Question

In opencast blasting the purpose of sub-grade drilling is

• A. To avoid the formation of toe
• B. To reduce ground vibrations
• C. To reduce consumption of explosive
• D. To improve over all blasting performance

Hint:

Imagine trying to break a block of concrete with a hammer.
If you only hit the top, the bottom might not break cleanly.
Sub-grade drilling is like making sure your crack goes all the way through the bottom, ensuring a clean break at the desired level.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks for the specific reason why blast holes in opencast mines are drilled deeper than the planned bench floor level.

Step 2: Detailed Explanation:
In opencast (surface) mining, rock is excavated in a series of steps called benches.
Blast holes are drilled vertically from the top of the bench to break the rock.
The

bench floor or

grade is the desired final level after the broken rock is removed.
-

Toe: The "toe" is the rock mass at the bottom of the bench face.
This region is the most confined and the hardest to break effectively with explosives.
If the blast holes are drilled only to the level of the bench floor, the explosive energy is often insufficient to fully fracture the rock at the toe.
This results in a hard, protruding ridge of unbroken rock, known as a "hard toe," being left behind.
This toe obstructs digging by excavators and requires costly and dangerous secondary blasting to remove.
-

Sub-grade drilling (or Sub-drilling): To prevent the formation of a toe, the blast holes are intentionally drilled a certain distance

below the planned bench floor level.
This practice ensures that there is sufficient explosive energy placed at and below the toe to completely break the rock mass, resulting in a clean, flat floor that is easy to dig.
While this may slightly improve overall performance (D), its specific and primary purpose is to deal with the toe problem.

Step 3: Final Answer:
The purpose of sub-grade drilling is to avoid the formation of a toe.
This corresponds to option (A).