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BLASTING IN TUNNEL

Know the topic "Blasting of tunnel" with basic approach considering its consequences

Blasting of tunnel

Definations of Blasting in tunnel

  1. Blasting in tunnel: Discharging operation of a explosive to loosen rock Blasting Powder=Salt petre + Sulphur + Charcoal
  2. Safety Fuse : Use to convey a flame to explosive .It contin black powder.
  3. Electric Squib : To ignite blasting powder
  4. Prima Cord : Very high explosive detonating fuse whose core is contained in the water proof with considerable tensile strength.  Used to detonate dynamite @ velocity of 6000m/sec
  5. Coyoto Tunnel : Tunnel of several metre diameter into which a large quantity of explosive placed for detonating purposes 
  6. Stemming: Process of adding inert material such as mud or rock dust in bore hole
  7. Primer:It is mainly used to initiate explosion.

Types of Explosive used in blasting in tunnel

1) Powder Explosive 
2) Liquid air Explosive 
3) Disruptive Explosive 
Grade and strength in form of cartridges of 2.5cm to 20cm in diameter and 20cm to 70cm in length .The strength of explosive specified as percentage of ratio of weight of nitroglycerine to total wt. of cartridge

Lebrun’s Relation between weight of charge and depth of hole

 W= c.m³⁄ cos³α for maximum volume:α=54°44̍

1) Straight Dynamite:

  • Not suitable for tunneling in rocks
  • Highly sensitive and water proof type of explosive
  • The percentage of nitroglycerine is 15 to 60%
  • Used in tight spots and for block holding

2) Ammonia Dynamite:

  • Suitable for soft rock
  • Also called Extra Dynamite
  • The percentage of nitroglycerine is 15 to 60%

3) Gelatin Dynamite:

  • Most suitable for rock tunneling
  • Their shattering effect is high
  • The percentage of nitroglycerine is 20 to 90% for straight gelatin and 30 to 90% for special gelatin 
  • The quantity of explosive varies from 13kg to 28kg per cubic metre of rock to be blasted   depending on nature of rock. 

DETONATORS:

Device used for firing explosive . Consist of  copper cylinder of small diameter 7mm closed at one end. The length of detonator be about   50mm. It contain dry mercury fulminate and  potassium chlorate.

Types of Detonators used in blasting in tunnel:

1)Half second delay detonator: Available in 10 days,interval being ½ second
2)Milli second delay detonator: Available in 15 days.
  • Delay detonator used to reduce ground vibration,improved fragmentation and reduction in over breakage.
  • The charge of explosive placed equidistant from each faces will break out 2.25 times as much as single phase and if charge can be placed equidistant from three faces it will break 3.5 times amount broken for single free face.

Method of Blasting in tunnel

1)Spring method of Blasting in tunnel:
This method is adopted in Bhakra project.Special gelatin of which 75% is useful.for average rock each cartridge used for springing should be provided with enough cavity to hold 6 to 8 cartridges.

References for Blasting in tunnel

  1. Railway bridge and tunnel enginnering by Rangwala 
  2. Harbour Docks and Tunnel engineering by R shrinivasan
  3. Roads,Railway Bridges and Tunnel engineering by Ahuja and Birdi
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