2008-02-22_APPLICATION CORRESPONDENCE - C2008086 ‎(43)‎

• Permit No: C2008086
• IBM Index Class Name: Application Correspondence
• Doc Date: 2/22/2008
• Doc Name: PDEIS Appendix C Subsidence
• Media Type: D
• Archive: No

4.3 Lithology and Angle of Draw
The purpose of the reasonably nearby drilling through the Mount Garfield formation for
Dorchester Coal Company's Fruita Project was to explore for potential mining of the
Main Cameo which ranged from 10 to 29 feet thick at depths of up to 1600 feet in their
proposed lease area. The reported lithologic distribution of rock types above the Main
Cameo from 19 drillholes, which individually penetrated between 67 and 1316 feet of
overlying rock, for a total of 13,880 feet of drilling is presented in Table 5. Lithologic
Distributions for Dorchester Project Overburden. The overall average percentage of
sandstone in the overburden is approximately 46%. Abel and Lee (1984) collected data
on the relationship between measured angles of draw and the lithologic distribution in
the overburden above several coal seams. Figure 14. Estimated Angle of Draw in
Relation to Percent Sandstone and Limestone presents the relationship. The
Dorchester Project drilling indicated 46% sandstone and no limestone and predicts a 19°
angle of draw (a). The Dorchester drilling indicated considerable lateral rock type
variation. Therefore, it should be anticipated that there will be a similar variation in the
angle of draw. The range of sandstone percentage as determined from the drillholes was
from 28 to 65%, suggesting a range for the angle of draw from just over 15° to over 25°.
Angles of draw were predicted at 25° at two coal mines in Colorado mining in the
Mesaverde Group based on drillhole lithology. Later survey measurements indicated
angles of draw of 21 ° and 22°.
A 19° to 22° angle of draw is on the low end of the range of values reported for the
countries listed on Table 6. Angles of Draw for Coal Mining in the United States and
Europe. The British National Coal Board's (NCB) conservative 35° angle of draw has,
however, been measured in Pennsylvania (Auchmuty, 1931). The larger NCB angle of
draw estimate will be used because it should overestimate the area outside a longwall
panel potentially affected by mining. In addition, the NCB maximum subsidence value
(Smax) calculated from the flatter English terrain measurements was 17% to 21 % greater
than what was measured for ridge tops over three longwall panels in Mesaverde Group
rocks and mountainous terrain at the York Canyon Mine west of Raton, New Mexico.
NCB predicted subsidence in topographic lows were 55% greater than measured at the
York Canyon Mine. This implies that the maximum tensile strain, compressive strain and
tilt estimated using the NCB method may be similarly greater than what will be measured
in the Project Area because the strains and tilt are directly proportional to the maximum
panel subsidence (Smax) value.
5.0 TOPOGRAPHIC FACTORS AFFECTING SUBSIDENCE
5.1 Rugged Terrain
The Red Cliff Mine Project Area is located in canyon-ridge topography. As shown on
Table 3, overall slope angles range from 21 ° to 41 ° (38% to 87%) for canyon walls
ranging from 400 feet to 920 feet high. Cliff sections are present on some canyon walls
where thicker sandstones outcrop. Because of this rugged terrain, subsidence related
surface impacts may change several times as the overburden depth changes along the
roughly 7,300-foot to 13,500-foot lengths of the longwall panels. Subsidence, strain and
tilt predictions will be less certain than would be the case in more gentle and flatter
terrain. For example, vertical displacement may be as much as 30 percent greater over
narrow ridge tops. The overburden ahead of a moving longwall face will move down
slope as the subsidence trough ahead of the longwall face approaches but will not be
C-16
DBMS 308