This lecture provides an overview of the issues influencing dilution in an underground production environment. The lecture reviews the dilution problem throughout the entire mining process, and provides a rational approach to underground mine design in order to minimize dilution. The stages contributing to dilution include orebody delineation, design and sequencing, stope development, drilling and blasting, production and mine management issues. This lecture provides an overview of the issues influencing dilution in an underground production environment. The lecture reviews the dilution problem throughout the entire mining process, and provides a rational approach to underground mine design in order to minimize dilution. The stages contributing to dilution include orebody delineation, design and sequencing, stope development, drilling and blasting, production and mine management issues. Nature of Mineralization Geological dilution Mining Methods and Dilution Underground Mine Design: Basic Input Global (Block) Design Issues Detailed design issues Geotechnical Monitoring Parameters Influencing Dilution: Orebody delineation Design and sequencing Stope development Drilling and blasting Production stages Issues for mine management

1. @Hassan Harraz 2019
Geotechnical Design for Dilution Control in Underground Mining
1
Prof. Dr. Hassan Z. Harraz
Geology Department, Faculty of Science, Tanta University
hharraz2006@yahoo.com
Spring 2019

2. OUTLINE
 Nature of Mineralization
 Geological dilution
 Mining Methods and Dilution
 Underground Mine Design:
 Basic Input
 Global (Block) Design Issues
 Detailed design issues
 Geotechnical Monitoring
Parameters Influencing Dilution:
Orebody delineation
Design and sequencing
Stope development
Drilling and blasting
Production stages
Issues for mine management

3. OBJECTIVE
 This lecture provides an overview of the
issues influencing dilution in an
underground production environment.
 The lecture reviews the dilution problem
throughout the entire mining process,
and provides a rational approach to
underground mine design in order to
minimize dilution.
 The stages contributing to dilution include
orebody delineation, design and
sequencing, stope development, drilling
and blasting, production and mine
management issues.

4. 1) INTRODUCTION
 Dilution is defined as the low grade (waste or backfill) material which comes into an ore
stream, reducing its value.
 The detrimental impact of dilution to the economics of the mining industry has been well
documented elsewhere:
Puhakka (1991) and Elbrond (1994) have recognized that waste rock dilution and
ore loss exist during geological modelling and evaluation, decisions regarding cut-
off grade, design of the mining method, stoping and ore concentrating.
 Dilution is a source of:
i) direct cost as waste or backfill material is blasted, mucked, transported, crushed,
hoisted, processed and stored as tailings.
ii) indirect cost as the dilution material may adversely affect the metal recoveries
and concentrate grades.
 A lost opportunity may result from directing resources at handling waste (as opposed to
ore) for the mill feed. Furthermore, ore processing facilities will be engaged for material
which contributes very little to final useful metal production. In most cases, mining and
milling capacity is limited; this capacity is affected by the displacement of ore by waste
within the overall mining and processing facilities.
 Dilution is always defined and quantified with respect to an idealized (planned) stope
boundary.
In order to quantify dilution, an orebody must be properly delineated and the extracted
volumes must be effectively measured.

5.  Dilution can be divided into three general categories,
namely; internal, external and ore loses (See
Figure 1).
i) Internal dilution usually refers to the low-grade
material contained within the boundaries of an
extracted stope. It can be caused by insufficient
internal delineation of waste pockets within an
orebody. It is also occur in situations where the
mining method dictates a minimum width of
extraction.
ii) External dilution refers to the waste material
that comes into the ore stream from sources
located outside the planned stope boundaries
(Villaescusa, 1995). Low grade material from
stope wall overbreak, contamination from backfill,
and mucking of waste from stope floors are typical
examples of external dilution.

6.  Ore loss refers to:
 any unrecoverable economic ore left inside a
stope (broken, in place as pillars or not properly
blasted at the boundaries), or to any valuable
ore not recovered by the mineral processing
system.
or
 the economical material that is left in place
within the boundaries of a planned stope.
Planned ore diaphragms (ore skins), unbroken
stope areas due to unsufficient blast breakage,
non recoverable pillars left to arrest stope wall
instability and insufficient mucking of broken
ore within stope floors are typical examples of
ore loss.

