BLAST DESIGN IN OPENCAST MINES

AND CONCEPT OF OPTIMUM

BLASTING

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BLASTING (TECHNO ECONOMIC CONSIDERATIONS)

***BY: PARTHA DAS SHARMA

(E.MAIL: sharmapd1@gmail.com)

DESIGN OF BLAST IN

OPENCAST

BLASTING OPERATION

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The fundamental concepts of an ideal blast design should be adopted for carrying out any blast, which are then modified as per

geologic conditions

There are two types of variables in blasting design

v Uncontrollable - The variables over which we do not have control, such the geology, rock - characteristics, and regulations or local specifications (such as the distance to the nearest structures) etc.

vControllable variables - The variables over which we have

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vControllable variables - The variables over which we have control . These are listed below:

§ Hole diameter

§ Hole depth

§ Sub-grade drilling

§ Stemming distance

§ Stemming material

§ Burden and spacing

§ Number of holes in the blast

§ Direction of rock movement

§ Delay Timing

§ Types of explosive and initiation system

BURDEN§ Burden is defined as the shortest distance to relief at the time of blast.

§ Fixing proper burden is one of the most important decisions made in any blast design.

§ If the burden is too small, rock is thrown at a considerable distance from the face, air -blast levels are high and the

In practice, Burden = ½ to 1/3 In practice, Burden = ½ to 1/3 In practice, Burden = ½ to 1/3 In practice, Burden = ½ to 1/3 of Bench height.of Bench height.of Bench height.of Bench height.

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distance from the face, air -blast levels are high and the fragmentation may be excessively fine.

§ If the burdens are too large, severe back-break occurs behind the last row of blast holes.

§ Excessive burdens cause over-confinement of blast holes, which result in high levels of ground vibration per weight of explosive being used.

§ Rock breakage can be extremely coarse and often causes bottom or toe problem, when burden is kept large.

SPACING

§ The distance between adjacent blast-holes, measured perpendicular to the burden, is defined as the spacing.

§ Spacing calculations are a function of burden.

§ If spacings are significantly less than the burden, it tends to

Spacing = 1.0 to 1.5 of Burden

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§ If spacings are significantly less than the burden, it tends tocause early stemming ejection and premature splitting betweenblast holes due to the rapid release of gases to the atmosphere,and result in noise and air blast.

§ When the spacings are too large, the fragmentation of rock maybe poor, and may cause an uneven floor.

§ Consequently, burden and spacing decisions are made bycareful analysis of geology, explosives, condition at the site, andexperience.

STEMMING DISTANCE

§ A stemming distance of 0.7 to 1.0 of Burden is adequate to keep material from ejecting prematurely from the hole.

§ But if the blast is poorly designed, the above stemming distance may not be adequate to keep the stemming from blowing out.

§ A delicate balance exists between not enough or too much stemming.

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stemming.

§ Fly-rock and excessive air blast can be caused when there is not enough stemming.

§ Too much could lead to excessive vibration and poorfragmentation.

Stemming Distance = 0.7 to 1.0 of Burden

OPTIMUM SIZE OF STEMMING MATERIAL

§ The most common material used for stemming is drilling cuttings, since they are economic and conveniently located at the collar of the blast hole.

§ If we want to minimize the stemming depth in order to break cap rock we should use the proper size of stemming material.

§ Very fine drilling dust will not hold in the blast hole and is easily ejected; very coarse material tends to bridge the hole when loading and may be ejected; round gravel or sand from river will not function as well

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may be ejected; round gravel or sand from river will not function as well as crush stone.

§ The optimum size of stemming material would be the material that has an average diameter of approximately 0.05 times the diameter of the blast hole.

§ Material must be granular to function properly, never use a coarse, large, or sharp rock that could damage the initiating system.

§ If the drill holes are wet to the collar, the use of crushed stone can result in better fragmentation and control.

SUB-GRADE DRILLING

§ Blast holes may not break to full depth, especially when blasting takes place in dense rock.

§ Too much sub-drilling is waste and may cause excessive ground vibration because of the increased confinement.

§ Insufficient sub-drilling can cause a high bottom, ultimately causing excessive equipment wear and the need for secondary

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causing excessive equipment wear and the need for secondary blasting.

§ As per rule of thumb, sub-drill holes depth equal to 20 % to 50 % of the burden, i.e., sub-grade drilling J = 0.2 to 0.5 of Burden.

Sub-grade drilling is approximately 0.3 of Burden

INCLINED HOLE DRILLING

Advantages of angle drilling :

§ Less back-break.

