SHORT DELAY BLASTING IN UNDERGROUND MINING

113MN0493

M BALA SUBRAHMANYAM

SHORT DELAY BLASTING A method of blasting in which the detonation of  several explosive charges takes place in a specified  sequence over certain intervals of time, usually  measured in milliseconds.

The idea of short delay blasting was put forth and realized in the USSR.

By the engineer in 1934 during the sinking of mineshaft; later, it was introduced in the USA (1945) and Great Britain (1949).

DELAY DETONATOR

WHY WE ARE DOING THIS ? Reduced consumption of explosive as blasting is more efficient due to availability of a free face for each row of round of shots . (i.e. blasting due to No.1 delay detonator gives a free face for the blasting effect of shots fired by No.2 detonator)

In short delay blasting, the initiation of each subsequent charge or group of charges occurs in an area of the rock mass that is under stress as a

 result of the preceding explosion, thus increasing the useful blast effect

. The use of shortdelayblasting increases intensity of fragmentation of the medium by the blast

Reduces disruption of the rock mass outside the crushing zone,provides localized disintegration of the rock mass, and decreases the seismic effect of the blast.

In addition, shortdelay blasting makes possible control of the direction of movement of the crushed rock, providing collision and additional crushing of the fragments.

With out Delay detonator If the delay between the rows is too short, the rock

from the back rows tends to take an upward direction instead of a horizontal.

With Short delay detonator control of the direction of movement of the  crushed rock is achieved.

CONDITIONS WHERE SHORT DELAY IS USED

In shafts withhazardous gas and dust conditions, shortdelay blasting makes possible the firing of an entire set of blast holes.

 In one stage instead ofseveral, with instantaneous blasting (charging and blasting of  holes and ventilation, charging, and blasting of auxiliary holes).

PERFORMANCE OF SHORT DELAY Short-delay blasting is performed by means

of shortdelay electric detonators or, when blasting with a detonator fuse, by means of pyrotechnic delay mechanisms (relays).

 

Singlerow and multirow shortdelay blasting arrangements are use The principal singlerow shortdelay  blasting arrangements are sequential row blasting  in which the charges are detonated one by one  from 

one end to the other.

Single row and Multi row short delay

And consecutive-counter blasting in which the detonation proceeds from the centre of a row toward its flanks.

Providing head-on collision of the fragments and formation of a pile against the 

centre of the face.

In cases of a large face, the method can be repeated, forming several collision centres along the front (wave arrangement)

DRILL PATTERNSWITH SHORT DELAY BLASTING

MULTI ROW SHORT DELAY The principal types of Multi row short delay blasting

are sequential row blasting, in which the charges are detonated consecutively from one flank with the same delay interval

  (for example, 20 milliseconds between charges and, so that the moments of detonation of adjacent charges do not coincide, 30 milliseconds between rows).

Short Delay Series detonators are designed for opencast and underground mines, for driving tunnels, shaft sinking and in quarries where multi shot rounds are required.

Long Delay Series detonators are designed for underground mining for driving shafts and tunnels, blasting raises and shaft sinking. They are used for stopping in metal mines, trenching and other blasting applications.

The longer delay period allows sufficient rock movement before firing the consecutive shot.

Patterns used in open & underground mines for short delay blasting

BENCH BLAST PATTERN

In open cast mines we use this pattern for Short delay blasting

Under ground Blasting provides a good over view for a Varity of underground blast design

A typical arrangement for blasting in VCR methods of mining.

REFERENCES encyclopedia2.thefreedictionary.com/short-delay +

blasting Deshmukh D.J(2012),Elements of Mining Technology,

Vol-1,Denett & co, Nagpur, Pg 8.24-8.27.

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