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 55
Table 8. Input Parameters Used for the LAMODEL Analyses
Young's modulus (rock) (psi)
E Seam
F Seam
Cover depth range (ft)
120 -400
142 -422
1 E to F interburden thickness (ft)
1
15
Design coal strength (psi)
900
900
Modeled coal strength (psi)
varies
900
Young's modulus (coal) (psi)
330,000
290,000
Poisson's ratio (coal)
0.30
0.25
Young's modulus (rock) (psi)
676,000
Poisson's ratio (cock)
0.25
Lamination thickness (inches)
600
Vertical stress gradient (psi/ft)
1
Design opening width (ft)
11.5
Modeled opening width (ft)
11.5
Element size (inches)
34.5
Number of yielding stages (material types)
10
Web pillar width (no. of elements, typical)
3-4
Barrier pillar width (no. of elements, typical)
9
pillars. As an extreme case, the E Seam in -situ strength was decreased to 605 psi to simulate
complete collapse of the highwall mining panels (Figure 36). Loads are transferred from the
collapsed pillars to intact abutment zones, either on the panel periphery, or in areas where
highwall mining did not fully penetrate. The resulting stresses imparted to the underlying F
Seam are shown in Figure 37. Stresses below the E Seam abutments are three to four times the
overburden stress. Due to the age of the E Seam mining (approximately 7 years) and the
inability to assess the current stability of E Seam pillars, AAI considers the risks of highwall
mining the F Seam below these workings to be high. Pillar design could be performed
accounting for the worst -case stress concentrations indicated in Figure 37; however, pillar widths
approaching 40 ft would be required.
6.0 OPERATIONAL CONSIDERATIONS
The design curves presented in Figures 29 through 33 provide Colowyo with a rational
starting point for highwall web and barrier pillar layout. By using these design curves to
determine the minimum pillar width for each panel as mining progresses, and adjusting that
width as conditions warrant, maximum resource recovery can be attained. Based on the
observations and analyses discussed in this report, the following recommendations for
implementation of the design are made.
Mining Sequence —The lowest seams in each pit will be mined first, with backfill placed
to the floor elevation of the next highwall mining target seam. Advancing highwall miner
openings uphill in the pit (from north to south in South Taylor and from east to west in the West
Pit) is recommended as it will leave any water drainage or accumulation behind the mining.
Agapito Associates, Inc.