2021-02-05_PERMIT FILE - C1980007A ‎(5)‎

• Permit No: C1980007A
• IBM Index Class Name: Permit File
• Doc Date: 2/5/2021
• Doc Name: SUBSIDENCE EVALUATION
• Section_Exhibit Name: Exhibit 60E Subsidence Evaluation for the South of Divide and Dry Fork Mining Areas
• Media Type: D
• Archive: Yes

Subsidence Evaluation for the
Exhibit 60E Southern Panels, Apache Rocks West, & Sunset Trail Mining Areas Page 19
Horizontal compressive strain ranges from-0.013 to-0.049(-1.3 to-4.9 percent)where
the extraction thickness equals 10 feet,and-0.016 to-0.061 (-1.6 to-6.1 percent)where
it equals 14 feet(Table 3).
Sunset Trail Mining A� Maximum tensile and compressive horizontal strains are calculated in
the conceptual model,using the values obtained from the West Elk Mine area, and as projected in
Table 4. These values are believed to be conservative, based on Dunrud's annual observations in
the Apache Rocks mining area.
• Panels SS1 to SS4: For the four panels that extend into the Sunset Trail mining area,
the projected horizontal tensile strain ranges from 0.005 to 0.029 (0.5 to 2.9 percent)
where the planned coal extraction thickness is 9 feet, and 0.008 to 0.045 (0.8 to 4.5
percent) where the extraction thickness is 14 feet.
Horizontal compressive strain ranges from-0.005 to-0.031 (-0.5 to-3.1 percent)where
the extraction thickness equals 9 feet (Table 4), and -0.008 to -0.048 (-0.8 to -4.8
percent) where the extraction thickness is 14 feet.
5.3.2 Maximum Projected Depths of Surface Cracks
Curvature, or differential tilt (curvature is the second derivative of vertical displacement with
respect to horizontal distance) of subsided rock layers causes horizontal strain. Comparison of
calculated curvature values and horizontal tensile strain derived from horizontal displacement
measurements, therefore, provides a means of calculating the depth of the neutral surface, and
hence the maximum depth of tension cracks from the surface. The neutral surface is the boundary
between tensile and compressive strain
In terrains with slopes less than about 30 percent,the depth of the neutral surface can be estimated
by dividing the maximum horizontal strain values by those of maximum curvature at a given
location. The calculated depth of the tension zone to the neutral surface—the boundary between
tension above and compression below—ranges from 50 to 100 feet in the subsidence monitoring
network at West Elk Mine. Crack depth may be much less than this projected 50 to 100-foot range
of maximum values because most of the monitoring network was located on slopes exceeding 30
percent. An unpublished study for the U. S. Bureau of Mines (Engineers International) indicated
that surface crack depth rarely is greater than 50 feet. Cracks will also be less extensive or
terminate where shale and claystone layers occur. Based on annual field subsidence observations,
maximum crack depth in bedrock in the Southern Panels, Apache Rocks West, and Sunset Trail
mining areas is estimated to be 1) 5 to 15 feet in terrain sloping less than, or equal to, 30 percent,
2) 10 to 35 feet in terrain sloping more than 30 percent, and 3) 40 to 50 feet in thick, brittle
sandstones in ridges (Tables 2 and 3).
Crack depth will likely be at a maximum value above massive coal barriers. The crack depth is
projected to be less (probably 10 to 20 percent less) above the panel chain pillars, where even the
rigid pillars are predicted to yield 10 to 30 percent of the coal extraction thickness (Tables 2 and
3).
831-032.912 Wright Water Engineers, Inc.
December 2020