2011-09-23_REPORT - C1980007 ‎(7)‎

• Permit No: C1980007
• IBM Index Class Name: REPORT
• Doc Date: 9/23/2011
• Doc Name: Spring 2011 Subsidence Report ‎(Part 1 of 3)‎
• From: Wright Water Engineers, Inc
• To: DRMS
• Permit Index Doc Type: Subsidence Report
• Email Name: TAK
• Media Type: D
• Archive: No

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Spring 2011 Subsidence and Geologic Field Observations
Box Canyon, Apache Rocks, and South of Divide Mining Areas
last six field visits. These maximum strain values are below the median strain value of
0.65 percent projected for B- and E -seam mining in Exhibit 60 and Appendix A.
Continuous annual observations find substantial weathering with crack edges rounding
and crack depths filling with eroded material. Overall, crack widths have remained
relatively constant.
4. The length of time that tension cracks are expected to be visible, before the effects of
erosion and deposition, mass wasting, infilling, and revegetation obliterate them (duration
of cracks), is a function of their location with respect to the mine geometry and type of
material in which the cracks formed. Crack duration in zones of permanent tensile stress,
such as above mine boundaries and unmined pillars between longwall panels, commonly
ranges: 1) from one to three years in colluvium, 2) from three to six years in soft, friable
sandstone, and 3) to many decades in hard, durable sandstone (such as at Apache Rocks
as discussed above). However, cracks which form in the zone of temporary tensile stress,
such as above moving longwall faces, commonly close again when the longwall moves
out of their area of influence. This has been particularly noticeable above the western end
of Longwall Panels El (along Horse Gulch Road) and E2 (along Dry Fork Road).
5. Observed mine - induced subsidence has been less in the SOD mining area than were
observed annually in the Box Canyon and Apache Rocks mining areas dating back to
1996 (see Table 2, Appendix A for predicted subsidence parameters). The more subdued
topography and the fewer cliffs and ledges of the SOD mining area reduce the potential
for rockfall /landslide areas. As of the time of this report, E -seam mining in the SOD area
has been underway since December 2008 with observations consistent with those cited
above.
6. Angle of draw calculations for the SOD and Dry Fork mining areas were performed using
data collected from the E -seam subsidence monitoring stations and included in the Spring
2010 subsidence report as an August 2, 2010 memorandum in Appendix B. This analysis
calculated angles between 12.0 and 14.9° and concluded with a recommended maximum
831 - 032.793
September 2011
Wright Water Engineers, Inc. Page 45