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Blasthole Length, Subdrilling and Stemming Analysis in an Iron Mine

Anneliese Botelho, Master Degree at Federal University of Rio Grande do Sul, Brazil.

Sarah R. Guazzelli, PhD candidate at Federal University of Rio Grande do Sul, Brazil.

Jair C. Koppe, Mining Engineer, MSc, PhD, Engineering Department, Federal University of Rio

Grande do Sul, Brazil

Enrique Munaretti, Mining Engineer, MSc, PhD, Engineering Department, Federal University of

Rio Grande do Sul, Brazil

Abstract

Drill and Blast (D & B) is one of the main operations in the mining production cycle, which aims to

fragment rock according to a pre-defined plan for proper loading, haulage and processing specifications.In this sense, data from a Brazilian iron ore mine was analyzed, given emphasis on blasthole length,

subdrilling and stemming. The D & B report, including mining data such as number of blastholes,

length, and diameter and stemming aspects, was analyzed in order to be able to compare these results

with the blast design. The study showed significant differences between the designed blasting patternand that one obtained by the operation. As a consequence, problems such as inadequate fragmentation,

incorrect size distribution and loss of energy, were identified. For example, when stemming has adifferent length than it should have, noise and flyrock can occur, including inadequate fragmentation and

vibration in both situations. Therefore, a comparison between the usual D & B procedures andrecommended D & B design based on literature reveals the most important deviations and their

consequences. The study intends to identifysolutions for simple problems, improving D & B operations

and safety practices.

IntroductionDrill and Blast (D & B) is one of the main operations in the mining production cycle, which aims tofragment rock according to a pre-defined plan for proper loading, haulage and processing specifications.

In this sense, data from a Brazilian iron ore mine was analyzed, given emphasis on blasthole length,

subdrilling and stemming. The blasting pattern was compared to most common geometric relations fromliterature.

The mining is carried out in a Banded Iron Formation (BIF) composed of five main rock types: Itabirito

friable and compact (the poorest iron ore type, containing between 50% and 63% of iron), Hematite

friable and compact, which contains between 63% and 66% of iron, and Blue Dust (a fine form of

hematite with grades between 66% and 69% of iron).

The resulting fragmentation of D&B usually presents many fines and boulders independent of the pitregion. Ore fragmentation can be improved just doing little changes in the procedures. These changes

will impact not only the drilling costs, but also in the subsequent operation.

Data were collected from January to May 2013, corresponding to 500 blast reports, which werecompared to theoretical and recommended blast design, making clear that the field practice is

completely independent from planning in this mine. Inadequate fragmentation, incorrect size distribution

and loss of energy, were identified.

Blast Design

Copyright 2014 International Society of Explosives Engineers

2014G - Blasthole Length, Subdrilling and Stemming Analysis in an Iron Mine1 of


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One of the goals of blasting is to provide material into fragments for excavation and handling to supplythe comminution plant. Thus, primary crusher is the stage that defines the ideal ore size distribution to

be fragmented in the blast operation. The ore size distribution will impact excavation, loading and

haulage operations (Bozoic B, 1998).

There are several consequences of an inadequate fragmentation, all of them directly affect mine cost and

efficiency. Poor fragmentation can produce boulders, and as consequence, induces secondary blast and

load and haulage difficulties. Too many fines will reduce crusher efficiency and will influence millstage. Because of that, it is important to know what factors contribute in the fragmentation process

(Bozoic B., 1998).

Rock mass is not homogeneous, and in this case, the deposit consists of five different rock types. The

mining area suffered several deformation phases generating many structural discontinuities. Moreover,this region has faults, joints and other discontinuities responsible for isolating large rock blocks allowing

gas penetration that can cause noise and vibration concerns.

In order to distribute the explosive energy as efficiently as possible and to promote adequate

fragmentation, an effective blast pattern must be developed. Thus, using the formulas suggested, it is

possible to establish an initial blast design. This plan will likely be changed to adapt to varyinggeological characteristics.

The drilling equipment used in this study mine are shown in Table 1. Blasthole diameters vary from101.6 mm (4 in) to 203.2 mm (8 in). One of the factors to determine the borehole diameter is the

geological structure. Large diameters are better in homogeneous rock that is easy to break, with few or

no weakness planes (Dick et al., 1983). In addition to rock characteristics, large holes are necessary tomine a large volume of ore in large-scale operations. The mine in this study produces a large volume of

ore, however, it is not homogenous.

