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