Question

In a mine working the roof fall which takes place soon after withdrawal of supports is called

• A. Local fall
• B. Main fall
• C. Air blast
• D. Rock burst

Hint:

Think of the roof as having two parts: a weak lower layer and a strong upper layer.
The Local fall is the weak layer collapsing right away.
The Main fall is the strong layer breaking later, after a large area is exposed.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks for the specific term for a roof collapse that occurs shortly after supports are removed in a mining operation, like in the goaf of a longwall or depillaring panel.

Step 2: Detailed Explanation:
In underground mining methods where pillars are extracted or longwall panels advance, an area called the "goaf" or "gove" is created where the roof is intentionally allowed to collapse.
This collapse happens in stages:
-

Local Fall (or Immediate Fall): This refers to the collapse of the immediate roof strata, which are the rock layers directly above the coal seam.
This fall happens relatively soon after the supports (like hydraulic chocks in a longwall) are advanced or withdrawn.
It involves the rock breaking and falling into the void under its own weight.
-

Main Fall: This is a much larger and more significant collapse of the stronger, thicker main roof strata that lie above the immediate roof.
The main fall occurs periodically, after a significant area of the goaf has been created, as the massive rock beds break under the immense stress.
An

Air blast is a violent rush of air caused by a large, sudden roof fall (like a main fall) that displaces a huge volume of air.
A

Rock burst is a violent, explosive failure of rock under high stress, different from a gravity-induced fall.
The question specifies a fall that takes place "soon after withdrawal of supports," which is the definition of a local fall.

Step 3: Final Answer:
The roof fall that occurs soon after support withdrawal is called a local fall.
This corresponds to option (A).