PERMFILE131380

• Permit No: C1981041A
• IBM Index Class Name: Permit File
• Doc Date: 6/16/2003
• Doc Name: TOC to Appendix 20-3
• Section_Exhibit Name: Tab 20 Subsidence Survey Monitoring & Control Plan
• Media Type: D
• Archive: Yes

~J
POWDERHORN COAL COMPANY
TABLE 20-1
FACTORS OF SAFETY FOR RETREAT PILLARS
•
•
PILLAR SIZE
DEPTH 10X80 12X80 12X80 15X80 18X80 20X80
200 1.15 1.40 1.65 1.77 2.15 2.39
300 0.95 1.16 1.38 1.48 1.80 2.01
400 0.85 1.04 1.24 1.33 1.62 1.81
600 0.74 0.92 1.09 1.18 1.43 1.62
800 0.68 0.85 1.01 1.09 1.34 1.50
1,000 0.65 0.80 0.96 1.04 1.28 1.44
1,200 0.62 0.77 0.93 1.02 1.23 1.39
1,500 0.60 0.74 0.89 0.96 1.19 1.34
~~