A great article written by H. K. Verma CSIR-Central Institute of Mining &Fuel Research Regional Centre, CBRI Campus, Roorkee
Pre-splitting blasting consists of a row of lightly charged, closely spaced holes adjacent to the final slope that is fired prior to the detonation of the other holes. This creates a breakage plane to vent explosive gases and reduce crack propagation.
A pre-split blasting is best carried out when the burden is composed of homogeneous consolidated rock. In a badly fractured rock unloaded guide holes may be drilled between the loaded holes. The light explosives charges can be obtained using specially designed pipe cartridges, part or whole cartridge taped to detonating cord down line.
In pre-split technique holes spacing and charge concentration is an extremely important factor. In most rock types the pre-split blasthole should be angled to achieve a more stable wall. The angle selected should be based on the slope design, rock structure, drill type and charging requirements of the blast holes. The key factors that control the success of pre-splitting are drill accuracy, geological structure, hardness, pre-split spacing, pre-split charging, standoff distance of inner buffer row, face burden (horizontal relief), bench width to height ratio (should be less than 2), timing configuration, overall energy level (Lyall, 1993).
As conditions become more challenging the pre-split design will have to be modified to produce satisfactory results. In hard rock masses a short “stab” hole is often required between the inner buffer and the pre-split to achieve adequate fragmentation.
Sub-drilling may be required to establish the proper bench grade when the rock is hard. If the rock mass is highly structured and relatively weak, air decks may need to be used in the buffer rows. The following illustration outlines some of the modifications required for pre-split blast design in unfavorable conditions.
One of the key elements of pre-split blast design is the charging of the pre-split row. Normally the charge is decoupled to reduce the borehole pressure to well below the compressive strength of the rock. This can be achieved by air-decking or using a charge diameter that is smaller than the blast hole diameter.
Air decking is the least expensive method and is appropriate when the rock mass is relatively massive. It typically consists of placing a small bulk charge in the bottom of the hole and leaving the remaining hole open to achieve decoupling. As the rock becomes more structured better explosive energy distribution is required.
To improve the energy distribution multiple small explosive decks, continuous small diameter packaged explosive, or in some cases detonating cord can be used. While continuous explosive is the most expensive option for pre-splitting, it also provides the best performance in unfavorable conditions.
Unless air blast is a concern, the pre-split holes should be left open to reduce borehole pressures and protect the crest region of the hole. Different charging methods using air-deck is illustrated in Figure 8. Pre-splitting can be the most expensive and labor intensive of the wall control methods.
However, the long-term benefits can outweigh the costs if a maximum slope angle is required. If the wall is so weak that even well designed pre-split techniques cause damage the next wall control consideration should be line drilling.
