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I
<br /> April .29, 1981
<br /> Mr. Douglas W. Bowman
<br /> Page 2
<br /> II
<br /> References : Gentry, D'. W. and Abel, J. F. , "Rock Mass
<br /> Response to Mining Longwall Panel 4N, York Canyon
<br /> Mine,". Preprint No. 77-AM-2, AIME Meeting, Atlanta,
<br /> Ga. , March, 1977, , 33 pp. Dunrud, C. Richard, "Some
<br /> Engineering Geologic Factors Controlling Coal Mine
<br /> Subsidence in Utah and Colorado," USGS Prof. Paper
<br /> 696, 1976, pp. 39, Brauner, Gerhard, -Subsidence Due
<br /> to Underground Mining," USBM Information Circulars
<br /> Nos. 8571, 8572, 1972, pp. 56, 53. Orchard, R. J.
<br /> and Allen, W.S. , "Longwall Partial Extraction Systems ,."
<br /> Mining Engineer, London, June, 1970, pp. 523-535.
<br /> Concerning point 2 (angle-of-draw) , in the case of the Coal
<br /> Basin application, Gentry and Abel (1977) and Dunrud (1976)
<br /> are cited as references since their observations seem most
<br /> applicable to your operations and conditions. Gentry and Abel
<br /> reported angles-of-draw varying from 15.70 to 250 in nearly
<br /> flat lying Mesa Verde strata; Dunrud lists a variety of
<br /> angles in various Mesa !'Verde localities ranging from 20 .to
<br /> 210. It would therefore appear that 150 would be acceptable
<br /> as an approximate average. If monitoring data subsequently
<br /> proves that 150 is inadequate, the- only effect--which
<br /> presupposes surface subsidence does occur-- is that the
<br /> width of barrier pillars will have to be increased at the .
<br /> lease perimeter. This "indication would be evident early in
<br /> permit life, if it is in fact going to happen.
<br /> Finally, regarding point 3, my hypothesis (on page 12,
<br /> 8/20/80 report) of bulking of caving over-burden which
<br /> prevents subsidence from reaching the ground surface is
<br /> based upon our subsidence inspection of Mines No. 1 and No.
<br /> 2 and upon personal experience with retreat caving .in metal
<br /> mines. This bulking action, in conjunction with bridging of
<br /> strong sandstone strata, is the only possible explanation
<br /> for lack of present surface expression above .such an extensive
<br /> area of influence. My statement may be in contradiction to
<br /> a "worst case" condition, but it seems to be substantiated
<br /> by the fact that no subsidence has reached the surface. I
<br /> have not been able to locate verification of this opini-ori
<br /> in the literature, but neither have I found literature
<br /> which deals with Coal Basin-type geology and topography
<br /> where longwalling or complete pillar extraction was accomplished
<br /> so deliberately as at Coal Basin.
<br /> Please contact us if you have further questions or if we
<br /> may be of assistance at. MLRB. We hope other conditions
<br /> have stabilized.
<br /> Very truly yours,
<br /> ohn G. oAge
<br /> angager, Mining
<br /> Engineering, Division
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