_GENERAL DOCUMENTS - C1981017 ‎(280)‎

• Permit No: C1981017
• IBM Index Class Name: GENERAL DOCUMENTS
• Doc Name: Bid Documents ‎(IMP)‎
• Media Type: D
• Archive: No

April 29, 1981
Mr. Douglas W. Bowman
` Page 2
References : Gentry, D. W. and Abel, J. F. , "Rock Mass
Response to Mining Longwall Panel 4N, York Canyon
Mine, " Preprint No. 77-AM-2, AIME Meeting, Atlanta,
Ga. , March, 1977, 33 pp. Dunrud, C. Richard, "Some
Engineering Geologic Factors Controlling Coal Mine
Subsidence in Utah and Colorado , " USGS Prof. Paper
696, 1976, pp . 39. Brauner, Gerhard, "Subsidence Due
to Underground Mining, " USBM Information Circulars
Nos . 8571, 8572 , 1972, pp. 56, 53 . Orchard, R. J.
and Allen, W. S. , "Longwall Partial Extraction Systems , "
Mining Engineer, London, June, 1970, pp . 523-535 .
Concerning point 2 (angle-of-draw) , in the case of the Coal
Basin application, Gentry and Abel (1977) and Dunrud (1976)
are cited as references since their observations seem most
applicable to your operations and conditions . Gentry and Abel
reported angles-of-draw varying from 15 . 70 to 250 in nearly
flat lying Mesa Verde strata; Dunrud lists a variety of
angles in various Mesa Verde localities ranging from 21 to
210 It would therefore appear that 150 would be acceptable
as an approximate average. If monitoring data subsequently
proves that 150 is inadequate, the- only effect--which
presupposes surface subsidence does occur-- is that the
width of barrier pillars will have to be increased at the
lease perimeter. This indication would be evident early in
permit life, if it is in fact going to happen.
Finally, regarding point 3, my hypothesis (on page 12,
8/20180 report) of bulking of caving over-burden which
prevents subsidence from reaching the ground surface is
based upon our subsidence inspection of Mines No. 1 and No.
2 and upon personal experience with retreat caving in metal
mines. This bulking action, in conjunction with bridging of
strong sandstone strata, is the only possible explanation
for lack of present surface expression above such an extensive
area of influence. My statement may be in contradiction to
a "worst case" condition, but it seems to be substantiated
by the fact that no subsidence has reached the surface. I
have not been able to locate verification of this opinion
in the literature, but neither have I found literature
which deals with Coal Basin-type geology and topography
where longwalling or complete pillar extraction was accomplished
so deliberately as at Coal Basin.
Please contact us if you have further questions or if we
may be of assistance at MLRB. We hope other conditions
have stabilized.
Very truly yours ,
alan nG. o cdegager, Mining
gineering, Division