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Blast Design using MWD parameters

Jorge Brown Segui

PhD Student

Julius Kruttschnitt Mineral Research Centre

Mike Higgins

Principal Consultant

JKTech

Abstract

Measurement While Drilling techniques can provide a useful tool to aid drill and blast

engineers in open cut mining. By avoiding time consuming tasks such as scan-lines and

rock sample collection for laboratory tests, MWD techniques can not only save time but also

improve the reliability of the blast design by providing the drill and blast engineer with the

information specially tailored for use.

While most mines use a standard blast pattern and charge per blasthole, based on a

single rock factor for the entire bench or blast region, information derived from the MWD

parameters can improve the blast design by providing more accurate rock properties for each

individual blasthole. From this, decisions can be made on the most appropriate type and

amount of explosive charge per blasthole or to optimise the detonation time of different

decks and blastholes. Where real-time calculations are feasible, the system could extend

the present blast design towards a more appropriate blasthole pattern design like

asymmetrical blasting.

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Introduction

In 1911, Schlumberger introduced downhole electrical logging to the oil industry. At

the time, its main aim was to reduce the “blindness” of drilling operations and improve the

knowledge of oil field geology and structural characteristics. Since then, logging systems

have been extensively developed and other features have been added, increasing the

probing capability. The system has proved to be extremely important for the oil industry, and

since the 1970s the technology had been extended to mining operations, mainly in open pit

bench drilling.

Early monitors were pen strip recorders adapted to mining. They generated long

reports of the results but the use of the data was minimal (Vynne, 1997). A major difficulty

was interpretation of the records, requiring hours of tedious scanning through the data and

the system was susceptible to frequent failures.

The use of computers has radically modified this scenario and today all interpretation

is done by software, allowing filtered information to be delivered to the different centres in the

mine. The scope of application of this information is extensive. For instance, geologists can

compare the inferred lithology to the mine model; scheduling engineers can receive

feedback on the timing and performance of each drill rig; mechanical engineers can

determine the condition of components of the drill rig; drill & blast engineers can produce

detailed plans describing the location and downhole details of each blasthole; and the plant

can obtain a description and quantity of the rock types present in a blast. This paper focuses

on how the use of the MWD systems can improve the knowledge of the drill & blast engineer

about rock mass variations within the bench and how this knowledge can be applied to a

blast design. As listed above, other aspects and capabilities of the system are also possible.

MWD systems have evolved from simple parameter recording to rock recognition

systems. The system relies on a set of parameters that have to be considered together and

cannot be taken individually. The parameters not only reflect the rock properties but also the

drill settings. As both vary along the blasthole, the approach is to consider them as an

interlocked set of data and filter them through an appropriate algorithm.

It is important to note that a MWD system must be calibrated for a mine site. It does

not have the ability for definitive recognition of rock types. Also, the accuracy of the results

are effected by the resolution adopted by the mine. Resolution is the interval length of the

drill hole over which data is measured and evaluated (Figure 1). In industrial applications, it

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can be set as low as 10 cm. At the CMTE research facility in Brisbane, a resolution of about

3 mm has been achieved.

Figure 1. Blasthole description in terms of Blastability Index (BI) from the Aquila system gives the amount of each type of rock. It can infer up to eight different rock types.

Some MWD systems can produce a set of normalised numbers related to the specific

energy (SE) from the default setup. Specific energy is the basic concept behind these

techniques. It is used in both calculated and inferred parameters, and ultimately leads to

determination of indices associated with rock properties, such as blastability index (BI) or

comminution index (CI).

Drilling versus Crushing

With mechanical methods of rock excavation, penetration into the solid rock mass is

limited by the geometry and the pull-down force at tip of the drilling tool, and the properties of

the rock. These determine the penetration achievable per revolution for a drill, measured in

the direction of drilling. The penetration rate is unlikely to be greatly influenced by any lateral

components of force on the tool, whatever they may be (Teale, 1964). Thus, the

fundamental action of a drilling tool can be considered that of an indenter, such as is

commonly used for measuring surface hardness, particularly of metals. Indenter penetration

in the surface of a semi-infinite solid of brittle material under a normal thrust is the basis of all

mechanical rock working processes.

