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DOI 10.1007/s10064-010-0298-7

ORIGINAL PAPER

a risk based approach

A. K. Raina • A. K. Chakraborty • P. B. Choudhury •

A. Sinha

Received: 22 September 2009 / Accepted: 16 April 2010 / Published online: 21 May 2010

Ó Springer-Verlag 2010

Abstract Flyrock, a rock fragment thrown to an exces- structures existent à moins de 100 m de distance. Une

sive distance, is a random event and an ongoing problem in approche statistique du problème est proposée et de nou-

opencast bench blasting. Existing criteria for a ‘Flyrock veaux concepts de facteur de sécurité, de niveau de risque

Danger Zone’ are rigid, such that blasting may not be et de risque de projection ont été élaborés afin de préciser

permitted where there are structures within about 100 m. des classes de risque pour différentes conditions d’exploi-

A statistical approach to the problem is proposed and new tation minière. Les nouveaux critères permettent à l’ing-

concepts of Factor of Safety, Threat Levels and Flyrock énieur des mines de préciser le niveau de confiance de

Risk have been introduced in order to elucidate risk classes pratiques d’abattage en termes de probabilité et de risque.

for different geo-mining conditions. The new criteria allow L’approche est unique, avec un accent mis sur les types

the mining engineer to work out the confidence level of d’abattage et les niveaux de risque qu’un ingénieur spé-

the blasting practice in terms probabilities and risk. The cialisé dans l’abattage peut accepter sans porter atteinte aux

approach is unique, with the emphasis on the categories of objectifs de production et en même temps en contrôlant les

blasting and degree of risk that a blasting engineer can distances de projection. La zone de danger dynamique

afford without sacrificing production and at the same time proposée donne à l’ingénieur la possibilité d’ajuster les

controlling the travel distance of the flyrock. The proposed opérations d’abattage prenant en compte les exigences de

dynamic danger zone gives the engineer flexibility to adjust sécurité et de production.

blasting operations to take account of safety requirements

and production demands. Mots clés Abattage à l’explosif en exploitation à ciel ouvert

Zone de danger de projection Facteur de sécurité

Keywords Opencast blasting Flyrock Danger Zone Analyse de risque

Factor of Safety Risk analysis

un événement aléatoire et un problème courant pour les

opérations d’abattage à l’explosif dans les exploitations Blasting in opencast mines is intended to both produce

à ciel ouvert. Les critères existants pour définir une zone smaller rock fragments from the bench and simultaneously

de danger dû à des projections sont rigides, de sorte que throw such material or muck an optimal distance for the

l’abattage à l’explosif peut être interdit lorsque des mucking and hauling equipment. Rock fragments which

travel beyond the desirable distance are known as flyrock

and may arise as a consequence of the blast design or its

A. K. Raina (&) A. K. Chakraborty P. B. Choudhury application in the field, or when the rock and face condi-

A. Sinha tions are not properly taken into account in the design or

Regional Centre, Central Institute of Mining and Fuel Research,

charging of a blast. Flyrock can result in injuries or even

3rd Floor MECL Complex, Seminary Hills,

Nagpur 440 006, India fatalities as well as damage to properties and/or equipment,

e-mail: rainaji@gmail.com here referred to as ‘‘object(s) of concern’’.

123

164 A. K. Raina et al.

engineers as it is a random phenomenon. However, it has

received relatively little attention from researchers due to

the complex nature of the interaction between the blast

design and rock parameters. To date, most research has

focussed on the prediction of the maximum throw of flyrock

and the initial velocity of the rock fragment projected from

the blast face. Such workers as (Bajpayee et al. 2004; Raina

et al. 2006; Bhowmik et al. 2004) detailed the major reasons

for and control of flyrock with an exhaustive literature

survey. The classical studies by (Ladegaard-Pedersen and

Holmberg 1973) worked out the travel distance and estab-

lished relationships between specific charge/loading density

of the explosive and flyrock.

To determine the value of k (travel distance), measure-

ments of /qv were made for different charge diameter (d)

values in granite, Lundborg et al. (1975) proposed Eq. 1:

/qv=2; 600 ¼ 10d ð1Þ

where / is the boulder diameter in metres, q the density of Fig. 1 Observed versus predicted flyrock distance (calculated from

rock in kg/m3, v the velocity of flyrock in m/s and d the Lundborg’s-1974 equation)

diameter of the drill hole in inches.

