Blasting works

Field of application

Blasting techniques are essentially applied to rocky materials.

Blasting may serve several purposes:
– excavating or leveling out rocky massifs;  
– stabilizing potentially unstable rocky areas;  
– excavating or expanding underground works (wells, tunnels…). 

Basic Principles

Blasting consists in fragmenting rocks by inserting explosives in a drill hole. Although a single charge may suffice to fragment a simple block, rocky massifs require more or less closely-packed multiple charges.
Two types of energy come into play when using explosives:  
– the shock wave;  
– gas pressure.  

Different types of explosives

There are two main types of explosives:   
– deflagrating explosives (black powder);  
– detonating explosives (the most common kind).

There are three main groups of explosives:  
– dynamite: a nitroglycerine-based product, it is suitable for massive rocks;  
– ANFO (aluminum nitrate – fuel oil):   larger quantities of this explosive are needed as it is less efficient. It should not be used when the rocks present cavities;  
– emulsion: this explosive may be used in a number of ways and therefore has a relatively wide field of action..

 Explosives ill only work when associated with a detonating device and a priming device (pyrotechnic, micro-timed electric, electronic (sequential) or non-electric detonator).

Explosives are classified according to various parameters :
– detonation speed: it is the speed at which the reaction process propagates in the mass of the explosive.  
– critical diameter: it is the diameter under which the explosive will not detonate.  
– specific energy, which is the sum of the gas-generated energy and the shockwave-generated energy.  
– density,  
– sensitivity to priming,  
– water resistance.

Firing plan

The firing plan is a document that must list certain elements of information elaborated by the engineer:  
– firing location;  
– drilling characteristics (diametre, depth, inclination, number of drilled holes, CUI – single-detonation charge…);  
– characteristics of the device including type of explosive, priming, packing…   
– priming sequence.

Drilling

Drilling operations matter because they will determine the quality of the firing plan.

Philippe's advice about Blasting :

"Pre-zoning detonations make it possible to individualize the masses to be mined and hence limit the impact of Blasting on the non-excavated areas bordering the zone of activity."

Caution measures relative to Blasting

Blasting is often a quite impressive technique that may generate a number of risks and nuisances which can be perfectly controlled if the company undertaking the works is competent.  
– Provisions can be made for trial detonations in order to validate the CUI (single-detonation charge);  
– Vibration recordings may be carried out in order not to exceed the noise threshold criteria;  
– Protections against projections may be installed (geotextiles, wire netting…).