WALL CONTROL BLASTING TECHNIQUES ----------------------------------------------------(ii) Presplitting - Presplitting is the most common controlled blasting technique and has proven successful in applications from large open pit mines to civil construction. This method involves the drilling of closely spaced holes at the planned excavation perimeter which are lightly loaded with explosives in order to generate an appropriate borehole pressure. Presplitting is being done using hole diameters ranging from 2 inches to 12Âź inches. Often, small diameter presplitting is preferred for technical reasons and because the cost per square foot of wall may be lowers. Other mines use large diameter holes in order to employ the same drills for presplitting as for production drilling. This approach has worked especially well in active highwall pre-splitting designs associated with blast casting operations. It has not always been as successful in other types of mining applications. In small diameters (<5 inches, 127 mm) spacings of 3 to 6 feet (0.9 to 1.8 m) have been common. When the decoupled borehole pressure can be permitted to significantly exceed the rock compressive strength, then spacings of 7 to 9 feet (2.1 to 2.75 m) have been used successfully in 3inch (76 mm) boreholes, greatly reducing the cost of wall control. In larger diameter (>6 inches, 152 mm) hole spacing of 5 to 18 feet (1.5 to 5.5 m) have been employed. As spacing become larger geological structure becomes an increasingly important control on this dimension. â&#x20AC;˘
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Spacing Between Holes - The spacing between the holes is a function of the hole diameter, decoupled borehole pressure and the tensile strength of the rock. The spacing between presplit holes may have to be varied in different areas of the pit if differing rock types exist with different uniaxial compressive strengths and tensile strengths. Therefore characterization of the geology is important. Not only do the rock properties affect the spacing, but the geological structure is also an important control. As a rule of thumb the hole spacing should not exceed twice the spacing between major, open joints. Pre-splitting on an Angle - Observations in open pit mines and quarries has shown that presplitting at an angle less than vertical contributes to a wall that remains in better condition for extended periods of time than one that is presplit vertically. This has been observed in iron mines, coal mines and quarries. Vertical presplit may be appropriate where the rock is particularly competent, or special circumstances preclude an angled wall. Presplit angles typically range between 70 and 80 degrees, with 80 degrees being perhaps the most common. In construction blasting a vertical presplit is likely to be more common. An angled wall may lead to greater construction cost. However, in deep road cuts for example an angled presplit should still be considered. A principal advantage to angle hole pre-splitting results from the toe of the presplit face being moved out from the crest. Therefore, if isolated blocks of rock fall from the face near the toe the entire face is not undercut, as would typically be the case for a vertically presplit wall. Another primary advantage occurs when steeply dipping joints or bedding planes dip back into the wall and strike near parallel to the face. Under these conditions the wall may be subject to toppling failures. The stability of a wall prone to these failures can be enhanced by the toe buttressing effect of an angled presplit wall. The third important advantage to angled presplit holes relates to the relative position of the presplit and buffer rows. When the presplit holes are angled and the buffer row is vertical it is possible to locate the toe of the buffer hole close to the presplit line for good breakage, while maintaining a greater standoff at the crest to avoid excessive crest fracture.
Author: Partha Das Sharma, (B.Tech-Hons., Mining Engg.), E.mail: sharmapd1@gmail.com, Website: http://miningandblasting.wordpress.com/ Page 10