Question

The type of Subsidence in which maximum subsidence occurs over an area is known as

• A. Critical width of extraction
• B. Sub Critical width of extraction
• C. Super Critical width of extraction
• D. Narrow width of extraction

Hint:

Think of the subsidence profile shape:
- Sub-critical: A gentle "U" shape. The bottom doesn't reach the maximum possible depth.
- Critical: A "V" shape. The bottom point just touches the maximum depth.
- Super-critical: A flat-bottomed "U" shape. A whole area at the bottom is at the maximum depth.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks to identify the term that describes an extraction panel width where the maximum possible subsidence occurs.

Step 2: Detailed Explanation:
Surface subsidence is the lowering of the ground surface due to underground mining.
The amount of subsidence at the center of a mining panel is directly related to the width of the panel relative to the depth of the workings.
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Sub-Critical Width: When the extraction panel is relatively narrow compared to its depth, the rock mass above the panel can bridge or arch across the opening to some extent.
The subsidence at the surface is less than the maximum possible value.
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Critical Width: This is the specific panel width at which the effect of the extraction just reaches the surface point directly above the center of the panel.
At this width, the subsidence at the center point reaches its maximum possible value for the first time.
The maximum subsidence is equal to the seam thickness multiplied by a subsidence factor.
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Super-Critical Width: If the panel is made even wider than the critical width, the subsidence at the center point

does not increase further.
It remains at the maximum possible value.
However, a trough of maximum subsidence develops over the central part of the panel.
The question asks for the condition where "maximum subsidence occurs over an area", which implies a trough or flat-bottomed subsidence profile.
This only happens when the extraction width is super-critical.

Step 3: Final Answer:
The type of subsidence where maximum subsidence occurs over an area is associated with a Super Critical width of extraction.
This corresponds to option (C).