Introduction to Types of Rock Blasting

Conventional blasting techniques include several different types of blasting that are commonly used:

Production Blasting is a blast that is intended to fragment and displace a designed volume of rock. The focus of this blast is the maximum volume of rock fragmented per amount of explosive used. This blasting technique by itself will produce a ragged rock face and does not provide protection against back break or overbreak at the new rock face.

Secondary Blasting is a secondary blast used to fragment rock that was not adequately fragmented by the initial production blast.

Presplit Blasting is a controlled blasting procedure that is used to produce a shear plane within the rock mass. Most often used to produce a clean, relatively solid rock cut face, presplit blasting involves the use of boreholes that are more closely spaced and lightly loaded than production blastholes. A crack propagates along this line of more lightly loaded holes that are detonated ahead of the main production blast. The crack is intended to protect the new rock cut face, or some other perimeter, by allowing the blasting gases to escape and for blasting cracks to terminate at the presplit creak. This has the effect of reducing backbreak, or overbreak in the new rock wall, thus preserving its structural integrity. This method is used extensively for roadway rock cuts, lock walls, and any other cuts to produce a solid wall with little or no backbreak is needed. It is used to reduce the amount of rockfall that can occur from the exposed face than could be expected using production blasting alone. When well executed, the exposed rock face may contain “half casts” of the boreholes used for blasting. Presplit blastholes are fired before the production blast, which is between the presplit blast and the free face. The production blast may follow the presplit blast with a connected delay or fire completely separate from each other. During the initial evaluation period of pre-splitting results, it is recommended that the presplit blast be its own blast that way the results can be evaluated.

Precision Presplit Blasting is a controlled blasting procedure that is used to produce a shear plane within a weak rock or one that is geologically complicated with the minimum amount of explosive and minimum overbreak. Used to produce a clean, relatively solid rock cut face where rock is weak, or rock has extensive geologic discontinuities such as closely spaced jointing. Precision presplit blasting involves the use of boreholes that are more closely spaced and loaded lighter than standard Presplit blastholes. A crack propagates along this line of more lightly loaded holes that are detonated ahead of the main production blast. The crack is intended to protect the new rock cut face, or some other perimeter. When well executed, the exposed rock face may contain “half casts” of the boreholes used for blasting. Precision Presplit blastholes are fired before the production blast, which is between the presplit blast and the free face.

Smooth Blasting, commonly called “Trim Blasting” is similar to presplit blasting, but the holes are detonated after the production blastholes are detonated. The purpose is to blast loose remaining burden with lighter charges while not causing any additional damage to the new rock wall face. Smooth blasting is commonly used underground.

Precision Trim Blasting is a controlled blasting procedure that is used to produce a shear plane within a weak rock or one that is geologically complicated with the minimum amount of explosive and minimum overbreak. It is used to produce a clean, relatively solid rock cut face where rock is weak, or rock has extensive geologic discontinuities such as closely spaced jointing. Precision trim blasting involves the use of boreholes that are more closely spaced and loaded lighter than standard Trim blastholes. Precision Trim blastholes are fired after the production blast, which is between the perimeter and the free face.

Buffer Blasting refers to a designated section of rock between a slope or wall to be formed by line drilling or presplitting during excavation and the production blast. The explosives in the buffer blasthole and the burden in the buffer zone are reduced to prevent damage to the final rock slopes or face. Buffer blasting can be fired after the adjacent production blast or as a separate shot.

Sinking Cut Blast is where a blast has only the top or horizontal face and has no vertical or sloped free face (Figure 6). Rock cannot be displaced sideways in this type of blasting and thus it must be expelled upwards. Flyrock is a particular problem with this type of blast as it is not possible to direct the blasting energy in any direction but up. This must be accounted for during design and monitoring.