7. Figure 1. Classification of dilution
MINE DILUTION
EXTERNAL INTERNAL ORE LOSS
UNPLANNED PLANNED GEOLOGICAL
INSTABILITY
CONTAMINATION
MINING METHODS
NATURE OF
MINERALIZATION
MINING METHODS
EXPLORATION
OREBODY
DELINEATION

8. Geological dilution
 Geological dilution refers to the waste rock or
ore-losses incurred during the exploration and
orebody delineation stages, where only an
estimated model of the orebody can be made.
 Geological dilution may comprise up to 1/3 of
the total dilution depending upon orebody
complexity (Lappalainen and Pitkajarvi, 1996).
 A geological model is based on limited
information, and is unlikely to coincide exactly
with the real orebody, therefore the delineated
orebody boundaries are likely to exclude ore
and also to include waste. The magnitude of
this problem is a function of the sampling pattern
for the mineralization type under study.

9. 2) NATURE OF MINERALIZATION
 The geometric configuration of an orebody and its spatial grade distribution play a
significant role during the selection of a mining method and subsequently influences the
amount of dilution experienced during the stoping operations. Geometric orebody
configuration is related to the shape, size and continuity of a deposit while grade
distribution defines the potential value of the deposit. Deposits such as seams, veins,
lenses, lodes and stratiform orebodies usually require selective mining in order to
minimize dilution, especially if the orebodies are not very continuous or if a heavily
faulted environment has created sharp changes in the spatial grade distribution.
 Selective mining methods such as room and pillar, cut and fill and more recently
bench stoping have the potential to allow the full recovery of high grade mineralized
zones within a deposit, while at the same time controlling dilution. These mining methods
are susceptible to external dilution from unstable spans, and sometimes to internal
dilution due to constraints on the equipment size.
 In large deposits, the orebody shape is usually regular and the grade distribution uniform
so that the application of mass mining methods such as sublevel stoping and
sublevel caving may be applicable.
 Both tabular and massive orebodies can be mined by sublevel open stope using
delayed cemented backfill and subsequent pillar recovery. In some cases, the
support from backfill can be used to achieve a complete extraction of an orebody. Open
stoping in large tabular orebodies is susceptible to dilution from host rocks at the footwall
and hangingwall stope interfaces as well as from backfill from previously extracted stopes
along the strike of an orebody. Stoping in massive orebodies involve the exposure of
multiple backfilled stope faces, such in tertiary stopes at the Mount Isa Mine in
Queensland Australia (Alexander and Fabjanczyk, 1982). In such cases, the capacity of
the backfill to sustain large exposed fill surfaces is critical to the control of dilution.
External (waste or lowgrade) dilution from unstable spans can also occur at the stope
crowns, or from the stope walls located at the orebody abutments.

10. 3) MINING METHODS AND DILUTION
 The geomechanical properties and geometric configuration of an orebody and its host rock medium often
dictates the type of mining method used for underground extraction. The prediction and control of the rock
mass behaviour within a stope and the surrounding rock is critical to ensure efficient geomechanical and
economical performance during stoping operations. Different magnitude of displacements are expected
within a rock mass, depending upon the mining method chosen to extract an orebody. A full range of
geomechanical strategies are available to the mining engineer selecting the most economical mining method,
in order to control dilution.
 A mining method characterized by high mining recovery and a low dilution can be considered to be an
efficient mining method. Deposits with weak host rocks and indistinct contacts are usually associated with
high dilution. The same applies to cases where thin ore veins are being mined - in which case dilution may
reach 80-100%.
 Brady and Brown (1985) analyzed the rock mass response to stoping activity in terms of displacements and
constitutive behaviour of the near and far field rock surrounding a stoping block. Their analysis classified the
mining methods into naturally and artificially supported as well as caving (unsupported) methods. Supported
and caving methods induce different stress and displacements fields into the rock mass and consequently
different degrees of dilution can be expected. Brady and Brown (1985) also provided a detailed analysis of
the most commonly used underground mining methods. However, dilution was not considered in their
analysis.
 By enlarge, dilution control may be more difficult in the caving methods where displacements of large
magnitudes within the host rock are experienced. Caving methods are not selective and the barren country
rock can contaminate the ore stream (Wright, 1983). Artificially supported mining methods rely on achieving
close control of the performance of the rock mass surrounding a stope. Cut and fill relies on passive support
from the applied backfill, while shrink and VCR stoping use the broken ore as a temporary support for the
stope walls. Srink stopes can be susceptible to external dilution due to time dependent failure of the exposed
walls, while excessive damage (external dilution) to the stope walls can be experienced during VCR mining,
specially when used for pillar recovery. Significant dilution and increased wall damage can be caused by
repetitive cratering blasting (Page, 1987) .
 The success of naturally supporting methods such as sublevel open stoping (for large tabular and massive
orebodies) relies on achieving large stable and mostly unsupported stope boundaries. Experience indicates
that wall behaviour and stability is a function of the opening geometry (shape and size), geological
discontinuities, stress concentrations, and the blasting practice (Villaescusa et al, 1997). The stand-up time
before backfill support is introduced as well as support provided by cablebolting is also an important factor
controlling stability.

