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

opposite direction when the second adjacent panel is mined, i.e. the tensile strains are
additive over rigid gateroads. The optimum situation is for the 80-foot wide rigid pillar to
be only temporarily rigid as the second longwall face passes. The rigid pillar can safely
be allowed to yield and then crush after it is roughly 100 feet out from the longwall face
between collapsed gob on both sides. This, in effect, reduces the tensile strain, and
fracture opening, that is directly proportional to the differential vertical subsidence
between the gateroad and the maximum subsidence (Smax) over the center of the
adjacent panels.
3.3 Previous Mining
There is no known previous mining within the proposed coal lease. However, the
McClane Canyon Mine is operating in the immediately adjacent existing coal lease on
most of Sections 15, 16, 21 and 22, T. 7 S., R. 102 W., 6 P.M. A small coal operation,
the Munger Mine now closed and reclaimed, operated in adjacent Section 27. Figure 1.
Red Cliff Mine Project and Coal Lease Areas shows the location of these workings.
The McClane Canyon Mine map, Figure 6. McClane Canyon Mine Workings,
indicates room- and-pillar advance mining, without pillar robbing on the retreat, to
overburden depths just over 1,500 feet (generally 80 by 80-foot pillars on 100-foot
centers for 36% recovery). Pillars were robbed on the retreat from other panels at depths
of very nearly 1,300 feet. Total coal extraction (recovery) appears to have been as much
as 78% within two small (roughly 520-foot maximum width) irregular shaped panels that
were retreat mined to depths of 1,100 feet. The mine map indicates that robbing the 100
by 70-foot advance pillars using a method called "christmas treeing" was incomplete and
erratic, with many 100 by 70-foot advance pillars and occasional 100 by 30-foot stump
pillars left within the panels. This method is no longer permitted by MSHA because of
safety concerns. In general, however, panel recovery was approximately 64% when
robbing pillars on the retreat, apparently leaving different shaped stump pillars when
retreating from different panels. The width of those panels ranged from 350 to 570 feet.
No observations have been reported of surface subsidence effects over the McClane
Canyon Mine. Estimates of maximum subsidence (Smax), tensile and compressive strains
and maximum slope changes were made over the five selected panels and are indicated
by number on Figure 7. Subsidence Predicted for Five Selected Panels, McClane
Canyon Mine. The method used combined the British National Coal Board (NCB, 1975)
method for longwall subsidence prediction and the room-and-pillar adaptation by Abel
and Lee (1984) presented on Figure 8. Subsidence Over Room-And-Pillar Workings
After Failure. Table 1. Predicted Maximum Subsidence For Selected Panels,
McClane Canyon Mine presents the estimated super-critical subsidence over each of
the selected room-and-pillar panels and the predicted NCB corrected subsidence for the
panel width with respect to depth. The predicted maximum surface subsidence for the
five panels ranged from 1.52 feet to 2.56 feet. Table 2. Predicted Maximum Strains
and Tilt for Selected Panels, McClane Canyon Mine presents the maximum tensile
and compressive strains and slope angle changes In addition, a rough estimate of
potential open surface fracture widths was made for each selected panel. This rough
estimate was based on the relationship between the vertical subsidence measured over
York Canyon Mine longwall panels with known widths and overburden depths, measured
surface fracture widths and the NCB predicted maximum surface tensile strains for these
known conditions permitted an estimate of the potential width of surface tension
fractures.
C-5
DBMS 297