This author also proposed a semi-empirical approach

a probability of occurrence for a specified initial fragment.

based on Eq. 1 to estimate flyrock throw distance. Based

He emphasised the importance of an assessment of risk, in

on conservation of momentum and the scaling laws of

order to make blasting operations more flexible.

spherical charge, a relationship between charge diameter

(d) and rock velocity (V) was obtained. The relationship for FðxÞ ¼ 1 eðR=R0 Þ ð5Þ

determining the initial velocity (V0) of fragments in crater

where R0 and R are parameters of the equation, F(x) the

blasting in granite blocks is given in Eq. 2 below.

cumulative distribution of the flyrock and R the distance

V0 ¼ ð10d 2; 600Þ=ðTb qr Þ ð2Þ from the blast (round).

Considering a 76 mm diameter blast hole and a distance

Lundborg et al. (1975) determined the flyrock range

of 600 m, the probability of being struck by a flyrock was

based on ballistic trajectories (Eq. 3) and the size of the

estimated by Lundborg (1974) to be the same as being

flyrock (Eq. 4) taking into account that the product

struck by lightening (1 in 10,000,000).

V0 9 Tb 9 qr depends on the diameter of the blast hole.

Gurney (1943) developed a model (Eq. 6) to describe

Lm ¼ 260d2=3 ð3Þ the expansion of a metal cylinder driven by the detonation

of an explosive. The model closely predicts the initial

Tb ¼ 0:1d2=3 ð4Þ

velocity of the fragments produced by the break-up of the

where Lm is the flyrock range in m, d the hole diameter in cylinder. He assumed the partition of energy between

inch and Tb the size of rock fragment in m. a metal cylinder and gases driving it and a linear velocity

There are several drawbacks in the above prediction gradient in the expanding gases.

method (Hustrulid 1999). In the present study, an estimate

fV=ð2EÞ0:5 g ¼ fðM=CÞ þ ð1=2Þg 1=2 ð6Þ

of the flyrock travel distance was made following Lund-

borg but, as shown in Fig. 1, the correlation between the where V is the velocity of metal, M the mass of metal, C the

predicted and observed flyrock travel distance was very mass of charge and 2E a constant, the Gurney’s velocity

low. This may be because the equation was developed for coefficient, which is specific to a particular explosive. The

granite and is not fully applicable to other rock types. equation assumed different forms for different explosives.

Lundborg (1979) also suggested that Eq. 3 may not However, a simple method to calculate the constant is

always hold good as there is a probability that some flyrock given in Eq. 7.

will be thrown a longer distance. He considered the proba-

ð2EÞ0:5 ¼ VOD=2:97 ð7Þ

bility of damage due to flyrock with reference to the expo-

nential distance (Weibull distribution, Eq. 5) assuming where VOD is the velocity of detonation of the explosive (m/s).

123

Flyrock demarcation zones in opencast mines 165

Roth (1975) attempted to determine the flyrock travel concern vary, hence a single fixed rule does not appear to be

distance by estimating the initial flyrock velocity (V0) and logical. Rather, there should be a method that allows the mines

a factor 2E (Gurney 1943), see Eq. 8. to specify their own danger zones depending on their priorities

and the level of confidence at which they want to work.

V0 ¼ ð2EÞ0:5 ðql=mlÞ ð8Þ

An attempt has been made here to establish a flyrock

where (2E)0.5 is Gurney’s constant, a function of explosive, risk evaluation methodology that helps to strike a balance

ql the linear charge concentration and ml the total mass of between flyrock occurrence and productivity and can

material per unit of length. The parameter 2E is difficult to take into account specific design parameters and the con-

calculate owing to the host of parameters that control the ditions which mitigate towards or against the occurrence of

motion of flyrock. flyrock. Alén et al. (2000) proposed a similar approach to

Fletcher and D’Andrea (1987) investigated three major establish a risk evaluation methodology for landslides.

causes of accidents due to blasting in surface mines

(inadequate blasting area security, excessive flyrock and

misfires) and defined different flyrock safety zones. Rich- Development of a risk method for demarcation

ards and Moore (2004) proposed a methodology to predict of a Flyrock Danger Zone

throw and flyrock taking burden and stemming into con-

sideration and determined the Flyrock Danger Zone around Basis of risk analysis

a blast. Previously, Davies (1995) had suggested a way of

determining the danger zone based on risk analysis. The The objective of a risk analysis is to establish the proba-

frequency of impact by ‘‘wild flyrock’’ (travelling [300 m bility that an event will occur and the consequences if it

from the blast site) for a single shot, is given in Eq. 9. does. From a statistical point of view, this is equivalent to

the expected loss as given in Eq. 10.