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Tricone drill bits used in mining are essentially arrays of indenters mounted in the

form of teeth on rollers. The rock working process this type of tool performs is repeated

indentation under static thrust. The fragments are removed by high-pressure air or water to

reduce secondary breakage inside the hole.

The effect of the drill bit forced into a hard rock surface can be described by two

actions:

1. it penetrates by compacting and crushing the rock in front of it;

2. after some penetrating depth, fragments of rock break out from the bottom of the

indenter hole (Fig. 2)

Figure 2. Indenter action on a rock surface

Teale describes indenters and drills as “primary” crushers because their product is

broken from a semi-infinite solid and is not, as in other crushers, the result of reducing one

existing size distribution to another. There have been many studies in crushing generally

relating applied energy to material properties and dimensions. However, with regard to the

differences in dimension and application, the concept of specific energy, determined from the

energy balance of actual drilling operations, can be used to estimate in situ rock properties

(Teale, 1964).

Specific Energy and Drill Performance

Specific energy is defined as the work done per unit volume excavated. The amount

of energy necessary to excavate a given volume of rock should depend entirely on the

properties of the rock. The difference between this theoretical amount of energy and the

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applied mechanical energy can be explained by dissipation in regrinding, friction, and

mechanical losses outside the rock-drill interaction.

In analysing the components of specific energy for rotary non-percussive drilling

(Teale, 1964), the work done can described by the formula:

��

���

���

���

�+��

���

�=R

NT

AA

FS

π2 N.cm/cm³ (1)

where

S = specific energy

F = thrust (Newtons)

A = cross-section area of the drill hole (cm2)

N = rotation speed (RPM)

T = torque (N.cm)

R = penetration rate (cm/min)

The formula can be separated into two parts – the thrust (or vertical) component St,

and the torque (or rotary) component Sr:

��

���

�=A

FSt N.cm/cm³ (2)

��

���

���

���

�=R

NT

ASr

π2 N.cm/cm³ (3)

Since A is constant for a drill hole, then St is directly proportional to F, and Sr is

proportional to T/R for a fixed N. Therefore, the torque-penetration curve for a rock type,

over a reasonable working range of rotation speed for a drill, approximates to a straight line

through the origin. So, for a given A and N, Sr and therefore S itself should show little

variation over the working range referred above.

As an alternative, the penetration per revolution P = R/N (cm per rev) can be used:

��

���

���

���

�=P

T

Ar

πε 2 N.cm/cm³ (4)

where T is the torque required to remove a layer of rock per revolution to a depth P.

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For a given rock type, torque is directly proportional to thrust, and can be up to ten

times or more the magnitude of the thrust. Since the amount of energy required to break

brittle material like rock is not much affected by the rate at which it is applied, the relationship

between T and P may not be significantly affected by changes in rotation speed. Therefore,

the ratio (T/P) can be a useful index for specific energy.

Specific energy attains its highest values at low thrust. Below a certain value of thrust

there will be no effective penetration into the rock. The volume excavated will be zero but a

finite amount of work will still occur against friction. This situation will yield infinite values for

specific energy. As thrust increases above the minimum value, particle size and penetration

rate will increase and specific energy will decrease. However, this situation will not continue

indefinitely. Eventually, the pull down pressure will cause the drill bit to stall in the rock or the

volume of cuttings will be too large to be removed and will lead to regrinding (Figure 3). This

will increase the specific energy at some point before stalling.

Figure 3. Variation in rate of penetration (ROP) vs. pulldown pressure (PDP) from MWD.

Therefore, for a fixed rotation speed and a particular rock type, there is a range of

thrust where the specific energy will reach a minimum value and the mechanical efficiency of

the drill will reach a maximum. This relationship offers a method of equating drilling

performance to a rock strength parameter. The consequences of this are significant for

operations such as blasting, comminution and scheduling. However, it must be noted that

this is also dependent on drilling conditions. Rock structure, such as open or closed joints,

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will generate unpredictable fluctuations in specific energy that are not related to rock

strength.

Rock Characterisation by MWD

As shown above, MWD systems are capable of highlighting variations in rock types

and thus improve the knowledge of the geological characteristics of rock domains for drilling

and blasting. From these parameters, with the addition of a global positioning system (GPS)

to determine hole collar position, it is possible to simulate a virtual bench for comparison and

enhancement.

