Announcing the Formation of

Itasca Consulting Group, Inc. (ICG) and

Hydrologic Consultants Inc. of Colorado (HCI) are

pleased to announce the consolidation of their com-

panies through the formation of a

parent company, HCItasca Inc.

The merger took affect on

October 1, 1999.

The combined companies

will continue to provide top-

quality consulting services and

software products related to the

reaction of the geotechnical and

hydrological environments to

the actions of man. The

emphasis will be on solving real

problems and offering practical

tools and solutions to clients in

the mining, civil, defense, waste

isolation and petroleum engineer-

ing industries.

Itasca’s record of expertise in

rock mechanics, mining engi-

neering and numerical modeling

is matched by HCI’s skills in

ground-water and surface-water

hydrology. By combining these

strengths, this merger promises to

provide customers with a truly

integrated, geotechnical consult-

ing service. More often than not, solutions to

geotechnical problems are dependent on both the

geomechanical and hydrological aspects of the site.

Now, through the merger of HCI and ICG, clients

can come to one source and be assured of obtaining

all the appropriate expertise for their projects.

HCI is currently very active in

Chile, Indonesia, South Africa, and the

western U.S. In addition to its office in

Minneapolis, Itasca maintains offices in

Sweden, France, Germany, Spain, South

Africa, Canada and Chile. Lee Atkinson,

President of HCI, notes that “The

opportunity to better serve our cus-

tomers by conducting projects from

and locating hydrologists in some of

the existing Itasca offices, especially

Chile and South Africa, is a big attrac-

tion to us.” Itasca, likewise, plans to

take advantage of the HCI office to

facilitate access to mining clients in the

western U.S. by locating mining and

geotechnical engineers in the

Lakewood, Colorado office.

The consolidation of HCI and Itasca

also should provide expanded career

opportunities for current employees and

help in attracting additional well-

qualified professionals.

We hope that you, our customers,

will feel that this joining of two well-

regarded consulting operations will pro-

vide added value to you. We are enthusiastic about

the future of our company and look forward to

working with you.

I

HC TASCA

HYDROLOGIC

CONSULTANTS, INC.

ITASCA

John J. Markham, Chief Executive Officer, HCItasca Inc.

The Source

’s consulting newsletter

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HC TASCA

Volume 5, Number 1

May 2000

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HC TASCA

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HC TASCA

Hydrologic Consultants, Inc. of Colorado

(HCI), the newest addition to the HCItasca compa-

nies, is currently conducting a multi-faceted

hydrologic and hydrogeochemical investigation at

P.T. Freeport Indonesia’s vast surface and under-

ground mining operations in Irian Jaya.

Significant aspects of the investigation have

included:

• designing and implementing a dewatering

system for the Grasberg pit,

• controlling inflows to and draining

drawpoints in the underground IOZ block

cave mine,

• developing dewatering and water quality

management plans for the DOZ and Kucing

Liar underground mines,

• defining the local water budget with respect

to increasing the water supply for mill expan-

sion and evaluating the potential effects of

drought caused by El Niño events, and

• understanding the role of the alpine karst

system on the storage and movement of

ground water, and predicting the effects of

mining on the quantity and quality of water

resources.

Specific tasks have included designing and con-

ducting underground airlift pumping tests using

multi-level piezometers, supervision of shut-in

pressure tests at multiple underground levels, design

of a unique passive drainwell system, use of water

chemistry to define water sources, and design and

supervision of installation of a network of flumes to

accurately and automatically measure large under-

Ground-Water and Water Chemistry Investigation at

the Grasberg Mine in Indonesia

David Bird, Senior Project Geochemist, Hydrologic Consultants, Inc.

The Source, Volume 5, Number 1

2

continued on page 4

Conceptual geochemical

model of source inflows

to Grasberg system

RAINFALL RAINFALL

WANAGON STOCKPILECARSTENSZ STOCKPILE

GRASBERG PIT

PRE-MINING TOPOGRAPHY

LIMESTONE LIMESTONE

AMOLE PORTAL

2500 PORTAL

GRASBERG INTRUSIVE COMPLEX

2828 EXTRACTION LEVEL

Grasberg pit

May 2000

3

Using The Particle Flow Code ( ) to Assess

Stability of Undercut Backfill

PFC2D

Matthew Pierce,

David Potyondy,

Mining and Rock Mechanics, Itasca Consulting Group, Inc.

