WALL CONTROL BLASTING TECHNIQUES ----------------------------------------------------Decoupling and decking are the techniques basically used for reduction of borehole pressure. A primary means of reducing the borehole pressure is to decouple the charge from the hole. This means that the diameter of the explosive charge is less than the diameter of the hole. Pressure may be further reduced by decking, whereby gasbags, wooden or cardboard spacers etc., are used between charges or the charges are taped to detonating cord with a gap left between individual cartridges. For a given hole diameter and explosive the usual approach is to decouple radially first. If this is insufficient to reduce the borehole pressure enough, additionally decks can be employed. When a charge is decoupled from the blasthole the explosion gases expand to fill the hole volume before exerting borehole pressure. Therefore the decoupled borehole pressure will be much less than the coupled value.
CALCULATION OF COUPLING RATIO AND DECOUPLED PRESSURE: Coupling Ratio (CR) can be expressed by: CR = (C)1/2 x ( dc/dh), where ‘C’ = the percent of explosive column actually loaded, ‘dc’ = charge diameter and ‘dh’ = hole diameter. Decoupled pressure may be calculated from the following formula: (Pb)dc = (Pb)c x (CR)2.4 where ‘(Pb)dc’ = The borehole pressure for a decoupled and/or decked charge, ‘(Pb )c’ = The borehole pressure for a fully coupled charge and ‘CR’ = Coupling ratio. In using these equations it is necessary to have an idea of what an acceptable decoupled borehole pressure will be. In pre-splitting it has been found that the pressure should be in the range of 2 to 5 times the uniaxial compressive strength. In larger hole diameters it is often better to set the decoupled borehole pressure near to the uniaxial compressive strength of the rock because of the greater radius of rupture that may result around larger diameter boreholes, when the borehole pressure exceeds the compressive strength of the rock. This potential for large rupture radius around the borehole can lead to a wall more prone to unravel over time.
Discussion on Borehole pressure on type of controlled blasting - In some presplitting applications a concentrated charge is used in or near the bottom of the hole with the remainder of the borehole left void. Upon detonation the explosion gases are free to expand up the hole and exert a suitable decoupled pressure on the surrounding rock. This method has been used extensively in active highwall pre-splitting when blast casting in dragline mines. It has also been used in other types of Author: Partha Das Sharma, (B.Tech-Hons., Mining Engg.), E.mail: sharmapd1@gmail.com, Website: http://miningandblasting.wordpress.com/
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