11. 4) UNDERGROUND MINE DESIGN
Underground mine design is an engineering process in which the key
performance indicators are: safety, dilution, recovery, productivity and cost
criteria.
A safe and economical design may require a combination of physical,
analytical, numerical, probabilistic or empirical excavation design tools
that must be appropriately calibrated with field observations.
Empirical methods are very popular and are often based on some
geomechanical-based classification system or local experience (Potvin et al,
1989, Laubscher, 1991, Villaescusa et al, 1997).
Figure 2 presents a rational methodology for underground mine design in
which three key stages are identified.
An initial orebody delineation and rock mass characterization stage,
followed by a global and a detailed design stages respectively.
Global Design issues are relevant and applicable within entire areas of
a mine, such an extension of an existing orebody, while
Detailed Design issues are applicable to the extraction of individual
stopes.
The methodology proposed involves an integral approach to excavation
design (from orebody delineation to stope extraction) in which the interaction
among geology, mine planning, rock mechanics and operating personnel is
required throughout the entire excavation process.

12. Figure 2. Underground mine design process.
Orebody
Delineati
on
Geolog
y
NO
YES
NO YES
Rockmass
characteriz
ation
Geology
Rock
Mechanic
s
Access &
Infrastruc
ture
Mine
planni
ng
Stope &
Pillar size
and location
Stress
analysis
(sequencin
g)
Scheduli
ng
Accepta
ble
design
Drill & blast
design
Economical
analysis
Rock
reinforcement
Extraction
monitoring
Acceptabl
e design
Docume
nt
results
En
d
G
L
O
B
A
L
D
E
S
I
N
G
D
E
T
A
I
L
E
D
D
E
S
I
N
G

13. 4.1. Basic Input
The orebody delineation and rock mass characterization stages provide the
input for the entire design process. The suggested approach is to obtain
representative (mine-wide) rock mass properties likely to be used in the global
excavation design and stability analysis. In most cases, this information is
obtained from diamond drill holes (core logging) and direct mapping of
underground openings. Geophysical tools can also used for orebody delineation
and rock mass characterization.
Diamond drilling, with geological core-logging, is the most commonly used
method for orebody delineation. Information obtained from drill intersections is
extrapolated hole-to-hole using geological assumptions to provide estimates of
deposit size, shape, grades, tonnage and geotechnical characteristics. The
advantages are the depth to which information can be obtained, and a relatively
routine data analysis and interpretation. On the other hand, diamond drilling
can be costly leading to limited sampling coverage across an orebody.
Orebody delineation can be potentially improved with the introduction of
geophysical logging. Increased sampling of an orebody boundary would occur by
designing an optimum percentage of a delineation drilling budget for
geophysical logging of percussion-drilled holes. Geophysical properties have the
potential to be extrapolated hole-to-hole in order to provide a better estimate
of the size and shape of an orebody. Once the geophysical tools are calibrated,
an increased logging productivity may be achieved since assaying is not
required. Unfortunately, geophysical logging is affected by the uncertainty in the
interpretation of lithology and the grade from geophysical data.

