Blast Hole Pattern

The blast hole pattern used at any particular site will be the result of a certain amount of trial and error. Konya and Walter (1990) suggest using Equation (4.3) ... 

Holmberg, R., Persson, P. A. (1980). Design of Tunnel Perimeter Blast Hole Patterns to Prevent Rock Damage. Transactions of the Institution of... 

The main principle of blast sequencing is to allow adequate dynamic relief of individual blast holes as the pattern is detonated, but at the same time maintaining confinement and controlling air blast and vibration. To control these aspects of the blast, a delay is built into the firing sequence between adjacent holes in the same row and also between adjacent rows. A rule of thumb for the timing of hole-to-hole delays is given by Equation (4.6) (Konya and Walter, 1990) ... 

A -> Contour Blasting method in which cracks for the final contour are created by firing a single row of holes prior to the blasting of the rest of the holes for the blast 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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