§ Less problem at grade, i.e., bench stability is better.

§ More throw, especially on low benches

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§ More throw, especially on low benches

§ Better fragmentation on low benches

§ Loose rock better held on face by gravity

Drilling of Inclined Holes depend mainly on availability of Drilling Machines in the mines

Selection of Blast Hole Size and Bench Height

For selection of blast hole size following points to be considered:

§ Drilling economics

§ The effect of fragmentation

§ Air blast, fly-rock, and ground vibration

§ The larger diameter of blast holes tend to be more economic, but would cause more problems with Air blast, Fly-rock, Ground vibration and

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cause more problems with Air blast, Fly-rock, Ground vibration and Fragmentation.

§ To gain the best result of blasting with the best fragmentation and the least result of unwanted effects, the operator would consider the Stiffness ratio.

§ Stiffness Ratio is ratio of Bench Height to Burden (H/B)

Stiffness Ratio must be between 2.5 and 4

DECKING

§ The technique of decking is to divide the explosive column into two or more charges.

§ This is accomplished by loading inert materials, such as drill cuttings or crushed stone, between the explosive charges.

§ The purpose of decking is to give confinement of explosive gases where a soft seam or void is encountered, or to assure a better energy

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where a soft seam or void is encountered, or to assure a better energy distribution.

§ This technique is also used to reduce the explosive weight per delay when blasting in an area of vibration constraint.

§ This decision is often made upon analysis of scaled distancecalculation or analysis of seismic records.

Minimal deck thickness Td = 6 times D; where Td = decking thickness (mm) and D = borehole diameter (mm).

POWDER FACTOR OR EXPLOSIVES FACTOR

§ Powder factor is the mathematical relationship between the weight of explosives (kg) and a given quantity of rock (Cum).

§ To calculate cubic metres per borehole, the following formula is used:

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formula is used:

§ V = B x S x H; where B = Burden (m.), S = spacing (m.), H = bench height (m.) and V = rock volume (cubic metres.).

§ To calculate tons of rock per borehole, multiply V by thedensity of rock (tons/cubic meter).

EXPLOSIVE CONSUMPTION

§ The amount of explosive required to break the rock into an average size is called explosive consumption.

§ There are many factors in the value of explosive consumption such as the types of rock (rock strength) to be blasted, geometry & geology of the rock, kind of explosive to be used, size of borehole, delay systems, initiation systems, product size etc.

§ Blasting with more free face consumes less than blasting with less

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§ Blasting with more free face consumes less than blasting with less free face. Loose rock needs less explosive than dense rock and so on.

§ In any blasting, the operators need to try and adjust the amount of explosive to achieve the optimum product size.

§ One must consider crushing and grinding cost compared with blasting cost.

§ In some operations, particularly when blasting in hard rock, drillingcost must also be considered.

BLAST HOLE INITIATION SEQUENCING

§ In this system each charge is given sufficient time to break its quota of burden from the rock mass before the next charge detonates.

§ By using proper delay sequence the ground vibration, air blast and fly-rock are minimized and the fragmentation is increased.

§ If sufficient free face is not available, an inner blast hole may

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§ If sufficient free face is not available, an inner blast hole maycrater upward, resulting in poor fragmentation, little forwarddisplacement, and an increase in the possibility of fly-rock & over-break, while increasing ground vibration and air blast.

§ The delay interval necessary for optimum fragmentation varies with the type of rock and burden distances.

§ It appears that delay intervals of more than 8 millisecondsbetween adjacent blast holes in a row provides the best result inrespect to ground vibration and fragmentation.

SOME EXAMPLES OF DELAY PATTERN

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FORMATION OF FLY-ROCK

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FLOW SHEET FOR BULK-LOADING EMULSION EXPLOSIVES

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Summary of Blast Design

Three Blast Design Keys To Optimum Explosive Performance

1. The explosive energy must be evenly distributed to achieve uniform fragmentation. This requires:

• Proper hole diameter to bench height

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• Proper hole diameter to bench height relationship

• Appropriate burden to spacing relationship

• Careful implementation of the design

• Angle drill in some cases to match existing face conditions

Summary of Blast Design

Three Blast Design Keys To Optimum Explosive Performance

2. The explosive energy must be confined long enough after detonation to establish fractures and to displace material.

• The explosive's path of least resistance must be

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• The explosive's path of least resistance must be controlled

• Blast holes must be loaded according to geology

• Use the proper stemming length and stemming material

• Match the timing configuration to the existing field conditions

• Use accurate delays

Summary of Blast Design

Three Blast Design Keys To Optimum Explosive Performance

3. The energy level must be sufficient to overcome the structural strength and mass of the rock, while providing controlled displacement.

