Grasberg block cave - InfoMine - Mining Intelligence and Technology
Transcript of Grasberg block cave - InfoMine - Mining Intelligence and Technology
When the Grasberg open pit is depleted,
the Grasberg Block Cave (GBC) and
East Ertsberg Skarn System (EESS)
mines will become the primary ore sources. Last
month’s article on Big Gossan explained how the
Common Infrastructure Project’s (CIP) AB Tunnels
will support the new underground mines providing
reliable gravity drainage, material handling,
ventilation, manpower and material access,
emergency egress, services line and maintenance
access. Currently, Redpath is developing the Lower
GBC mine terminal and DMLZ mine access spur.
The GBC mine terminal is a multi-heading
installation, providing a broad range of services to
mine development, construction and operation
groups. It lies at the underground end of AB
Tunnels’ GBC spur, some 6,000 m from its Ridge
Camp portal. This is the terminus for AB Tunnel rail
operations and the bottom station for the GBC
service shaft. Functions accommodated by the
terminal include:
■ Personnel handling and queuing for on/off
loading the man-cage at shift-change
■ Marshalling of material flat cars on the AB line
and shaft dollies in the shaft station
■ Handling loaded and empty material transport
containers and ISO-tainers, using the terminal’s
travelling bridge cranes
■ Intake ventilation, routed from AB Tunnels
through the terminal.
Redpath continues to work on the Grasberg
Terminal and had excavated more than 4,400 ‘D’
equivalent m to December 2009. ‘D’ is the
standard drift size that Freeport Indonesia uses to
calculate equivalent metres for budgets, planning
and actual accomplished – 4.4 m (wide) x 4.0 m
(high).
Secondary access to the GBC mine workings will
be provided by the inter-level ramp. This ramp is in
progress and will extend from the northwest end
of the GBC terminal at the 2,535 m elevation, up
to the Extraction Level near the shaft station at the
2,830 m elevation. From there, a short connection
extends the ramp access up to the Undercut Level
and hoist room. The inter-level ramp is also the
primary access to the crusher stations for
development, operations and maintenance. From
January to December 2009, Redpath excavated
more than 1,500 m ‘D’ equivalent of inter-level
ramp access.
The GBC area will be supplied with air via
primary intake and exhaust drifts, collectively called
the Grasberg Ventilation Declines (GVDs). The goals
of these are to provide:
■ Sufficient ventilation for the GBC initial mine
development
■ An alternate access for GBC development
activities, notably during the nine- to 12-month
period where the GBC Spur will be blocked off
for rail installation
■ An alternate emergency egress from the AB
Tunnels
■ Knowledge of ground conditions for the benefit
of conveyor drifts to be developed later
■ Incorporation of the ventilation drifts into the
long term Ventilation Drift layout proposed.
This development is crucial for initial and long-
term operation of the GBC mine, which will be
ventilated with airflow of 1,700 m3/s via four intake
and four exhaust GVDs. These declines will come
to a portal close to the existing Amole drifts, each
with a length of 2,800 m and dimensions of 6.8 m
wide x 9.0 m high. The mine uses a negative
pressure system, pulling air through rather than
forcing it in from the intakes. Each main exhaust
fan motor is rated 2,200 kW. The GBC ventilation
system is both extensive and complex. It must be
implemented and expanded as mine development
proceeds so as to provide adequate mine ventilation
at all stages of mine development and production.
Of the eight ventilation drifts, GVD#1 and
GVD#2 will form the two most westerly drifts of
the ten drift arrangement. So far, GVD#2 has been
used as an air exhaust way, and GVD#1 as an
intake airway. After additional drifts are excavated
and permanent exhaust becomes operational,
GVD#2 will take on an intake role.
Redpath started this project on January 27, 2007
with the development of the first two – GVD#1
and GVD#2. The declines provide access for
development of the remainder of the ventilation
and conveyor declines required for the GBC mine.
Total development expected for the ventilation and
conveyor declines is about 30,000 m.
