1981-11-13_PERMIT FILE - C1981013 ‎(40)‎

• Permit No: C1981013
• IBM Index Class Name: PERMIT FILE
• Doc Date: 11/13/1981
• Doc Name: Subsidence Control Plan
• Section_Exhibit Name: Volume 3 Exhibit 21
• Media Type: D
• Archive: No

2" SURFACE SUBSIDENCE
Final face position
0 _
10 Residual
subsidence
20
e
40 Subsidence profit
when face stoppec
50 ,
E wFinal subsldonc
a 70 Profile 21 days
after face stop
0
90
100
2.0 1.5 1.0 0.5 0 0.5 1.0 1.5 2.0
Face advace
Depth
FfS. 9.2.7 Residual subsidence profile(12). Courtesy U.K. National Coal Board.
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surface point and reduces to 2 to 3% if the face has passed the critical
width. The rate of face advance determines the time needed to pass
through the critical width. As long as the face is still within the critical
width, further subsidence is expected.
However, Brauner (13). citing Russian data, stated that in addition to
the rate of advance and size of critical area, the duration of surface
subsidence depends on strata conditions, depth of extraction, kinds of
packings, and previous extraction and that it ranges from several days to
several years. The duration is longer for thick-bedded or stronger over-
burden, for greater depths, and for complete caving. For a small extrac-
tion area of a relatively mobile overburden mined in a very short time,the
rate of subsidence is proportional to the difference between the final
subsidence S and the instantaneous subsidence S,:
dSL = c(S — S,) (9.2.1)
dt
the solution of which is
(: SO = C = 1 — e-0 (9.2.2)
where a is a time factor and c is a constant characteristic of the strata. A
typical a curve for soft strata is illustrated in Fig. 9.2.8.
In the United States, earlier studies (10, 11) showed that subsidence
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