The parameters can be divided into three categories:

�� measured

�� calculated

�� inferred

Measured parameters are, for example, RPM, bailing air volume, vibration, pulldown

force, depth of drill bit and rotation pressure.

Calculated parameters include direct values such as rate of penetration and torque,

and indirect values such as specific energy.

Inferred parameters are those which are dependent on the operating conditions, and

whose calculation must be calibrated for the drill equipment, local rock properties and

structure and drilling performance. Two examples of these are the Blastability Index (BI)

generated by the Aquila system, and the Comminution Index (CI) currently being investigated

at JKMRC. The Blastability Index aims to quantify the potential response of the rock to

blasting, similar to the rock factor in the Kuz-Ram fragmentation model. A high BI indicates

rock that is difficult to break (high strength, low joint frequency) and a low BI indicates softer

rock (low strength, many joints). The Comminution Index is seeking to extend this concept

in terms of the crushability or grindability of the rock, by comparing MWD parameters

recorded at higher rates with known values such as point load test, drop weight test and

Bond work index, from information accumulated and developed at the JKMRC. In both

cases, these values are derived from models using both measured and calculated

parameters as inputs, and the calibration is highly dependent on the relationships between

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the measured parameters at any point in the rock. It will also be affected by the resolution

adopted for measurement of the downhole values.

There is a misguided tendency to use direct parameters (measured and some

calculated values) as an indication of rock properties – for example, using the rate of

penetration (ROP) as a direct measurement of the rock strength, or by association with other

rock properties, in blast design. Although ROP is proportional to rock strength, it can be

influenced by other factors, the most common of which is variation in pulldown pressure

(PDP), which will be reflected directly in the ROP readings (Figure 3). Fractures and voids

can affect the ROP as well. In highly fractured zones the ROP can increase but can also

create a stall. A typical example of this generally occurs in the upper part of a bench, where

the MWD parameters will indicate a “softer” rock, possibly up to 2 m depth, even though this

rock is the same as that above and below. In fact, the parameters are being affected by the

pre-conditioning from the bottom charge of the previous blast. Most of the time this will not

have a significant influence in terms of blast design because it occurs in the stemming zone.

Nevertheless, it can be useful for the prediction of blast fragmentation.

Blast Design Using MWD

Mines are now starting to apply MWD parameters to blast design, such as in

determining explosive charging after the holes have been drilled. Three cases presented

below illustrate this application.

BHP Iron Ore in Newman, WA, was using a simple MWD system producing only a

few parameters for each blasthole. With some modelling work done by the JKMRC, a crude

specific energy index was calculated as a normalised ratio of ROP and PDP, expressed as a

weighted average of the indices generated from composites in each blasthole. Previously,

the only system in use was the Pilbara Iron Ore Classification (PIOC), which has been

applied for many years (Kneeshaw, 1984). PIOC is an empirical index of the physical and

chemical attributes of the ore assigned by the mine geologist. This system was initially

developed for exposed ore faces for stratigraphic modelling and refinement of the block

model.

In this case, the blasting engineers use a simplified version of the PIOC index applied

to each blasthole in the pattern based on inspection of drill cuttings. This is much more

accurate than considering the whole bench as one rock type and using one set of charging

parameters for all blastholes. However, comparison with the drilling index shows that the

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results are not consistent, depending on the subjective interpretation of the drill cuttings or

which section of the cuttings cone was inspected. For example, cuttings from the bottom of

the blasthole may entirely cover the cone and give a false result of the index. In all cases, if

large discrepancies occurred when comparing PIOC with the drilling index, the PIOC value

was adjusted.

The resulting values for each hole were plotted as contour plans, showing variations

in “rock strength” across the bench that could be used to determine an optimal quantity or

type of explosive for hole charging.

Figure 4. Contour map of bench WB24N (BHPIO Newman) of normalised MWD parameters (from JKSimBlast)

Ernest Henry Mine in Queensland is currently implementing a MWD system with the

aim to determine a comminution index for downstream crushing and grinding, in terms of

volume of each rock type per blast. However, there is no existing rock mass classification

system for comparison on a hole by hole basis, so results to date have been compared with

geological maps. A contour map of Blastability Indexes since the beginning of the trial is

shown below.

