Subdrill refers to the length of a blast hole that is drilled below the intended final floor or grade elevation of a bench. It is typically charged with explosives and serves a critical role in ensuring complete rock breakage at the base of the blast.
By extending the hole beneath the desired grade, subdrilling helps minimize the occurrence of high toes or uneven floors, which can hinder excavation efficiency, reduce equipment productivity, and increase secondary blasting requirements.
The appropriate subdrill length depends on factors such as rock hardness, bench height, hole diameter, and the desired fragmentation quality.
Table 1 shows the different approaches to estimating subdrilling (Sd).
|
Langefors & Kihlström (1963) |
Sd = 0.3 × B (B= Burden) |
|
Ash (1963b) |
Sd = 0.3 × B (minimum) |
|
Jimeno, Jimeno, & Carcedo (1995) |
Sd = 0.3 × B (B= Burden) |
|
Hustrulid (1999) |
Sd = 0.3 × B & 8 × D |
|
ISEE (2011) |
Sd = 0.2 – 0.5 × B |
Subdrill design recommendations
Use minimal subdrill and if the rock mass properties are favourable, use no subdrill at all.
Negative effects of using long or excessive subdrill include:
- Wasting drilling and blasting time, resources and expenditure.
- Increasing ground vibration due to over confinement of explosives in the subdrill.
- Accentuated rock movement and displacement.
- Damage to the rock mass at and below the target floor elevation.
- Difficult drilling conditions in the underlying bench, including collaring, leading to problematic drilling conditions.
Author: Stephen Mansfield, Principal Drill & Blast Engineer at First Quantum Minerals.
