2008-02-22_APPLICATION CORRESPONDENCE - C2008086 ‎(43)‎

• Permit No: C2008086
• IBM Index Class Name: Application Correspondence
• Doc Date: 2/22/2008
• Doc Name: PDEIS Appendix C Subsidence
• Media Type: D
• Archive: No

7.6 Rate and Duration of Subsidence
The first potentially detectable subsidence at a given point on the ground surface ahead
of an approaching longwall face begins when the longwall face is something less than
approximately 0.75 times the overburden depth of the seam, has subsided about 15% of
Smax when the longwall face passes under the point, is approximately 50 percent
complete when the longwall face is 0.2 to 0.3 times the overburden depth beyond the
point, and appears to have stopped subsiding when the face is between 0.5 and 0.6
times the overburden depth beyond the point. However, there is still 5% to 9% of
residual subsidence to take place after the longwall face has either mined beyond the
influence distance or the panel has been completed. Residual subsidence is probably
the result of consolidation of the gob and closure of some overlying bed separations in
the overburden. Measuring the time until residual subsidence is complete requires
extremely precise leveling to measure subsidence. Collins (1977) reported the results of
an eight year program in the South Wales Coalfield. He reported the results from six
panels at depths from 207 feet to 2330 feet. Longwall mining of the six panels was
completed over periods from 0.5 to 1.5 years and measurable residual subsidence
continued for 2.0 to 4.5 years afterwards. Complete stability is not significant because
the potentially damaging strains and tilt are directly dependent on the magnitude of the
subsidence and the magnitude of residual subsidence is small in relation to the
subsidence that takes place during the active period.
Shortly after the advancing longwall face has opened up enough area to initiate the first
major roof cave behind the shields, the wave of surface subsidence accompanying face
advance will start. The movement of the longwall face and the ground surface are so
closely tied together that when the advance of the face stops the advance of the
accompanying wave of surface subsidence advance may stop in less than a shift, but
definitely over a weekend. Stopping the advance of a longwall face will, however,
potentially increase the loads on the face supports. Sloughing from the coal face can
also occur during stoppages. Restarting face advance after holiday periods, etc. can be
difficult.
Peng (1992, p. 20-22) reports maximum dynamic tilt and horizontal strain decreases with
increasing speed of longwall extraction. Peng presents graphical data for the rate of face
advance for various longwall faces in a West Virginia coal mine which increased from
roughly 10 feet/day to roughly 43 feet/day:
Maximum dynamic tilt appears to have decreased an average of
approximately 44 percent (Peng, 1992, Fig. 3.6). The scatter of the dynamic
tilt data is so large and the contradictory indication of an increasing maximum
dynamic tilt for the single most rapid 43 feet/day face advance indicated on
Fig. 3.6 that it appears statistically only possible to state that the tilt probably
decreased with increasing face advance rate.
2. Maximum dynamic tensile strain decreased by an average of approximately
28 percent (Peng, 1992, Fig. 3.7). The scatter of the dynamic tensile strain
data indicated on Fig. 3.7 is less than for the dynamic tilt data and it may be
statistically possible to indicate a rough numerical relationship between
decreasing tilt and increasing face advance rate.
C-38
DBMS 330