Surface blasting - Blast design parameters

The controllable parameters of blasting are divided into three groups - 

• (A) Blast design parameters

• (B) Physicochemical parameters of explosives

• (C) Initiation and delay timings

(A) The basic blast design parameters are:

• H – Height of bench

• D – Blasthole diameter

• L – Drilled length of blasthole

• J – Sub drilling

• B – Burden

• S – Spacing

• I – Charge length

• T – Stemming length

H – Height of the bench

The vertical distance between the crest and toe of the bench is called the height of the bench. 

Figure 1  Opencast bench view

D – Blasthole diameter

The drill holes or the blasholes are drilled with the help of a drill machine. The blastholes are normally drilled in a circular shape. The diameter of the blasthole is the blashole diameter.

Figure 2  Opencast bench view with blasthole showing the diameter of the hole.

L – drilled length of blasthole

The length of the drill hole from top to bottom is the drilled length of the blasthole or drill hole. 

Figure 3  Opencast bench view with blasthole showing the length of the drill hole.

J – sub drilling

Subdrilling J is the length of the blasthole underneath the floor level which is needed to break the rock at bench height and achieve adequate fragmentation.

If the subdrilling is small, the rock will not be completely sheared off at floor level, which will result in toe appearance. 

Figure 4  Opencast bench view with blasthole showing the length of the drill hole and subdrilling length.

B – burden

The burden B is the minimum distance from the axis of a blasthole to the free face.

Figure 5  Opencast bench view with blasthole showing Burden distance.

S – spacing

Spacing S is the distance between blastholes in the same row.

Figure 6  Blastholes showing Spacing distance.

I – charge length

Charge length is the length of explosive charge inside the blasthole. 

T – stemming length

Stemming is the portion of blasthole that has been packed with inert material above the charge so as to confine and retain the gases produced by the explosion, thus improving the fragmentation process.

Figure 7  Blastholes showing Charge length and stemming length.

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