• Determine energy level on the degree of

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• Determine energy level on the degree of fragmentation and displacement required

• Account for site sensitivity

• Evaluate high energy explosives for special site conditions (i.e. large toes, poor floor)

• Maintain explosive quality control with routine testing

CONCEPT OF OPTIMUM BLASTING

TECHNO ECONOMIC

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TECHNO ECONOMIC CONSIDERATIONS

OPTIMUM BLASTING

INTRODUCTION:

In most mining operations the ore undergoes several processes such as drilling, blasting, loading, hauling, crushing and grinding to become the final stable product.

Drilling and blasting is an important step and it’s result such as fragmentation, muck-pile shape & looseness etc. effect the efficiency and productivity of downstream processes.

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Drilling and blasting costs constitute nearly 20 to 30 percent of total cost of drilling, blasting, shoveling, hauling and crushing operations.

Because of high proportion of cost of shoveling, hauling and crushing, better fragmented well shaped and loosened muck-pile has greater shoveling, hauling and crushing efficiency; and lower the over all cost of mining.

At the same time for achieving better fragmentation and loosened muck-pile the drilling and blasting cost has to increase.

OPTIMUM BLASTING

Therefore, the optimum blast design is a holistic approach by identifying and measuring the leverage that blast results have on different downstream

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results have on different downstream processes and then designing the blast to achieve the results that maximize the overall profitability rather than just minimizing the drilling and blasting costs.

OPTIMUM BLASTING

The concept developed by Mackenzie describes the total blasting cost as: Ct= Cd + Cb + Cl +…+ Cc

P

Where,

Ct = Total blasting cost per tonne of finished product,

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Ct = Total blasting cost per tonne of finished product,

Cd = Cost of drilling , Cb = Cost of blasting,

Cl = Cost of loading, Ch = Cost of Hauling,

Cc = Cost of Crushing,

P = Total tonnage of production.

OPTIMUM BLASTING

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IMPORTANCE OF DEGREE OF PERFORMANCE

Fragmentation levels required depend on the type of the excavating and hauling equipment.

Fragmentation coarser than optimum results decreases the loading efficiency and increase in the downtime of loading equipment with added cost of maintenance.

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equipment with added cost of maintenance.

On the other hand, to achieve fragmentation finer than optimum, cost of drilling and blasting may rise disproportionately, which may not be offset by the additional advantage in loading.

The blast design should, therefore, be aimed at optimum fragmentation to economise the overall cost of mining.

IMPORTANCE OF DEGREE OF PERFORMANCE

Once the optimum fragmentation requirements for a particular mining equipment is decided, the question arises as to determine which would need secondary breaking.

Any attempt to produce "Zero Boulder Blast" would severely affect the drilling and blasting cost.

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The optimum ratio of percentage of oversize boulders against the volume of rock may be taken as 1-2%.

Efficient explosives application is the least expensive method of fragmenting and casting rock. Blast casting the over burden off coal dramatize the economic advantages of using explosive energy to increase production rate and enlarge over all operating profit margins.

IMPORTANCE OF DEGREE OF PERFORMANCE (BLAST CASTING)

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ENERGY FACTOR

Energy factor describes the energy distribution within a given unit rock. Energy distribution within a shot is measured by Energy factor, which compares the explosives energy to a quantity of rock broken. The explosive energy

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quantity of rock broken. The explosive energy distribution within the entire blast is then evaluated with its resultant fragmentation and its effect on operating costs. Blasting analysis now become a function of the Energy factor, Explosive cost, Fragmentation results and subsequent production.

ENERGY FACTOR

Proper Energy distribution is important in obtaining the desired fragmentation and movement of bottom or toe portion of the shot.

Higher Energy explosive such as Emulsion based product can be substituted for lower energy explosives (ANFO) to increase energy distribution within the rock, thereby increasing fragmentation and elimination of toe & secondary blasting problem.

This become eminent when hard rock / tough rock mining is done.

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The additional incremental cost associated with the use of higher energy explosive in the bore hole is more than offset by easier digging of a well-layout and fragmented muck pile.

In fact, in hard rock mining upgrading to an explosive with higher energy can have extremely positive result when we look at total production cost.

Underutilizing explosive energy shows up not only right after the shot as with poor fragmentation, but months later when operation is faced with excessive equipment maintenance costs.

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