Table 1Blasthole drills and their respective diameters

Blasthole Drill Diameter (mm) Diameter (in)ROC D7 Atlas Copco 101.60 mm 4 in

ROC L825 BAR Atlas Copco 146.05 mm 5 in

SKSW Terex 203.2 mm 8 in

1190E Sandvik 152.4 mm 6 in

The burden is defined as the distance from the borehole to the nearest free face at the moment of the

detonation (Dick et al., 1983). It can be calculated as a function of explosive diameter, borehole

diameter, confinement degree, explosive density, depending on the author. This mine has borehole

diameters ranging from 101 mm (4 in) to 203 mm (8 in). Since the literature shows several methods to

set the burden length and this will depend on the free face, it is assumed that the burden designed isadequate. Once the burden is defined, it becomes the basis for subdrilling, stemming and spacing

calculations.

Borehole depth depends on the bench height. Here, bench height and hole depth are considered together.Subdrilling is the distance drilled below the ground level, ensuring that all the rock will be removed




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(Dick et al., 1983 and Konya, 1995). To calculate the subdrilling as a first approximation, it is used

thirty percent of the burden (Equation 1).

Equation 1

Where:

U = subdrilling (m) (ft) Bmax= maximum burden (m) (ft)

Borehole depth, as mentioned, is the sum of subdrilling to the bench height considering the inclination(Equation 2). It is important to remember that it is impossible to drill exactly according to the calculated

theory because of inherent limitations in drilling equipment and operator ability (Olofsson, 1990).

Equation 2

Where:

H = hole depth (m) (ft)

K = bench height (m) (ft)U = subdrilling (m) (ft)

If the bench inclination is considered, Equation 2 can be changed to Equation 3, as follows:

Equation 3

Where:H = hole depth (m) (ft)

K = bench height (m) (ft)

U = subdrilling (m) (ft)

= bench inclination (degrees)

The stemming is the part of the hole that is not filled with explosive material. Stemming consists of inert

material to confine the gases from the blasting and to reduce air overpressure. Therefore, it is the non-charged part of the hole. To calculate, the stemming is equal to the burden, as in Equation 4.

Equation 4

Where:T = stemming length (m) (ft)

B = Burden (m) (ft)

Case Study

The company records all information from blasts using an ordinary blast report. The Blast Reportcontains data such as lithology, bench height, number of holes, burden, spacing, borehole depth, type

and quantity of explosive. Using this information, it was possible to analyze the borehole depth and the

stemming length and compare them to the fragmentation obtained.

It is assumed that the blaster has the freedom to choose the borehole diameter, even if it is limited by theequipment size. According to the literature, the burden which was assumed as correct, depends on the




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borehole diameter, type of explosive and rock. All of the other factors are based on burden, as

previously mentioned.

Analysis of the Borehole DepthSince the borehole depth involves both bench height and subdrilling, these factors will be considered

together. Initially, the differences between bench height and borehole depth as reported in each blast

report were compared, as seen in Figure 1. It is presented only the data from January, 2013, as it is

representative and similar to the other months.

Figure 1 - Bench height and borehole deep, January, 2013.

The Figure 1 shows a huge scatter between the boreholes in each blast report, as can be seen when the

difference between the larger and the shallower borehole is observed. Nonetheless, this difference in theboreholes depth is not dependent on the bench high, once it is not fixed. As demonstrated in the graphic,

in some blasts, the minimum borehole length is slower than the bench height. In this case, the

possibility to form toes is extremely high, due the lack of subdrilling. When the maximum borehole

length is analyzed, many times is deeper than should be expected, causing gaps in the level below.

Figure 2 shows only the deeper boreholes. It can be observed that 35% of the deeper boreholes aresmaller than calculated. It means that all the other boreholes in these reports are smaller than the

calculated. On the other hand, it is clear that almost 40% of the reports have the deeper boreholes are

larger than expected.

0

5

10

15

20

25

0

5

10

15

20

25

0 5 10 15 20 25 30 35

BenchHeight(

)

HoleDepth(m)

Day

Maximum Borehole Length per Report Minimum Borehole Length per Report

Bench height




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Figure 2 - Blast patterns where the deeper hole is larger, smaller or equal to the theoretical depth.