14. Table 1. Global (block) design issues.
 Exploration drilling requirements for orebody delineation for the
designed area
 Quantity and grade of ore required with respect to scheduled
metal targets
 Area wide rock mass characterization from borehole data and
direct access
 Access and infrastructure development requirements - ore
handling systems, workshops, …etc.
 Production scheduling, details and timing
 Induced stresses from scheduled sequences, including extraction
directions
 Primary and secondary stope dimensions (including regional
access pillars)
 Backfill system requirements
 Equipment requirements
 Ventilation
 Global economic assessment

15. 4.2. Global (Block) Design Issues
Global design issues are related to the design and
stability of large sections of a mine, such a new
extension at depth or at an orebody abutment.
Global design involves several issues including mine
access, infrastructure, pillar and stope span designs.
Global design issues are schematically represented in
the upper loop on Figure 2, and listed in detail in Table
1.
 Stress analysis of the global production schedules
must be undertaken in order to determine the loading
conditions (stress and displacement) that result from
the proposed mine-wide stoping sequences.

16. 4.3. Detailed design issues
Detailed design is related to the extraction of individual stopes within a global
area. The process is schematically presented in the lower loop on Figure 1.
This stage begins when the geological team undertakes detailed orebody
delineation for stope extraction. In-fill delineation drilling, mapping, sampling
and geological interpretations on a stope scale are then completed. The mine
planning engineer uses geological sections from a mine design package to do
a preliminary stope design, while the rock mechanics engineer completes a
rock mass characterization program, provides guidelines for dilution control,
reinforcement and blast sequencing.
At this stage extraction factors that account for dilution as well as back
analysis of performance from any adjacent stopes are taken into account. Drill
and blast design is undertaken considering the equipment capabilities, to
ensure that the designed stope shape is achievable. This is then followed by
an economic analysis that determines stope viability by considering the break
even revenue cut-off figures including a calculation of net revenue versus
total mining, concentrating and overhead cost. Finally, a stope design
document that include plans of sublevel development, sections showing blast-
hole design concepts and drilling and blasting parameters, ventilation, rock
mechanics and overall firing sequence is issued to the operating personnel.

17. 4.4. Geotechnical Monitoring
Geotechnical monitoring of rock mass response to the individual stope extraction
including stope performance reviews that documents all the relevant information
such as actual dilution figures, issues influencing productivity and overall economic
results is required during the mine design process. Geotechnical measurements are
required to assess the response of the rock mass to the excavation process and are a
key component of the mine design optimization process required to achieve safe and
most economical extractions. The measurements can be classified into three phases:
Prior, during and after excavation (Windsor, 1993).
Measurements prior to an excavation are usually concerned with the characterization
of the geotechnical environment as an input to the excavation design. Such
measurements include borehole/core logging data to determine rock type, structure,
rock material properties and hydrology conditions.
Measurements during excavation are used to provide warning of hazards such as
excessive rock stress, deformation and extent of damage envelope around the
underground openings. These measurements are required for validation of design
assumptions made when some of the data was not available prior to excavation such
as in-situ stresses and material properties (i.e., numerical models of rock and fill
behavior). The measurements suggest the type and timing of remedial measures such
as modification to extraction rate and sequencing of excavations and to optimize rock
support and reinforcement schemes.
Measurements following an excavation are undertaken to obtain data required for
optimization of future excavation designs (back analysis of performance). These
measurements are required for dilution control and to minimize ore loses. They are
also needed to provide data on long term stability, safety and environmental effects.

18. 5) PARAMETERS INFLUENCING DILUTION
 The most common parameters
influencing dilution and ore losses in
underground mining are listed in Table
2. Five key stages ranging from the
initial orebody delineation program to
the final extraction stages have been
identified within the mine design
process. Management issues were also
included, given that in some cases they
represent the most critical factor
controlling dilution (Ashcroft, 1991).

