Sub-Drilling

Sub-drilling is defined as the length of blasthole drilled below the design floor or grade level. Sub-drilling is required in most bench blasts, to improve fragmentation, displacement and loosening of rock near blasthole toes.

A standard recommendation for sub-drilling is 8 to 12 blasthole diameters, for typical bench blasts fired to an open free face. This length is usually sufficient to "break out" the toe and enable excavation to the design floor level.

The length of sub-drilling required depends on many factors including:

• blasthole diameter and inclination
• face inclination and toe burden
• rock mass properties, especially discontinuities near grade level
• effective energy and in-hole density of explosives

Note that Subdrill is normally positive (so the hole extends below floor level). Negative values are not allowed.