Drilling Blast-hole pattern

The design of the blast includes the types, quantity and distribution of explosive placed in each drill hole. Before the design of the blast is finalised, an accurate survey of the profile of the quarry face and detailed logs of the drill holes are required to identify how the blast deviates fi-om the target pattern so that corrections to the standard design can be made. 

The type of blasthole stoping technique presented in this case study mine varies according to stope sequence and ore zone width. For each type of stope, blasting design practices have been standardized in terms of drilled hole diameter, range of powder factor, and the type and pattern of the explosives used. 

In the pre-splitting method, a series of holes is drilled around the perimeter of the tunnel, loaded with explosives that have a low charging density and detonated before the main blast. The initial blast develops a fracture that spreads between the holes. Hence, the main blast leaves an accurate profile. The technique is not particularly suited to slates and schists because of their respective cleavage and schistosity. Indeed, slates tend to split along, rather than across their cleavage. Although it is possible to presplit jointed rock masses adequately, the tunnel profile still is influenced by the pattern of the Jointing. 

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