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
Table 7. Normalized Coal Strengths
Area Seam
Average UCS
In -Situ Strength Estimate
(psi)
(psi)
South Taylor G7 and G78
1,672
727
E and 132 /E
2,767
1,203
West Pit F
2,931
1,274
E
2,767
1,203
D2
1,498
651
C
2,004
871
Page 38
Therefore, web and barrier pillar design for the E and D2/E seams in South Taylor, and the F and
E seams in the West Pit is based on 900 -psi in -situ coal strength. The remaining target seams
use the in -situ strength estimate from Table 7. Because of past failures of highwall mining
pillars with low width -to- height ratios, AAI recommends that a minimum pillar width-to- height
criterion of 0.8 be applied.
Once pillar strength is determined, an estimate of pillar loading is required to calculate a
safety factor. Pillar loading was estimated using tributary area load theory as follows:
LP = SV(W + WE)l W (Eqn. 4)
where Lp = average vertical load on the pillar (psi)
Sv = in -situ vertical stress (psi)
W = pillar width (ft)
WE = entry width (ft)
Finally, safety factor is calculated as:
SF = SPILP (Eqn. 6)
4.2 Web and Barrier Pillar Design
Using the more conservative of the modified van der Merwe formula and the Mark -
Bieniawski formulas, tributary area loading, and a 1.5 safety factor criterion, minimum web
pillar widths were calculated for each seam. Web pillars are designed to remain stable and to
reduce the probability of subsidence. In determining pillar loads, an average overburden density
of 144 pounds per cubic feet (pcf) was applied based on results of physical property testing. For
barrier pillars, the same 1.5 safety factor criterion was applied, and barrier pillar loading was
calculated using a 21' abutment angle (the pillar supports all overburden directly above it and
within a 21° angle over the adjacent web pillars as measured from vertical).
Design tables and charts for the web and barrier pillar designs developed, along with raw
recoveries (based on taking the full available seam height), are shown in Figures 29 through 33.
Web pillar widths are given in inches in the tables, and in both inches and feet in the charts.
Barrier pillar widths are given in feet in both. Barrier pillars were designed assuming that they
would be placed after every 20 highwall openings. AAI recommends this criterion for normal
operations.
Agapito Associates, Inc.