, Itasca Consulting Group, Inc.Structural and Fracture Mechanics

PFC2D recently been used to simulate min-

ing of sill pillars under weakly cemented rockfill.

The modeling approach is more appealing

than the continuum modeling approach because it

allows one to explicitly model the physical separa-

tion and caving of fill that occurs when it fails dur-

ing undercutting.

The first step in the modeling process involved

calibrating the material to reproduce repre-

sentative laboratory-scale properties of cemented

backfill such as elastic modulus, unconfined com-

pressive strength, and dilation. Following this, a

field-scale model of the backfilled stope was used to

model the stope-filling process and simulate under-

cutting of the backfill. The stope-filling process was

modeled explicitly in in order to take account

of the effects of stope geometry (length, width and

dip), filling method (continuous pour or lifts), and

fill height and fill properties (density, cohesion and

friction angle) on the initial stress distribution in the

fill. It was shown that the synthetic material could

account for the effects of stress arching in the

rockfill and reproduce reasonable pre-mining stress

distributions in the rockfill.

Once the pre-mining state was established in

the field-scale model, undercutting of the rockfill

was simulated. Sections of the lower wall represent-

ing the sill pillar were deleted in sequence from the

hangingwall to the footwall to simulate a transverse

sill pillar mining sequence. After each section was

deleted, the model was run until displacement of

the rockfill reached a near-zero value. The next

section was then deleted, and the process continued

until continuous caving of the rockfill occurred.

When exposed from below, the condition of the

model rockfill could be described by one of three

states: (some bond failure but the fill mass

remains stable); (more widespread breakage

has

PFC

PFC

PFC

2D

2D

2D

Stable

Caved

of bonds; sections of the rockfill detach from the

base of the stope and displace downward; caving

occurs to some height then stabilizes); and

(continuous caving of the rockfill occurs).

The figure below shows the change from stable

to caved to failed predicted by the model as

the exposed span was increased. The nature and

mechanism of the failure is similar to what has been

observed in situ.

The model was used to investigate several fac-

tors influencing the stability of undercut rockfill

including degree of cementation, stope width and

undercut dimension. The results of several simula-

Failed

PFC2D

tions were combined with the results of simulated

core-tests, in which the unconfined compressive

strengths of the as-built synthetic backfill were

obtained, to generate a series of stability charts.

These charts illustrate the relations between stope

width, exposure span and unconfined compressive

strength of the rockfill. When combined with esti-

mates of fill strength from characterization of

the rockfill, the model-derived design charts may be

used to obtain estimates of the stability of the fill at

various stages of sill pillar mining.

in situ

From left to right: Stable, Caved, and Failed model rockfill

The Source, Volume 5, Number 1

ground flows in the underground workings. A

major ongoing task has been development of a fully

three-dimensional, finite element, regional ground-

water flow model, with two “window” models to

evaluate near-mine conditions in more detail, using

HCI’s code MINEDW.

To fulfill requirements of the 1996 AMDAL

(the Indonesian equivalent of an Environmental

Impact Statement), HCI was part of two major

environmental investigations. A comprehensive

hydrogeochemical investigation of the region sur-

rounding the Grasberg mine was conducted to pre-

dict the water quality during mining and after mine

closure. Additionally, HCI conducted an investiga-

tion to evaluate the amount of infiltration through

waste rock piles. HCI conducted field infiltration

tests utilizing state-of-the-art equipment. Based on

the results of these field tests and mapping of waste

rock types and degree of compaction, the amount of

infiltration was calculated. The conceptual

hydrologic model of the waste rock piles was then

used to evaluate different options for water quality

control.

(published with permission of P.T. Freeport Indonesia)

Ground-water flow model grid in MINEDW

SourceThe

is published twice annually for

friends and clients of HCItasca.

For past issues, visit the

HCItasca web site:

www.hcitasca.com/pubs.html

ADDRESS

708 South Third Street, Suite 310

Minneapolis, MN 55415

PHONE

(612) 371-4711

FAX

(612) 371-4717

EMAIL

icg@itascacg.com

hcico@hcico.com

WEB

http://www.hcitasca.com

Model 1 Detail

Large-ScaleRegional Model

Model 2 Detail

Ground-Water and Water Chemistry Investigation at

the Grasberg Mine in Indonesiacontinued from page 2

4

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HC TASCA