Mine Subsidence Subsidence is a time-dependent deformation either natural or man induced , in which there is a lowering of ground surface .
TROUGH Subsidence - occurs over openings in relatively thin bedded deposits.
POT HOLE Subsidence - Due to sudden failure of overburden into an underground void.
Causes -
Shallow depth cover
Nature of overburden
Geological discontinuities
Method of working
Thickness of seam
Dip of seam
Method of goaf treatment
Geological discontinuity
Time
Rainfall
Removal of hydrostatic support
Earthquakes.
SUBSIDENCE BASED ON TIME -
Active Subsidence
Residual Subsidence.
Active Subsidence :- When Subsidence occurs simultaneously with the Mining Activities.
Residual Subsidence :- Subsidence occurs after the mining activities.
Important 👉
NEW (non effective width of Extraction) is the underground width of extraction which does not cause any movement on the surface.
CW (Critical width) is the minimum width that needs to be mind before the maximum possible substance is observed at the centre of the trough.
Sub Critical width (< CW) , less then Critical width of extraction.
Super Critical width (>CW) , More then Critical width of extraction.
Angel of Draw - The angel between the vertical line and the line of zero Subsidence.
Angel of Break - The angel between the vertical line and the max tensile strain (Max cracks are developed in this zone Due to max tensile strain).
Inflection point - The point divided the concave and convex portions of the Subsidence profile.
Width of Subsidence(W) = W= w + 2H tan(a). w= width of opening H= Depth of working (a)= Angel of Draw
Critical width (cw)= 2H tan(a).
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Nicely explained