Figures 2 and 3 shows the impressive difference between depths of the holes. Inthemost unfavorablesituation, the deepest hole of a pattern is 30% more than the set while the shallower can be up to 20%

less than the theoretical estimation.

39%

27%

35%

Deeper Hole is larger than the

maximun depth calculated

Deeper Hole is equal to calculated

depth

Deeper Hole is smaller than the

minimun depth calculated

31%

69%

More than 20%

10% - 20%

Up to 10%




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Figure 3Blastholes less deep than theoretical estimated: 69% of them are up to 10% shallowest

than calculated and 31% of the holes are between 10% and 20% shallowest than expected.

Drilling more than necessary is wasteful. Besides increasing the quantity of explosives used, it increasesthe vibration due to the high degree of confinement of the explosive. In mines encompassing multiple

benches, too much subdrilling can cause fracturing in the upper part of the bench below. Insufficient

subdrilling can cause accumulation of explosive above the bench below. It will reduce the life cycle of

the excavation equipment, while will increase secondary blasting (Dick et al., 1983). Beyond that,insufficient subdrilling may cause irregular ground, toes, flyrock and step between the levels.

In Figure 4 it is possible to observe the consequences of poor drilling. In the mine, irregular ground, in

addition to hindering the next drilling, cause damage to load and haulage equipment and increase

maintenance costs. A toe will wear load equipment and reduce its life cycle.

Figure 4 - Left side, irregular ground caused by uneven difference in depth of the holes. Right

side, figure shows a toe caused by shallow hole.

Analysi

s of StemmingBesides the depth of the hole, also the stemming was studied. As following in Figure 5, 56% ofstemming is larger than the calculated and almost 30% of stemming is equal (Figure 5, period from

January, 2013 to May, 2103).




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Figure 5 - Comparison between stemming length executed and stemming length calculated.

In the graph presented in Figure 6, it is possible to observe that 31% of blast reports have all the

stemming larger than planned, which represents 4191 holes. One reason for this can be the fear ofannoying the community near the blasting area. Nevertheless, 69% of the pattern has mixed length of

stemming (equal, larger and smaller stemming in the same pattern), which includes 5519 holes longer

than expected and 7631 stemming smaller or equal to theoretical stemming. As the company has no

standard for conducting stemming, these must be carefully analyzed.

56%

16%

28% Stemmings larger

than expected

Stemmings smallerthan expected

Stemmings equal to

the expected




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Figure 6 - Percentage of blasting patterns where stemming are larger than calculated and mixed

stemming length (larger, equal and smaller length together).

Selection of stemming length is usually a compromise between fragmentation and the amount of air

overpressure and flyrock that are acceptable. Too short of stemming will result in violent air

overpressure and flyrock and probably back break. Too big of a collar will generate a boulder on top ofthe bench. It happens, particularly, when the top of the bench consists in different types of rock that have

to be fragmented.

An example of inappropriate fragmentation is Figure 7, in which fragmented ore is compared to abasketball. This figure represents a large part of the blasts in the mine. A large amount of boulders can

be verified.

31%

069%

Pattern that all thestemming are larger

than the expected

Pattern that thestemming is larger,

equal or smaller than

expected




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Figure 7 - Boulder results from blasting compared to a basketball.

Conclusion and Future StudiesMonitoring and analyzing field operations are important tools to compare what is being performed to the

results obtained. Observing field practices in the blasting process is of fundamental importance toachieve an adequate blast design and to obtain desired fragmentation. With better blast control the

blasting will be improved, generating better results and reducing the cost of subsequently operations.

This study showed that the mine has too much difficulty in planning and field practice. It is normal to

have such discrepancies, however, the data showed impressive differences, which, probably, means a

lack in management of D&B. The company needs to evaluate its Blast Design and find why the blasters

are not following it.

ReferencesOlofsson (1990),Applied Explosives Technology for Construction and Mining(pp 62118). Sweden.

Konya, J.C. (1995),Blast Design(pp 6894). USA, Ohio: Intercontinental Development Corporation

Montville.

Bozoic, B. (1998), Control of Fragmentation by Blast. Croatia.

Dick, R.A. (1983),Explosive and Blast Procedures Manual(pp 5774). USA: Bureau of Mines

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