Explosives and blasting
An explosive is a substance or mixture of substances, which with the application of a suitable stimulus, such as shock, impact, heat, friction, ignition, spark etc., undergoes an instantaneous chemical transformation into enormous volume of gases having high temperature, heat energy and pressure.
Causes disturbances in-
the surrounding-air, gas, solid, liquid or combination of these.
The disturbance in the solid structure results in its shattering and demolition.
In mining works - used for dislodging, breaking or fragmentation of the rocks for quarrying, mining, tunneling, or excavation works
The energy released by an explosive does the following operations:
• Rock fragmentation
● Rock displacement
● Seismic vibrations
● Air blast (heard as loud bang).
DETONATION AND DEFLAGRATION
Detonation is the process of propagation of the shock waves through an explosive charge. The velocity of detonation is in the range of 1500 to 9000 m/sec. well above the speed of sound.
Deflagration is the process of burning with extremely rapid rate the
explosive’s ingredients, but this rate or speed of burning, is well below
the speed of sound
COMMON INGREDIENTS
CLASSIFICATION
BLASTING PROPERTIES OF EXPLOSIVES
• AN is mixed with 5–6% fuel oil-ANFO
• ANFO has become an indispensable explosive for most of the surface mines and underground non-coal mines
STRENGTH
• It is the energy released/unit weight (known as
weight strength); or per unit volume (known as
bulk strength) of an explosive. It is now a days
expressed relative to ANFO at 100%
STRENGTH is measured by:
● Shock generated (VOD and speed of chemical reaction)
● Gas volume
● Energy
● Detonation pressure
● Explosion temperature
● Shock generated (VOD and speed of chemical reaction)
Velocity of detonation (VOD)
VOD is the measure of the shattering effect of an explosive, an important parameter for hard rock blasting. It changes with change in diameter and density of explosives.
Gas volume-
Larger the gas volume of an explosive large will be the throw obtained. If throw is to be minimized its ingredients should be adjusted to get minimum volume of gas and maximum heat output. ‘Ballistic Mortar’ test and ‘Trauzl block’ test generally measure it.
Energy
The oxygen balance and reactive ingredients determines the energy output of an
explosive. This energy represents the temperature of explosion and hence the maximum work that can be done by an explosive is indicated by this value.
Detonation Pressure
Based on detonation velocity and density of explosives a shock wave pressure that is built ahead of reaction zone is known as detonation pressure. Higher the detonation pressure, higher would be the brisance capability (i.e. the ability to break or shatter rock by shock or impact). Its value varies from 5 to 150KB. Due to this property a primer having higher detonation pressure should be selected. Given below is the mathematical relation to express this parameter:
Above the critical density, detonation pressure is zero, as the cartridge does not explode.
