2021-05-27_PERMIT FILE - C1981019 ‎(10)‎

• Permit No: C1981019
• IBM Index Class Name: Permit File
• Doc Date: 5/27/2021
• Doc Name: Geotechnical Stability Report For Highwall Mining At the Colowyo Coal Mine
• Section_Exhibit Name: Exhibit 23, Item 1
• Media Type: D
• Archive: No

March 21, 2014 Page 54
In applying the design charts of Figures 29 through 33, it is recommended that the highest
anticipated mining height in the panel be used. The depth of cover can be chosen based on a
weighted average as follows:
Design Depth = 0.75(maximum depth) + 0.25(minimum depth) (Eqn. 7)
In determining the minimum depth, engineering judgment should be used, for example
the minimum depth could be chosen based on the minimum depth beyond the highwall crest, or
beyond the first bench in the highwall, as appropriate.
5.0 NUMERICAL MODELING ANALYSIS
Due to the extensive history of highwall mining at Colowyo, and the fact that many
similar scenarios to the proposed mining have been modeled previously, AAI did not perform
numerical modeling for sample panel designs in the current study. However, one area of planned
mining was of sufficient concern that a simple numerical analysis was performed. In the West
Pit, Colowyo has projected highwall mining panels in the F Seam below E Seam panels that
were previously mined at right angles to the planned mining (Figure 14). Depending on the
current condition of the E Seam web and barrier pillars, a range of stress conditions in the
unmined F Seam below (with an interburden thickness between 15 ft and 25 ft) are possible. To
explore these stress conditions, LAMODEL numerical modeling of the as -mined E Seam
highwall miner openings, and the unmined (pre- mining) F Seam was performed. Overmining of
the previous E Seam panels does not pose a great concern, as the D2 Seam is too thin in this area
to highwall mine, and the C Seam is distant from the E Seam (the C -E interburden ranges from
70 to 120 ft)
LAMODEL (short for laminated model) is a non - linear boundary- element displacement
discontinuity code for estimating stress, displacement, and yielding in tabular deposits such as
coal (Heasley and Salamon 1996). LAMODEL was developed by the former U.S. Bureau of
Mines (USBM), now part of the National Institute of Safety and Health (NIOSH). It has the
ability to handle in -seam materials based on both linear and nonlinear mechanical (stress - strain)
behaviors. The geometry of the model is variable; thus, incremental model changes can be used
to determine optimal mining sequences, as well as the impact of varying pillar sizes, variations in
cover load, and seam interaction effects. The programs perform an iterative procedure to solve a
set of equations representing the stress -strain state of each element in a grid representing the
mine geometry, until a steady -state equilibrium is reached (loads no longer transfer).
For the analysis, a two -seam (F and E) model was run. Actual cover depth over the area
was used. Model parameters are given in Table 8.
In the models, the strength of the E Seam coal was reduced from its design strength of
900 psi to simulate what may occur with pillar deterioration over time. With an in -situ strength
of 684 psi, (Figure 34), web and barrier pillar remain stable. Stresses imparted to the underlying
F Seam (Figure 35) increase slightly compared to the overburden stress that would occur if the E
Seam were not mined. These stress increases are on the order of 20 psi beneath E Seam barrier
Agapito Associates, Inc.