Question

In Board and Pillar method of mining with Hydraulic stowing, the preferred method of extraction of pillars is

• A. Step diagonal
• B. Vertical line
• C. Diagonal line
• D. Straight line

Hint:

The choice of extraction pattern is often determined by the method of goaf management.
For caving, a long, straight or diagonal line might be preferred to promote roof fall.
For stowing, a method that creates contained, easily-barricaded sections, like the step diagonal line, is superior.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks for the preferred pillar extraction (depillaring) pattern when hydraulic stowing is used to fill the void.

Step 2: Detailed Explanation:
Hydraulic stowing involves filling the goaf (the void left after extraction) with a sand-water slurry to control roof subsidence.
This requires the goaf to be systematically and securely barricaded to contain the slurry.
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Diagonal line of extraction is a common method, but it can create a long, continuous goaf line which is difficult to manage with stowing.
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Step diagonal line of extraction is a modification where pillars are extracted in a staggered or "stepped" pattern.
This method has significant advantages when combined with hydraulic stowing:
1. It breaks the long goaf line into smaller, more manageable sections.
2. It makes it easier to erect strong barricades to contain the sand.
3. It improves overall roof control and safety during the simultaneous process of extraction and filling.
For these reasons, the step diagonal method is the preferred technique when depillaring with hydraulic stowing.

Step 3: Final Answer:
The preferred method of extraction is the step diagonal line.
This corresponds to option (A).