I ¼ Nfpd pp pe ð9Þ

Risk ¼ Expected loss ¼ Probability Consequence ð10Þ

where I is the target impact frequency (impact/year), N the

total number of blasts per year, pd the probability of wild However, evaluating the risk is often associated with a

flyrock travelling the target distance, pp the probability of number of problems such as accuracy in estimating the

wild flyrock travelling on an impact trajectory, pe the probabilities, expressing different consequences in the same

probability of target exposure. reference units etc. Risk can also be expressed as Eq. 11:

This method requires extensive data to establish the risk Risk ¼ Probability of failure of a safety rule

and may not be viable as flyrock data are rarely docu- Cost of the failure of the safety rule ð11Þ

mented or reported. In addition, it does not take account of

A conceptual representation of the facts related to

blast practices and geo-mining conditions and is based on

blasting and the basis of possible risk treatment to flyrock

distances more than 300 m from the blast site. Currently,

is given in Fig. 2.

the maximum flyrock distance should be no more than half

An attempt has been made to define the risk criteria for

of the distance to the object of concern. With such a rigid

the demarcation of a Danger Zone for flyrock with the

definition, it is imperative that the risk criterion for flyrock

following methodology.

should be simple and easy to use and evaluate.

St George and Gibson (2001) used a probabilistic 1. Devise a ‘‘Safety Rule’’ for flyrock. The ‘Factor of

approach based on Monte–Carlo simulation and with the Safety’ for flyrock (a dimensionless parameter) is

analytical model and risk analysis estimated safe standoff introduced for this purpose.

distances in relation to flyrock. 2. Classify the Factor of Safety into different categories

As noted above, as flyrock is a random phenomenon, the to define the safe and failure values.

approach adopted by earlier researchers either do not hold 3. Define the probability density function of the Factor of

good or do not present a complete solution. As a conse- Safety.

quence, a method to both evaluate the probability of the 4. Define a parameter for ‘‘Consequence’’, which would

flyrock occurrence and demarcate the Flyrock Danger Zone represent the cost of failure of the Safety Rule in

is urgently required. The Danger Zone for blasting in indirect terms, as direct cost estimation is not possible.

respect of flyrock is a relative one as the priorities for A new parameter ‘‘Threat Level’’ or ‘‘Distance Ratio’’

different mines may be different. Previously, damage to is devised for this purpose.

persons or property not belonging to the mine owner was 5. Define the risk criterion in terms of a Factor of Safety

the prime consideration while today the safety of personnel and Distance Ratio and possible modalities for ascer-

and equipment associated with the mine is also considered taining confidence levels for blasting in general and

important. Clearly, the distances to the various objects of flyrock in particular.

123

166 A. K. Raina et al.

toxic fumes/dust and their effects.

While most of the above can be determined due to their

regular nature, flyrock is a random phenomenon and hence

only probabilities can be determined. A Factor of Safety

for flyrock is proposed, which can account for the varia-

tions in throw for similar blasting conditions. Such a Factor

of Safety must be a dimensionless parameter so that it

represents a balance of opposing factors/forces in the

process of blasting. This requires identification of different

blast design parameters. The interactions which such

parameters represent should be logical. Such an attempt is

presented in Table 1.

To ascertain the reliability of the conceptual model, test

studies were conducted on concrete models (Raina et al.

2006). A comprehensive field investigation was also

undertaken in nine different metal mines in India. All pre-

and post-blasts parameters were measured; flyrock distance

was measured manually and by a laser range finder. The

verification of the flyrock distance was done using a high-

speed motion camera, which aided in finding the initial

velocity of the flyrock as well as calculating the flyrock

distance. Cross-checks were also made with normal video

cameras. A summary of the data from the 100 real-time

blasts used for the empirical modelling is given in Table 2.

The data were categorised in the following main fields.

1. Rock parameters: joint spacing, joint orientation,

Fig. 2 Systematic approach to flyrock risk evaluation density.