19. Table 2. Parameters influencing dilution.
 Orebody delineation:
Under sampling of orebody boundaries
Errors in decisions regarding cut-off grades
Down hole survey errors
Lack of geotechnical characterization
 Design and Sequencing:
Poorly designed infrastructure
Poor stope design (dimensions)
Lack of proper stope sequencing
Lack of economical assessment
 Stope development:
Non alignment of sill horizons
Poor geological control during mining
Mining not following geological markups Inappropriate reinforcement schemes
 Drilling and Blasting :
Poor initial markup of holes
Set-up, collaring and deviation of blast holes
Incorrect choice of blasting patterns, sequences and explosive types
 Production stages:
Mucking of backfill floors
Mucking of fall offs and stope wall failures
Contamination of broken ore by backfill
Leaving broken ore inside the stopes
Poor management of waste rock (tipped into the ore stream)
 Mine Management :
Lack of supervision and communication
Excessive turn over of personnel
Limited time for planning
Lack of stope performance reviews
No documentation and proper training
Performance indicators based on quantity (focus on tonnes as opposed to metal content)
Lack of leadership and vision

20. 5.1. Orebody Delineation
 Orebody delineation is the process which establish the size, shape, grades, tonnage and mineral
inventory for the ensuing mining process. Efficient, effective, and accurate delineation of a deposit is
required to design a mine in a manner that maximizes recovery, minimizes dilution and increases
safety. Dilution can not be planned or minimized if detailed geological and geotechnical information is
not available. Experience indicates that increasing the information density is likely to decrease
dilution and ore losses (Braun, 1991, Lappalainen and Pitkajarvi, 1995). In cases where the stope
geology is not well delineated, the interpreted ore outlines are usually regular; the presence of waste
inclusions is then likely to remain unknown.
5.2. Design and Sequencing
 At this stage, several extraction strategies to minimize dilution/ore loss can be studied in advance to
choose the best design alternative. Engineering, geology and operating personnel should have a
direct input into this stage of the design. Extraction factors that account for dilution, should be
applied at this stage. Back analysis from adjacent stopes based on laser (Cavity Monitoring System,
Miller et al, 1992) surveys, drill and blast design and general experience in the area should be used.
Proper design means that the planning engineer receives an optimized block thus leaving more time
for drilling, blasting and ground support optimization, schedule modifications and other issues.
 At this stage, the stable stope and ore outlines are superimposed in order to detect volumes of
waste rock inside and ore outside the stope limits. Wall instability and any relevant remedial
measures are also identified. A stope shape must be drillable and stable, and the walls must insure
proper flow of broken ore to the stope drawpoint. Economical studies in conjunction with stability
analysis can be performed to evaluate different design options (e.g. stope sequencing, dimensioning,
etc.).
5.3. Stope Development
 Drive location has been shown to be critical for dilution control. Undercut of stope walls by the
access drill drives is likely to control the mechanical behaviour at the stope boundaries. Drive shape
and size also influence stope wall undercut. Incorrect positioning of sill drive turnouts off access
crosscuts, may also create stope wall undercut leading to dilution. Cross cuts need to be mapped,
sampled and interpreted prior to developing the sill drives along an orebody. In cases where assay
information is required prior to sill turnout, a prompt assay turnaround is critical to maintain
development productivity. Quality (and quantity) geological face mapping of development is critical to
minimize stope wall undercuts. Geologists should highlight any overbreak beyond an established
mining width. Promt feedback to the operating personnel undertaking the development mining is
required. Routine geotechnical mapping of development faces must be also undertaken. Perimeter
blasting techniques can be used to reduce wall damage in development access in order to minimize
stope wall undercut.

21. 5.4. Drilling and Blasting
 If dilution and ore losses can be minimized during the block design stages,
drilling and blasting can be done without problems and focused on better
fragmentation and damage control within the stope boundaries. Nevertheless,
dilution and ore loss can also planned and evaluated during the drilling and
blasting stages, where the blasting outlines can be designed to optimize
extraction.
The blasting process involves the interaction of the rock mass, the explosives,
the initiation sequences and the drill hole patterns. Consequently, a blast design
should account for the interaction of the existing development, equipment,
orebody boundary and stope outline. Geological, geotechnical, operational and
extraction design issues must be considered. Blasting performance is also
affected by the orebody geometry and drilling limitations (hole length and
accuracy).
Explosive consumption and performance determines the quality of fragmentation.
However, an increase on the specific consumption of explosive may also increase
the damage to the host rocks, increasing external dilution.
The effects of blasting on stability can be determined based on measurements of
blast vibrations, hole deviation, hole angle and distance of the holes to the
exposed stope walls. A consideration of the most suitable drilling technology for a
range of hole sizes and drilling patterns in order to minimize damage and hole
deviation is needed. Suggested drilling and blasting patterns for long-hole
stoping are presented in Table 3.
Hole Diam
(mm)
Burden
(m)
Stand-Off
Distance (m)
Drilling Technology Hole Depth
(m)
51 1.0 - 1.5 0.4 rods 10-15
63 1.3 – 1.8 0.6 rods 10-15
73 2.0 - 2.5 0.8 Rods + stabilizers 12-20
76 2.0 – 2.5 1.0 Rods + tubes 20-25
89 2.5-2.8 1.1 Tubes – top hammer 25-35
102 3.0 1.2 Tubes – top hammer 25-40
115 3.0-3.5 1.3 In the hole hammer 40-60
140 3.5-4.0 1.5 In the hole hammer 40-60
Table 3. Drilling and blasting patterns for
sublevel stoping

