1987-12-01_PERMIT FILE - C1981017 ‎(194)‎

• Permit No: C1981017
• IBM Index Class Name: PERMIT FILE
• Doc Date: 12/1/1987
• Doc Name: Subsidence Study by DR. Bruce Collins
• Section_Exhibit Name: Chapter IV Appendix IV-F-4
• Media Type: D
• Archive: No

3 _
ratio of 0.55 was assumed in the permit application (Vol . 7 ,
Chapter IV, p. 26 and Appendix IV-F-3) . As will be discussed
in more detail below, subsidence ratios for affected MCR
control stations (TP series) and monitoring points (MC
series) have been calcuated roughly in the range of 0 . 02
(TP-4) to 0. 25 (MC-2) . Points MC-4 and MC-6 appear to be
the only points which were initially stable or nearly so
(1981) ; that subsided significantly as a result of recent
mining (since 1981 ; MC-4 within subsidence zone of 7 South,
No. 3 mine , extracted early 1975 , and longwall 203 , No. 2
mine, mined spring of 1984 ; MC-6 over 3 North, No. 3 mine ,
pillared 1979) ; and now appear to have stabilized, i.e. no
movement detected 1984-1985. Total coal removed within the
MC-4 subsidence zone is estimated at 16 feet, with total
subsidence of 2 .10 feet, for a ratio of 0 . 13 . In this case ,
subsidence from earlier 7 South No. 3 mine pillaring may
have stablized, with the movement observed since 1981 being
due almost entirely to longwall 203 No. 2 mine extraction.
If this is the case, assuming 8 feet of M bed coal removed,
the subsidence ratio would be 2 . 10/8 or 0 .26 . Total coal
removed below MC-6 is estimated at 8 feet in 3 North of the
No. 3 mine, with total subsidence measured at -0 . 48 feet,
for a ratio of 0. 06 . It is possible in this case that some
subsidence occurred prior to monitoring, as the area was
mined in 1979 while the monitoring program did not begin
until 1981 ; however, it should be noted that initial movement
recorded at MC-6 was positive, indicating that compression
prior to subsidence was occurring between 1981 and 1982 .
The greatest calculated ratio, 0.25 , occurred at point MC-2 ,
located over longwall 102 and within the subsidence zone
of longwall 101 , No. 1 mine (B bed) . This part of lw 101
was mined in September of 1977 , and MC-2 was nearly stable
by 1982; movement since 1982 is assumed to be due almost
entirely to extraction of lw 102 , begun in November of that
year.
From this data, it would appear that the ratio of 0. 55 con-
tained in the permit is too conservative. The two points
for which all subsidence appears to be due to recent mining,
MC-4 and MC-6 , and that have apparently stablized, have ratios
of 0.26 and 0. 06 respectively (assuming that all movement of
MC-4 is due to recent longwall 203 mining as discussed above) .
A value of 0 .30 would therefore appear to be sufficiently
conservative. This correlates well with the 0 . 31 ratio
determined by Dunrud in the area of the Somerset mine, 17
miles southwest of Coal Basin. Using this figure, the maxi-
mum expected subsidence, assuming 16 feet of coal extracted
from the A and B beds and 8 feet from the M bed for a total
of 24 feet, would be 7 . 2 feet, rather than the 13 .2 feet
predicted in the permit (Vol. 7 , Chapter IV, p. 27) .
The lack of observed subsidence features on the surface sub-
stantiates the proposal made in the permit that "plastic"
deformation of the surface will occur as opposed to random