2. Blast design parameters: burden, spacing, hole diam-

6. Identify risk classes for different combinations of the eter, charge configuration.

Factor of Safety and Consequence parameters. 3. Explosive parameters: density and type.

7. Demarcate a flyrock ‘Danger Zone’ around a blast that 4. Post blast results: throw, flyrock distance (both hori-

varies with geo-mining conditions and the blasting zontal and vertical), fragmentation.

practices of a mine, i.e. one which is dynamic and not

The findings of analyses of the data obtained from

rigid, in contrast to the available criteria.

models and field trials is summarized below:

1. The blast design parameters can be explained in terms

Development of models for Factor of Safety

of the drill diameter.

2. The blast design parameters can take a range of values

The primary objective of blasting is to fragment the rock

for different rock types.

mass into an economically viable size, produce a muck

3. The blast design parameters thus can present a

profile that is convenient for effective lifting by machines

variation which can result in different distances of

and simultaneously reduce the environmental impacts due

throw of the material and eventually flyrock occur-

to explosions. Blasting is a complex process that involves

rence or its travel distance.

the interaction of different parameters in time and

4. Blast design parameters can be resolved into two

space. Such interactions manifest in different forms

categories viz. favouring and resisting flyrock.

which may be favourable or unfavourable to the objective

5. A correlation of parameters with the flyrock distance

of blasting.

was obtained which defined the relative importance of

(a) The favourable include: fragmentation, throw, and the parameters vis-à-vis flyrock distance.

muck profile. 6. Partial correlation of the parameters helped to establish

(b) The unfavourable are: ground vibration and related the factors that were most important for flyrock and

structural damage, air overpressure, noise and related their nature while blasting (Fig. 3).

123

Flyrock demarcation zones in opencast mines 167

Type Parameters Nature Quantification

Joint properties Inherent

Joint characteristics

Uncontrollable 2 Presence of weak zone Occasional Improbable, important for flyrock

Presence of voids Random/probabilistic

Controllable 1 Drill diameter Systematic Can be measured with accuracy and derivation

of their statistics is possible

Burden Inherent

Spacing

Stemming

Charge length

Controllable 2 Initiation type Systematic Can be measured but with certain reliability

Initiation sequence Mostly inherent

Uncontrollable 3 Face condition Unsystematic Difficult, partially possible

Over-confinement Occasional

Parameter Mean Standard error Median Mode Standard deviation variance Sample Min Max

Average joint spacing (m) 0.53 0.03 0.51 0.60 0.27 0.07 0.10 1.20

3

Specific charge (kg/m ) 0.48 0.03 0.38 0.59 0.30 0.09 0.11 1.75

Charge/hole (kg) 62.68 5.04 44.63 64.98 53.60 2,873.00 3.71 180.30

Burden (m) 3.72 0.11 3.87 4.84 1.17 1.38 0.82 6.05

Spacing (m) 4.22 0.15 4.00 4.00 1.57 2.48 1.25 7.50

Stemming length (m) 3.39 0.11 3.19 2.94 1.17 1.38 0.39 5.88

Charge diameter (m) 0.12 0.00 0.15 0.08 0.03 0.00 0.08 0.17

Bench height (m) 8.16 0.30 8.50 7.00 3.20 10.27 0.90 14.00

Drill depth (m) 8.65 0.33 9.00 10.00 3.46 11.94 0.90 14.50

Charge length (m) 5.26 0.24 5.57 4.56 2.55 6.50 0.28 11.36

Density of rock (kg/cm3) 2.46 0.03 2.40 2.40 0.34 0.12 1.80 3.50

Density of explosive (kg/cm3) 0.96 0.01 0.90 0.90 0.08 0.01 0.85 1.10

Horizontal flyrock, LN (m) 3.19 0.10 3.30 2.89 1.02 1.03 0.69 4.98

mathematical expression so that a dimensionless

parameter is obtained.

Based on the statistical correlations, the following param-

eters were identified for the development of a model for the

Factor of Safety. The classification of the parameters vis-à-vis

their relative importance to flyrock is given in Table 3.

The parameters identified (Table 3) were thus resolved

into a Factor of Safety (FSH) for flyrock (Raina et al. 2007)

defined as a balance in the factors that are contributing and

resisting the occurrence of flyrock. The mathematical

expression is given in Eq. 12.