Project challenges:
■ Development rates are high
■ Development is passing through areas of known
difficult geotechnical ground conditions
10 International Mining MARCH 2010
OPERATION FOCUS – Indonesia
John Chadwick concludes the mining side of the Freeport Indonesia story. Access to the wealth of proven undeveloped underground deposits is
the key driving force to future production
Grasberg block cave
A rig move for PT Pontil (acquired by Major Drilling in
1997), which has a long-term contract to supply drilling
services to the mine and the exploration projects
■ Intercepting areas with high pressure and high
volume water inflows
■ Numerous geological contacts between
limestone, sandstone and diorite
■ Decline development, increasing complications
from any water flows encountered
■ Breakthrough for ventilation at the highly
congested Amole portals area.
GBC – the stalwartThe GBC mine will be the main source of mill feed
after the open pit becomes exhausted. At that
time, the district-wide underground ore reserves
will be on the order of 1,700 Mt, of which the
GBC mine comprises more than half – 1,000 Mt at
a grade of 1.0% Cu and 0.8 g/t Au. As we have
already seen, development of the access adits was
initiated in 2004 in order to access and develop this
mine in time for the open pit completion.
Block cave mining was chosen as the most
applicable method for mining the deposit, with
production rates of up to 160,000 t/d being
considered. It is a large footprint (1 km by 1 km) to
develop. The ore’s average uniaxial compressive
strength is about 110 MPa, with some variation
depending on mineralogy and alteration between
70 and 140 MPa. The principal in-situ stress is in
the northeast direction and corresponds to the
regional structural geology (about 40 MPa). The
principal horizontal stress is approximately equal to
the overburden stress.
The rock mass rating (RMR), used to establish
mine design parameters (hydraulic radius and
ground support), is ‘good’ for the development
levels, the lower 50 m of the mineable zone, and
for the central Kali intrusive. In general, primary
fragmentation in the GBC is expected to be coarse,
with more than 76% of the tonnage being in
blocks greater than 2 m3. The secondary
fragmentation curves indicate that at ore column
heights greater than 250 m, 23% of the rock will
have a volume of more than 2 m3. The estimated
hydraulic radius required for sustained caving is 33
m. Rock burst potential does exist and has to be
evaluated in more detail. The assumed draw rates
range from 0.13 m/d during the initial 100 m of
column height, up to 0.30 m/d at a column height
greater than 200 m.
Wet muck has been a problem in the DOZ mine
(January article) but at GBC, the risk from wet
muck is thought to be relatively low, particularly for
the early years of mining. The predicted coarse
in-situ grain size should inhibit significant wet
muck, despite the relatively high volumes of
predicted passive groundwater inflow. However,
comminution of the High Sulphide Zone (HSZ – see
February article), fines from adjacent open pit
overburden stockpiles reporting to the cave, or
secondary fragmentation of limestone derived from
pit walls may all eventually report to the drawpoints
and present a wet muck problem.
GBC’s Extraction Level is to be at the 2,815 m
elevation. The primary access to the mine will be
via the AB adits, 6 km in length, that have been
developed from surface at the 2,500 m level by
Redpath, as noted earlier. Undercutting is to be
initiated in 2015, peak production is forecast by
2021, and closure estimated in about 2041.
The Extraction Level will use an ‘El Teniente’ style
layout, used previously in the Ertsberg District at the
now-exhausted IOZ mine. Spacing of the panels is
30 m with drawpoint spacing of 20 m. The current
mine layout is a very large footprint with a
diameter of about 1 km (700,000 m2) and contains
more than 2,500 drawpoints.
In order to develop and mine this size of
footprint, the panels and drawpoints are to be
broken down into five to six manageable
production blocks, each containing between 250
and 550 drawpoints. Each panel section within
those blocks contains an orepass and a vent raise,
located in adjacent pillars. Extending the drawpoint
through the drawbell to the next panel allows the
creation of an open access and can be used as a
short-term or pseudo fringe drift.
The sections allow for an LHD to access both
orepasses located north and south of the section.
This improves tramming efficiencies, with the
longest tram being about 150 m. It also gives the
flexibility of a back-up orepass in case one is down
for repair or maintenance.
