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: Picture a blast hole drilled exactly to bench floor level.
If a blast hole stops right at the planned floor grade, the explosive charge near the bottom of the hole has very little rock confining and surrounding it below that level, so the rock at the very base does not get broken cleanly.
Step 2: See what this leaves behind.
This unbroken rock at the bottom of the bench forms a hard ridge called a toe, which blocks the excavator bucket and often needs a separate, costly secondary blast just to remove it.
Step 3: See how drilling a bit deeper fixes this.
By extending the hole a little below the intended floor level, called sub grade drilling, we place explosive energy right at and below the floor line, this ensures the rock breaks cleanly down to grade and no toe is left standing.
Step 4: Conclude.
So the whole reason for sub grade drilling is specifically to prevent that toe from forming.
\[ \boxed{\text{To avoid the formation of toe}} \]