Page 525 - Rock Mechanics For Underground Mining

CONTINUOUS SUBSIDENCE DUE TO THE MINING OF TABULAR OREBODIES

Figure 16.22 Typical section thro- classes of panel width. The vertical displacement, s, of any point of the surface, is
ugh workings, illustrating standard called the subsidence. The maximum subsidence in a given profile is denoted by S.
symbols for subsidence and slope (af- For a given area and corresponding width of extraction known as the critical area and
ter National Coal Board, 1975).
critical width, S will take the maximum value possible for the particular seam, S max .
Areas or widths for which S < S max are described as subcritical.For supercritical
areas or widths, a subsidence of S max is achieved over a finite width rather than at a
single point as in the critical case.
The angle of draw, , is defined as the angle made with the vertical by a line drawn
from the base of the seam to the point of zero surface subsidence. Its values vary with
the mechanical properties of the rocks, being lower for stronger rocks and higher for
weaker rocks and soils. The value of will also vary with the resolution of the instru-
ments used to measure subsidence and with the cutoff value taken as being equivalent
to ‘zero’ subsidence. Assuming a cut-off subsidence of 20 mm, Holla and Barclay
(2000) found that the average of 74 measured values of angle of draw in the Southern
◦
Coalfield, New South Wales, Australia, was 29 . Seventy per cent of the values were
◦
below 35 , the value used for the UK coalfields (National Coal Board, 1975).
For a horizontal seam at depth h, the mining of a critical area having a critical
width, W c , as its diameter, will produce complete subsidence at a single point on the
surface vertically above the centre of the area. In this case
W c = 2h tan

In addition to this vertical movement, each point in the subsidence trough will also
undergo horizontal movement known as displacement. It is clear from the shapes of
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