PTFI selected an advance undercut approach,
with the leading edge of the undercut a minimum
of 20 m ahead of drawbelling on the Extraction
Level. A ‘crinkle cut’ undercut system is planned as
is being used at the currently operating DOZ block
cave mine. Undercut headings will be directly
above and parallel to the extraction level panels.
The draw column heights are expected to average
460 m, and great care will be taken to protect the
integrity of the extraction level drawpoints.
Each drawbell will be mined out once the
undercut has passed by a minimum of a single
drawbell ahead and the bell is clear of abutment
12 International Mining MARCH 2010
OPERATION FOCUS – Indonesia
GBC ramp access layout
stress. This will be achieved with a portable Alimak
nest and climber used to drive a 2 x 2 m vertical
raise, 9.5 m long. Once the raise is in place and
secure the drawbell will be drilled and a mobile
emulsion charging unit will be employed to load
the blastholes. Variable density emulsion and
electronic detonators will be used to fire the bell in
a single shot.
Maximum drawbell opening rates are expected
to be eight per month and maximum draw rates
0.30 m/d. The plan assumes that the cave will be
developed initially in two separate mining blocks
simultaneously. The sequence has been designed to
maximise grade and minimise dilution effects from
the toppling of the pit material.
Simulations were performed using Arena
software to confirm production and secondary
breakage equipment requirements, ore haulage
and requirements for the AB adits to handle all
development activities. The results to achieve
output of 160,000 t/d produced the following:
■ 45 operating LHDs
■ >1,000 active drawpoints
■ Six medium hang-up rigs
■ 36 secondary blasting rigs
■ Six haulage trains consisting of 24 x 30-t ore cars
■ Three 60" x 89" crushers.
The service level (2,785 m) provides intake and
exhaust to the undercut and extraction levels and is
the primary ore handling level. The rail haulage
system has four lines that run under the orebody in
a generally northwest orientation that match the
geometry of the production blocks and mining
sequence. The lines gather into a northeast-
trending fringe drift that runs over the coarse ore
bin dumps. There are two intake and two exhaust
drifts parallel to each of the lines.
Grade controlSampling underground has been very important
and will be so in the new mines. Last year, proper
sampling had resulted in a decrease of the grade
variance between the Underground report and the
Mill report from 12% in 2008 to 4% in 2009.
Sampling results are also used to calculate the
Underground grade actual reconciliation with the
Mill.
Sampling in the underground mines is done
twice a day, involving six Underground crew
members. It is undertaken to control and optimise
the economic value of the mineral reserve during
production planning by providing the actual grade
value, which provides clear direction when drafting
the production plan.
Average samples collected amount to 100 bags
per day; they are then taken to the Mill 74
laboratory. The results will help provide the value of
actual grade, assist in drafting the production plan,
and aid in determining the actual economic value
of the mined ore.
OPERATION FOCUS – Indonesia
MARCH 2010 International Mining 13
Grasberg block cave layout
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Searching for moreThese underground mines should have attractive
operating profiles when compared with other
mines around the world. Their development will
provide the opportunity for Freeport Indonesia to
realise significant value beyond the life of the open
pit, allowing the operation to leverage synergies of
existing mill facilities and infrastructure and
providing long-term incremental cash flows.
As the mine plan is driven by exploration,
exploration activities could add near-term
production and/or extend mine life even further.
Boart Longyear has been involved with
exploration at Ertsberg and Grasberg for many
years. Between 1974 and 1978, Longyear was
drilling on the original Ertsberg project, using three
drills rigs (one LY44 and two LY38s) the project was
managed by Longyear Australia.
Between 1979 and 2003, PT Freeport Diamond
Drilling Division operated with Boart Longyear as its
‘preferred supplier’ for tooling and for underground
equipment – four LM37s, seven LM55s and seven
LM75s. Boart Longyear was also the preferred
supplier for surface exploration equipment, with
five LF70 drilling rigs in operation.
From 1997 to 2003 PT Pontil (acquired by Major
Drilling in 1997) was contracted to supply both
underground and specialised surface drills to
support Freeport Indonesia’s own Diamond Drilling
Division, PT Pontil supplied both LM underground
drills and Universal drill rigs for surface operations.