Bd 1

!

dc Jfr

FSH ¼ Cf ð12Þ

rc

Fig. 3 Regression coefficients for different parameters analysed

123

168 A. K. Raina et al.

Sl. parameter Symbol Correlation Horizontal Vertical

1. Burden Bd -ve Y N

2. Charge diameter dc ?ve Y Y

3. Joint/rock factor rating Jfr -ve Y Y

4. Linear charge concentration rc = ql/hd ?ve N Y

ql Length of the explosive charge, hd hole depth

given in Table 4.

Figure 4 explains the relationships for FSH with the

distance of flyrock. The data in Fig. 4 have been limited by

upper and lower bound lines corresponding to the 95%

confidence level. It can also be observed from the plot that

the flyrock distance assumes a range of Factors of Safety,

hence a different classification system for different ranges

of the FSH is proposed that gives a range of flyrock dis-

tances (Table 5), in contrast to earlier equations which give

a fixed value. It will be noted that there is a limit to the

flyrock distance observed during these trials; the equation

Fig. 4 Flyrock distance (m) versus Factor of Safety (FSH)

may need to be re-assessed for higher ranges of flyrock.

The reliability of the model vis-à-vis related interactions

of the factors that define Eq. 12 can be worked out with the

Table 5 Classification of Factor of Safety for flyrock (Raina et al.

help of the following expression (Eq. 13).

2007)

l

a ¼ ln P ¼ 0:798 ð13Þ FSH Flyrock safety Flyrock distance (m)

rln P

where llnP and rlnP represent the respective mean and \0.5 Unsafe [40

standard deviations of the individual components. As is 0.5–1.0 Likely unsafe 40–15

evident, the reliability of the model is significant i.e. *0.8. 1.0–2.0 Safe 15–5

The Factor of Safety (FSH) defined above has the fol- [2 Very safe B5

lowing advantages.

1. The FSH is easy to work out in terms of blast design

and rock parameters. 3. FSH incorporates extreme conditions of blast design

2. Both blast design and rock conditions are reflected in and rock mass characteristics for which corrections can

the FSH. be applied.

Role Condition Correction

Special techniques applied factor

Favourable 1. Decking (bottom, top, middle—solid or air decking that reduce charge concentration) 1.1–1.2

2. Use of Nonel-Shock tube combination that reduces flyrock due to bottom initiation

3. Use of in-hole multiple delay technique that divides charge in several segments in a single hole and that blast at

different times

4. Stemming methods (nicely tamped, use of stone crusher chips etc.)

Unfavourable Choke blast or Solid blasting 0.7

Weak zones (both in horizontal and vertical direction (if no measures are taken)

1. Karst features (presence of cavities in the rock mass) 0.5–0.6

2. Weak layers within competent rock 0.67

3. Use of detonating fuse as in-hole initiation system 0.80

123

Flyrock demarcation zones in opencast mines 169

to objects of concern

Targets Nature, scope CCost Dobj(r)

Category A Belonging to the owner of the mine

Consequence

Personnel Injury Y Nearer

Fatality D/N

Machinery Damage Y

Equipment Damage Y

Buildings Damage Y

Roads Damage/blockade Y

Others Damage Y

Category B Not belonging to the owner of mine

Consequence

Fig. 5 Distribution of Factor of Safety People Injury Y Farther

Fatality D/N

4. FSH can take into account special blasting practices Livestock Injury Y

that help in controlling the flyrock. Fatality D/N

5. The variations in blast design parameters and rock Houses Damage Y

conditions produce a significant range of FSH to Roads Damage/blockade Y

evaluate the reliable statistics in field blasts through Others Damage Y

trials and probability of the factor being less than CCost feasibility of cost analysis, Dobj(r) relative distance of object of

minimum acceptable value of 1.0. This gives a method concern, D difficult, N not possible

to devise a safety rule for flyrock. This implies that in

order to establish the statistics of the FSH, it is

new term—Threat Ratio which can be calculated in terms

imperative for the user to undertake field trials.

of distance and hence can also be called Distance Ratio.

6. If the Factor of Safety is[1.0 the blast practice may be

‘‘Threat Ratio’’ or Distance Ratio (Rd) in the present

considered safe but the standard deviation will deter-

context has been defined as the ratio of the permissible

mine the level of confidence.

distance of flyrock and the distance of the object of concern

7. The risk due to flyrock decreases exponentially for

(Eq. 14).

increasing values of Factor of Safety from 0 to 1.

