Civil Engineering Reference
In-Depth Information
In summary
,
pumped emulsion explosives have the following charac-
teristics:
•
no explosive component and hence safe to handle;
•
no toxic components;
•
drill and blast using large drill holes containing water are easily possible;
•
full utilization of the drill hole volume;
•
very efficient loading process possible due to the pumping of the
emulsion explosive.
POWDER MATERIALS (BLASTING AGENTS)
Ammonium nitrate-carbon carrier explosives (ANC or ANFO) are powder
explosives. Approximately 6% diesel oil is added as a fuel to the porous
and absorbable ammonium nitrate. This composition renders the explosive,
amongst other things, able to run, very safe to handle (it can be blown in)
and gives it more of a crushing effect, which means a low detonation speed
of approximately 2500 to 3000 m/s and a high gas cloud volume of nearly
1000 litres/kg. Powder explosives are generally not water resistant. They
only find limited application in tunnelling.
In summary, powder (ANFO) explosives have the following charac-
teristics:
•
for use in dry boreholes;
•
initiation is with an amplification charge;
•
high plume (vapour) volume;
•
good exploitation (use) of the borehole;
•
high toxic fumes.
5.6.5.3 Detonators
A requirement of safety explosives is that they do not detonate easily and
they should only do so intentionally. Charges are therefore required to set
off the explosion, which themselves contain explosive material - these
charges are called detonators.
There have been significant advances in the type of detonator since the
invention of the mercury blasting cap by Alfred Nobel in 1865. In the
1880s the
electric detonator
was invented. With this type of detonator,
the electrical circuit is closed, the contact-element heats the filament bridge
and this ignites the primer material (Figure 5.39a). In the first half of
the twentieth century, several developments took place, culminating in the
millisecond delay detonator in the 1940s. Electrical detonators have many
advantages, such as total control of the initiation time, reduction in the air
blast and ground vibration and better blasting results with the delays, which
is particularly important in tunnel blasting. However, they have a serious