During this period PT Pontil successfully completed
a number of 700 m Surface to Underground
directionally drilled holes that were cased with 203-
mm stainless steel casing on completion.
In 2003 PT Pontil acquired the assets of
Freeport’s Diamond Drilling Division and signed a
long term contract to supply drilling services to the
mine and the exploration projects. This contract has
been recently extended. PT Pontil has an office
situated in Jakarta and is fully established in
Indonesia to support its operations there.
Boart Longyear still provides the majority of
equipment and consumables on site as the
“product of choice” from the current contractor.
Major Drilling is a Global Account of Boart
Longyear and receives 100% support worldwide.
Major Drilling has committed to use modern and
technically sound equipment in this remote
environment. PT Pontil uses a variety of drills to
support its Freeport operations, including Sandvik,
Boart Longyear and Duralite drills.
Freeport Indonesia’s mineral exploration in Papua
poses unique challenges due to the limited geological
data, severe terrain, lack of road access, and the
wet and rapidly changing weather conditions. To
effectively explore large areas Freeport pioneered
14 International Mining MARCH 2010
OPERATION FOCUS – Indonesia
Boart Longyear LF70 exploration drill at surface
Sampling underground has been very important and
will be so in the new mines. Last year, proper
sampling had resulted in a decrease of the grade
variance between the Underground report and the
Mill report from 12% in 2008 to 4% in 2009
Freeport Indonesia provides direct economic
benefits for the central, provincial, and local
governments of Indonesia, as well as the local
economy of Papua and the nation as a whole.
PTFI is the largest private employer in Papua
and one of the largest taxpayers in Indonesia.
From January to December 2009, PTFI
paid its financial obligations to the
Government of Indonesia in the sum of $1.4
billion. Under its financial obligations as set
out in the 1991 Contract of Work, from
1992 to 2009, PTFI has paid a total of $9.5
billion to the Government of Indonesia.
PTFI also has a strong commitment to the
local community and a portion of revenues
goes to benefit the local community through
the Freeport Partnership Fund for
Community Development. In 2009,
contributions to the Partnership Fund
totalled nearly $69 million, with total
contributions to the fund reaching nearly
$398 million since its inception in 1996.
Financial benefits to Indonesia
the use of helicopter hoisting to allow geologists to access and sample remote
areas quickly and safely. To date, Freeport affiliates have invested over $300
million on exploration in Papua and made several discoveries that are being
evaluated.
Continuous pilot core drilling is currently being used in the high speed drill
and blast excavation of twin AB access tunnels for the CIP.
These deep access tunnels will have a total planned length of 18.6 km. An
innovative technique allows them to safely probe ahead of the face to warn of
bad ground and high pressure water compartments. They are using 650 m long
HQ core borings drilled from remucking alcoves along the tunnels. They are able
to achieve 30 m/d of core drilling using Boart Longyear LM75 hydraulic drills. IM
Thanks are due to Redpath for a great deal of the input to this article.
References1.Freeport McMoRan Copper and Gold Inc., Common Infrastructure – Phase 3 Grasberg
Block-Cave Ventilation and Access Decline Feasibility Report, Unpublished report for PT
Freeport Indonesia, October 2005.
2.Freeport McMoRan Copper and Gold Inc., Common Infrastructure Study, Unpublished
report for PT Freeport Indonesia, August 2003.
3.Freeport McMoRan Copper and Gold Inc., Feasibility Study for the Grasberg Block Cave
Mine - Volume I Executive Summary and Report, Unpublished report for PT Freeport
Indonesia, May 2005.
4.Freeport McMoRan Copper and Gold Inc., Grasberg Block Cave Feasibility Update
Presentation, March 2007
5.Brannon, C., Casten, T. and Johnson M. Design of the Grasberg block cave mine, Massin
2004, Santiago, Chile, August 2004.
For more on the great achievements of Freeport Indonesia, this series of
articles started in August 2009 and there has been an article on aspects
of this operation published every month since.
MARCH 2010 International Mining 15
OPERATION FOCUS – Indonesia
Continuous pilot core drilling is currently being used in the high speed drill and
blast excavation of twin AB access tunnels for the CIP