Hence, the FSH is resolved into a linear factor by using Rd ¼ Dperm =Dobj where Dperm Dobj ð14Þ

Ln(FSH) for ascertaining the distribution. As seen in where Dperm is the permissible distance of flyrock and Dobj

Fig. 5, the Ln(FSH) shows a normal distribution. the distance of object of concern.

8. The probability of ‘FSH \ 1’ (Ps) defines a Safety Rd = 1 is the maximum possible ratio owing to the

Rule, that indicates unsafe conditions with respect to condition Dperm B Dobj. This also indicates that with a

flyrock. This safety rule can be used in evaluating risk. reduced value of Rd there is an increased level of confi-

dence. It gives mine owners a flexible criterion, as the

Defining the consequence: threat level ‘Distance Ratio’ can be varied to achieve a condition where

a flyrock zone can be defined with an anticipated level of

Cost or consequence, which is an integral part of any risk confidence.

analysis, is not possible in the case of flyrock (at least in the

present study) due the number of factors involved in cost Risk criterion for flyrock

analysis. In addition, the wide range of objects of concern

presents a very difficult situation for cost analysis, as Equation 11 can be reproduced or re-written here for

shown in Table 6. The process is further complicated by obtaining a risk criterion (Eq. 15) for flyrock.

the limited amount of data available on the occurrence of Risk ¼ Probability of failure of a safety rule for flyrock ðPs Þ

flyrock and related costs and the policies of different threat level or distance ratio ðRd Þ

mining companies regarding documentation issues and

legalities. Hence, the term ‘Consequence’ is replaced by a Risk ¼ Ps Rd ð15Þ

123

170 A. K. Raina et al.

Class Risk Category

B Negligible risk Desirable

C Calculated risk Acceptable with trials

D Moderate risk Extreme caution

E High risk Not acceptable

(c) b = (l/r).

(d) Ps = Z score of b on normal curve (from

Standard tables). Ps has been calculated for

some examples in Fig. 6.

6.

Work out the distance of objects of importance, Dobj

7.

Assume a safe distance, Dperm (1/2 of actual Dobj m

Fig. 6 Risk criterion for horizontal flyrock with examples of field

data in the case of mines in Fig. 6).

8. Distance Ratio (Rd) = Dperm/Dobj

9. Plot the Ps (Y) and Rd (X) on the graph yielding a

Risk can be identified with the help of FSH, the single point in the graph (Fig. 6, shaded circles).

probabilities of which can be worked out for a mine with 10. Identify the risk class and trade off Dperm to achieve

ease. The factor Ps is derived from the probability density desirable class and confidence level for flyrock.

function of the FSH, as explained earlier. Rd is very easy to

calculate and can be simulated over a wide range of 0.1–1.0. Demarcation of Flyrock Danger Zone

Figure 6 explains the risk criterion for flyrock together

with actual field blast data for illustration. Based on Fig. 6, The risk criterion defined above can be used to demarcate

a risk classification (Table 7) has been devised for flyrock. the Flyrock Danger Zone in the following manner

In view of the above, a new definition of flyrock is

proposed: 1. Define the plot of Ps (Y) and Rd (X) for a mine (see

A probabilistic phenomenon with definable risk in terms above and Fig. 6).

of Factor of Safety and distance of object of concern, fly- 2. Identify the risk class (Table 9) from the plot.

rock is a rock fragment projected in different directions 3. Select the risk class suitable for an operation and

beyond expected or desired distances from the blast face, change Dperm values to fit the desired risk class (dark

owing to improper blast design and/or its application and/ circles in Fig. 6)

or presence of adverse rock conditions that favour venting (a) If the point falls in the suitable class lower

of pressurised blast-hole gases. boundary then Dperm can be increased to trade off

the danger zone; unless decided otherwise, Dperm

Method of Flyrock risk determination is the danger zone limit.

