported immediately after blasting. The
ground conditions also mean that stopes are relatively small - 4,000 to
6,000 tonnes.
Initially, Barrick started bolting using pusher leg rock drills and both
resin grouted rebars and friction bolts. But although the resin would
initially seem to have anchored the bolt, the ground around the resin
would later crumble and the bolts would fall out.
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"The
Swellex bolt creates better anchorage." |
| Denis
Thibodeau, Senior Rock Mechanics Engineer at the Rodeo site.
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"Furthermore,"
explains Denis Thibodeau, Senior Rock Mechanics Engineer at the site,
"rebar bolt installation is a very slow, labour-intensive process
in these conditions because it's difficult to inject the right amount
of resin. The friction bolts may have been easier to install, but they
did not achieve satisfactory long-term anchorage."
In conjunction with rock mechanics studies, Barrick then decided to
try another type of bolt in the worst conditions at Rodeo - Swellex
and Super Swellex from Atlas Copco.
Excellent
results
Rock reinforcement design principles showed that it is possible to use
8ft-long Swellex bolts to support the back of a 20ft wide drift. These
bolts will have a maximum transfered load capacity of around six tonnes,
given a useful embedment length of 1.3 ft. The wedge apex will sometimes
be higher than 8 ft, requiring 12 ft Swellex bolts to be used instead.
"All in all, the extremely poor quality of the rock mass imposes
very tight limits on panel sizes making thorough support essential,"
continues Mr Thibodeau.
"Pull testing of the Swellex bolts gave excellent results. The
Swellex design achieves more confinement in the soft, friable rock because
of the water injection, creating more grip in the rock and better anchorage
than friction bolts."
However, there were still problems with the ground between the bolts,
so a shotcrete layer was placed before bolting and meshing. Scott Herr
explains: "It was the widespread and rapid variation in ground
strength in the mine that convinced us to standardize on Swellex. The
bolts hold in all conditions encountered and they substantially reduce
the risk of roof failure."
The width is the main factor affecting bolt density, while the type
of Swellex used - non-coated or coated - usually depends on whether
the drift will be mined through or not. Occasionally, 16 ft Super Swellex
may be used.
In 15 x 15 ft headings, the bolts are typically placed in rings on 5
ft centres, with the rings on a 5 ft spacing. Placing mesh and Swellex
bolts at the same time, the operator starts close to the floor, inserting
6 ft friction bolts in the rib, then 8 ft standard Swellex bolts across
the roof.
In 25 ft wide headings, a staggered pattern is created by placing alternating
rings of five and four Swellex bolts on 5 ft centres. The patterns are
still fairly constant, except where the ground is especially weak.
Comfortable
from Day One
"You could say we started with an overkill support scenario and
are now refining the approach," says Kevin Melong, Mine General
Foreman at Rodeo. "From the miners' point of view, Swellex is the
bolt that made us feel comfortable from day one."
The bolter operators and stoping supervisors are allowed to use their
own judgement in deciding bolt type, size and precise pattern. While
refining its rock reinforcement regime, the rate of bolt use has been
reduced by about 30%.
Says Robbie Traynor, bolting rig operator at Rodeo: "Typically,
we can install up to 110 8 ft Swellex bolts per shift. You can always
rely on them once they are inflated, and they are way faster than the
others."
Co-operative
effort
In addition to Swellex bolts, Goldstrike also relies on Secoroc drill
steel and bits for its underground operations and continuous technical
service support.
Atlas Copco began a "cost-per-foot-drilled" contract with
Barrick Goldstrike for the Secoroc products in October 2000. "We
provide Secoroc supplies to both of the mines seven days a week,"
explains Clay Gremel, Secoroc Area Manager. "It has become a very
efficient and effective co-operative effort. While we are refilling
inventory at the mine, all the used steel and bits are picked up and
taken to our service shop in Elko for reconditioning and then returned
to the mine."
The Atlas Copco technical service representative at the site, Troy Thiel,
functions as the Master Driller, trouble-shooting not only the Atlas
Copco Secoroc and Swellex applications, but the drilling function as
a whole.
Barrick is working with Atlas Copco to try out 10 ft Connectable Swellex
bolts as a potential replacement for 20-25 ft cable bolts and is also
testing the new PVC coated Swellex.
Click
here to see overhead
and cross section views of the mines
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Click
here to see overhead and cross section views of the mines
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Goldstrike
has a guaranteed cost per foot contract with Atlas Copco Secoroc,
which provides the Meikle and Rodeo mines with a full line-up
of hammers, adapters, rods and bits. These are:
DTH drilling: Secoroc COP 34 hammers and 95 mm SpeedBits.
Drifting: R38 shankadapters, Drifter rods (Lengths 10’,
12’, 14’) R38 x Hex32 x R32, 48mm button bits, 3” Dome bits (reaming).
Bolting: R32 shankadapters, Drifter rods (Lengths 10’,
8’6”) R32 x Hex25 x R25, 38mm button bits.
Cable Bolting: R32 shankadapters, R32 Speedrods, R25 Extension
rods, 48 mm and 51 mm button bits.
Bench Drilling: T45 shankadapters, T45 Speedrods, 76 mm
Retrac button bits, T45 Secoroc Backhammer, 5” Dome bits (reaming).
Jackleg Drilling: Tapered rods 11 degree (Lengths 2’6”,
4’6”, 8’6”, 10’6”), 35 mm and 38 mm tapered cross bits.
More
details at:
www.
rockreinforcement.
com
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