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

Maximum Strain and Slope Prediction Graph in Affected
Environment/Subsidence). The final graph provides various proportions of the
maximum tensile strain (+E) and maximum compressive strain (-E) along a
profiles from the center of a panel, across the side of the panel to the limit of
subsidence outside the panel (Figure 12. NCB Horizontal Strain Profile
Graph in Affected Environment/Subsidence). The method has been modified
by others to extend its application to room-and-pillar panel mining and to
consider the impact of varying proportions of sandstone, limestone and shale
or mudstone in the overburden.
• The NCB subsidence method is used directly for longwall mines and has
been modified for room-and-pillar mines. Reported subsidence predicted and
measured over room-and-pillar workings at the Roadside Mine east of
Palisade Colorado predicted 1.61 feet, measured 1.02 feet in the Cameo
Seam; the Eagle No. 5 Mine southwest of Craig, Colorado reported having
measured 10% more subsidence than predicted and the Southland Mine near
Canon City, Colorado predicted 1.51 feet, measured 0.89 feet. The NCB
method has been applied for subsidence prediction over a longwall panel in
the Mid Continent Mine west of Redstone, Colorado predicted 4.99 feet and
measured 1.71 feet; the York Canyon Mine west of Raton, New Mexico
predicted 8.1 feet and measured 7.09 feet. Subsidence was predicted using a
modified NCB method at the Chimney Rock coal augering mine east of
Pagosa Springs, Colorado predicted 2.59 feet, measured 0.49 feet.
6.1 Subsidence Zones
There are approximately four overburden zones to consider and analyze in the trough
subsidence process over a longwall panel. Figure 5. Conceptual Representation of
Subsidence Deformation Zones in Affected Environment/Subsidence presents one
such representation (Peng, 1992). There a four generally agreed zones of overburden
response to longwall mining. They are (1) the caved or collapsed zone, (2) the fractured
zone, (3) the continuous deformation zone and (4) the near-surface zones. These zones
are really transitional from one to another, and not sharply bounded.
6.1.1 Caved Zone
After the removal of the coal under the roof of a longwall panel, the immediate roof
collapses and caves upward to fill up the mined void. Piggott and Eynon (1977)
calculated the height of the collapse zone over a longwall panel in coal measure rocks
as 2 to 3.3 times the mining height based on a typical range of percent swell of 30 to
50%, see Figure 10. Potential Collapse Heights Above Different Mine Opening
Geometries. The collapsed rock is a jumbled mass of rubble that will be partially
reconsolidated by the overburden load. The collapsed rock no longer gives the
appearance of having been part of a bedded or stratified sedimentary formation. H.F.
Schulte (1957) reported that the height of the rubble zone exposed in a winze excavated
down into the center of a worked area was 2.4 times the mining height above the seam
floor. P. Kenny (1959) reported observing and measuring the active height of caving into
the original roof above a longwall panel to range from two to four times the mining
height, depending on the angle of repose, fragmentation, bed thickness and swell of the
immediate roof rocks. S. Peng (1992) reported the height of the caved zone is normally
2 to 8 times the mining height, depending on the properties of the immediate roof and
C-22
DBMS 314