(b) If the point does not fall in the suitable class,

Table 8 provides field blast data for illustration purposes. reduce Dperm until it does, thus defining the

1. Determine the possible variations made in the danger zone.

controllable parameters of the mine(s) based on field (c) If Ps is too high and does not fall in the suitable

data. class, reconsider blast design and conduct new

2. Determine the geology of the area, whether weak trials by modifying blast pattern.

zones are present and type of corrective measure to Nine real-time examples are shown in Fig. 6 using the

be adopted. Calculate factors Cf as given in Table 4. basic data in Table 9 to illustrate the trade-off for Flyrock

3. Calculate the Factor of Safety using Eq. 10, over a Danger Zone demarcation; see Raina (2008) for more

range of individual parameters. examples. The examples in Fig. 6 cover most conditions

4. Calculate natural log, Ln (FSH).

for risk classification. The methodology, conclusions

5. Calculate:

and remedial measures based on the analysis are given

(a) Mean (l) of the Ln (FSH). in Table 10. It can be seen that using the proposed

123

Flyrock demarcation zones in opencast mines 171

Mine no. Range of parameters FSH statistics

Joint frequency rating Charge dia. (m) Burden (m) Charge length (m) Drill depth (m) l r

2 30–40 0.11 3.0–4.0 0.5–7.0 3.0–10.0 0.592 0.242

3 50–70 0.083 2.5–3.5 2.0–2.1 3.3–5.0 0.58 0.513

4 30–60 0.083 1.8–2.8 1.1–4.1 2.5–7.0 0.855 0.42

5 40–70 0.125–0.150 3.0–5.4 2.5–9.0 4.5–11.0 0.444 0.342

6 20–100 0.165 3.2–5.2 7.4–11.4 12.0–14.5 1.027 0.792

7 30–100 0.083–0.110 1.5–4.6 0.3–7.6 1.0–11.0 0.184 0.527

8 17–60 0.15 2.7–6.1 2.0–7.3 5.5–10.0 0.202 0.605

9 40–90 0.083–0.110 3.6–5.1 2.6–7.8 5.5–10.5 0.039 0.44

Mine Ps Dobj Existing norm Traded trade off results Comments

Dperm Rd Risk category Dperm Rd Risk category

2 0.0071 300 150 0.5 E 12 0.040 C–B 2

3 0.0084 400 200 0.5 E 10 0.025 C 2

4 0.0207 400 200 0.5 E 25 0.063 C 2

5 0.0701 400 200 0.5 E 25 0.063 D 3

6 0.0750 1,500 750 0.5 E 50 0.033 D 3

7 0.2550 350 175 0.5 E 50 0.143 E 4

8 0.3257 250 125 0.5 E 50 0.200 E 4

9 0.4191 300 150 0.5 E 50 0.167 E 4

Comments: 1 Objective achieved, 2 slight modification is required in blast design, 3 moderate modification and further trials are needed, 4 major

changes in blast design are anticipated

Reason Cause Throw Throw (horizontal) Impact Corrective measures

direction distancea

Low burden at top of blast Care/previous Front/top of Moderate Threat to nearby Check blast design, take

blast hole equipment care while charging

Low burden at top Care/previous Top and sides Moderate but Threat to people, Check face before drilling,

blast significant height equipment modify charge length

Less stemming Blast design Top and sides Low to moderate Threat to Increase stemming height

equipment

Face Indentation Previous blast Towards free High Threat to people/ Solid or air decking in the

side equipment zone of low burden

Solution cavity Natural Towards free Excessive throw Lethal to all Make judgements while

face sideb and flyrock drilling (deck this region

of blast hole)

Weak beds (out of face) Natural Towards free Moderate Threat to all Deck this region of the blast

face sideb hole

Weak beds (into the face) Natural Towards free High Threat to all Deck this region of the blast

face sideb hole

High burden of front row or Previous blast Top Moderate but Threat to Corrective drilling, increase

inappropriate delay/watery holes significant equipment stemming height

a

Relationship with the charge type and quantity (assertions are relative)

b

May change with location of the cavity (top if cavity is present at top)

123

172 A. K. Raina et al.

methodology it is possible to find a best permissible range the MOM, GACL and MOIL, the help provided by different mines

for flyrock while minimising the effect on production. In and A. Halder, P. Sahu, S. Bhowmik and P. Srinivas. The advice

rendered by Dr. C. Bandopadhyay, Sri. S. J. Sibbal, Dr. Asim Sinha

addition, it is possible to identify whether a practice is safe and Sri R. Guha, Dr. VMSR Murthy and Sri M. Ramulu is gratefully

or not for a particular operation. When the FSH has been acknowledged.

determined, a qualitative analysis of the blast design

parameters can be undertaken in order to establish which

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123