The map shows three distinct zones. In the central part of the pit there is a

supergenic weathered rock (blue), which is considered soft by the mine; in the NE and SW

are medium strength rock types; and in the southern part of the pit, felsic volcanic and

mafelsic volcanic rocks are rated as hard to very hard material. Blast engineers use this

information to determine explosive charge for each hole in a way that previously was not

possible, particularly for transition within a blast from one rock type to another.

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Figure 5. Contour map of the whole pit at Ernest Henry Mine showing hard (red), medium (green) and soft (blue) rock strength zones (from JKSimBlast).

The third example, from a copper mine, shows the downhole variations in BI for a

particular blast. Note that there are a significant number of holes showing “soft” collars,

which can be directly attributed to pre-conditioning by previous blasts. When the total length

of each rock type in each hole is plotted as a contour plan for the blast, distinct zones

become apparent, with a ridge of soft material extending up from the south and a second

concentration in the northern part of the blast area. It is then a simple step to select those

holes that exceed a certain quantity of soft rock and charge them with a lower energy

explosive. Also, those holes that show a harder toe can be individually selected for charging

with a higher energy explosive to ensure consistent breakage at floor level.

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Figure 6. Sectional view of downhole values of Blastability Index for a copper mine.

Figure 7. Contour plot of total length of soft rock in each blasthole. Note the zoning of rock types.

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These examples illustrate the possibility of using MWD parameters to modify the

charging aspects of blast designs to cope with variations in rock properties within a blast.

When rock mass properties are consistent, blast design is straightforward and performance

is predictable. But when the properties vary, such as rock strength and joint frequency, the a

calculated or inferred parameter will clearly show this.

There are also potential alternatives to fine tuning the explosive charge. As the

usage of electronic detonators increases, it may be possible to optimise initiation timing with

regard to energy distribution within the rock mass from the explosive column and burden

relief performance during the detonation sequence. With the increased sophistication of

MWD systems, it is now possible to provide results in real time, which means that the on-

board system could automatically determine the optimum placement of subsequent

blastholes while drilling (Segui, 2001).

Conclusions

MWD systems provide a better – more accurate, detailed and timely – description of

the rock mass inside the hidden volume of the bench than traditional assessment methods.

The consistency behind the MWD system removes the human error in manual

classification systems.

Cost savings and improved blast performance are possible by utilisation of the

automation of drilling, such as prevention of over and under drilling and increased rate of

penetration. With appropriate site developed guidelines, there is significant potential to

automate many aspects of the blast design process.

MWD systems are not absolute rock recognition systems. However, with in situ

calibration and proper interpretation, they have considerable potential for not only drill and

blast design and implementation but also for short term scheduling and mineral processing.

Acknowledgments

We would like to thank Mine Superintendent Derek Miller, Training Supervisor Chris

Dunbar, Drill & Blast Engineers Andrew Theobald, Jason Kohn, Claye Bickers, and Derrick

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Barden (IT Services) from BHPIO Newman for their tremendous support for our constant

demands for “data” and for help. Geoff LeJuge was a tremendous help in fieldwork and we

would like to thank him for the wise guidance.

Our work would be not possible at Ernest Henry Mine without the support of Mine

Technical Services Manager John Moore, Drill & Blast Manager John Flynn, Drill & Blast

Engineer Shaun Barker, and Project Geologist Max Ayliffe

References

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Sons, NY., 1977

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[3] J.F. Vynne, “The application and economic benefits of blasthole drill monitors”,

Proc. ISEE 23rd Annual Conference on Explosives and Blasting Technique, Las

Vegas, 1997

[4] R. Teale, “The concept of specifc energy in rock drilling”, Int. J. Rock Mech.

Mining Sci., vol 2, pp. 57-73, 1964.

[5] B.A. Wills, Mineral Processing Technology, 5 ed. Pergamon Press, 1992.

[6] F.C. Bond, “The third theory of comminution”, Trans, SME-AIME, vol. 193, pp.

484-494, 1952.

[7] M. Kneeshaw, Proc. Australasian Institute Mining & Metallurgy, pp. 157-162,

1984.

[8] J.B. Segui, “Asymmetrical Blasting: a rock dependent blast design method”,

Explo 2001, Hunter Valley, 2001