22.  Training modules for drilling crews, including an understanding of blasting are required to minimize blast damage. Clear,
concise drilling plans from planning to the drilling crews are required. In cases where the orebody contact does not
coincide with the sill development walls (narrow orebodies), painting of the orebody boundary would help the drilling
operators to obtain a proper hole breakthrough.
5.5 Production stages
 Even at this relatively late stage, dilution and ore losses can still be minimised. Information from percussion blastholes
can be used to locate zones of waste within an orebody, thus enhancing orebody delineation. The blast design could be
revised based on detailed information regarding zones of ore and waste. Some holes might not be blasted (i.e. leaving a
pillar), or additional holes may be drilled. Drillcutting data can be used to identify the orewaste contact in production
holes. However, these task-intensive operations (sampling, bagging, and assaying) are prone to inaccuracies, and the
turn-around time for the data analysis is often too slow for practical use. In practice, information about the ore-waste
contact at the production stages is seldom acquired without the use of properly calibrated geophysical tools.
 The potential exists for geophysical logging (single hole techniques) of production holes to identify the ore-waste contact
for optimal blast design. An advantage of single-hole geophysics is that information would be immediately available;
therefore significantly reducing turnaround time. This is particularly beneficial in situations in which severe blasthole
deviation is occurring, and the exact location of the orewaste contact is undefined.
 Inspection and floor preparation before firing and mucking commences, minimizes ore contamination during mucking.
Mucking units may dig holes and dilute ore with fill. Mucking units may also ramp up and leave broken ore in the stope
floors. A training program on draw point inspection for grade, ore contamination and stope status (stability) is required to
control dilution. The stopes must be inspected several times through a mucking shift to check the LHD tramming route
and the state of a stope. The condition of the hangingwall, footwall and back must be assessed during these inspections.
Any significant falloff, overbreak or underbreak should be recorded, given that variations from planned designs could
affect stability and place at risk further extraction in adjacent stopes.
 Stope performance review must be undertaken following the completion of production blasting. These reviews are needed
to improve performance and to determine what lesson can be learnt and what improvements can be made. Geology, mine
planning and operations personnel must be involved. The performance review compares the laser (CMS) surveyed void
with the planned void. The differences can be due to blasting overbreak, stope wall failures, pillar failures, insufficient
breakage, etc. The variations from the planned volumes are used to determine actual tonnage and to estimate the
extraction grade for each stope. These can be used to undertake the final economical analysis and to optimize future
extraction in similar conditions.
5.6 Issues for mine management
 Although geologists, engineers and operators are involved in the mine design process, mine managers must be ultimately
accountable for the success of a dilution control plan. Dilution control and ore losses must be managed within a global
program of optimization for cost control and increased safety. The choice of an option that minimises dilution may disrupt
scheduling and low levels of dilution could be sometimes justified in the context of a particular total mining scenario.
 In some cases, dilution and ore loss are not assessed because the geology and related costs are not sufficiently well
known. At best, critical decisions are simply based on the experience of the drilling and blasting designer. In other cases,
when a decision is taken, experience and rules-of-thumb are used instead of calculations based on grade. This is often due
to lack of real data and the level of judgement for dilution prediction.

23. REFERENCES
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Ashcroft, J.W., 1991. Dilution: A total quality improvement opportunity. Inco Limited, Thompson Manitoba, Canada.
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Windsor, C.R., 1993. Measuring stress and deformation in rock masses. In: Szwedzicki (Ed), Instrumentation and
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