PERMFILE52001

• Permit No: C1980007
• IBM Index Class Name: Permit File
• Doc Date: 6/2/2006
• Section_Exhibit Name: Exhibit 60B Subsidence Evaluation for the South of Divide Mining Area
• Media Type: D
• Archive: No

Subsidence Evacuation For
Exhibit 608 South ~ Divide Mining Area Page 4

clacks, construction cracks occur in a continuous zone where weathered and/or &aclured
bedrock is encountered during road constroction.
Construction cracks may be wnfused with subsidence cracks, particulazly where mining has
occurred in the area, and where local bedrock is weathered and fractured, or where bnrsh and
trees have been ripped out of soil and/or colluvium during the wnstruction process. The most
diagnostic features of construction cracks are that they 1) have a less regulaz pattern, 2) are
related to the material they occur in, and 3) they lack of any spatial relationship to the underlying
longwall mine geometry.
4.3 Desiccation Cracks
Desiccation cracks tend to occur in claystones and siltstones of the Mesaverde and Wasatch
Formations in the West Elk Mine area, particularly where the rocks are weathered to clays and
silts. The process.of desiccation involves the shrinking of the clays and silts after a dry period
that follows a wet period, when the material swells (the shrink/swell process).
Desiccation cracks can often be recognized by their irregular, branching and diverging pattern-
less regular than typical subsidence cracks. Some of the largest desiccation cracks in the West
Elk Mine area were observed in clays of the Barren Member of the Mesaverde Formation in the
Horse Gulch-Minnesota Reservoir area and in the weathered claystones of the Wasatch
Formation on West Flatiron, where there has been no mining. The larger, more regular
desiccation cracks and construction cracks may be confused with subsidence cracks in areas
where mining has occurred. However, transverse and longitudinal subsidence cracks have a
defurite spatial relationship to the longwall mining panel causing the cracks.
4.4 Pseudo Subsidence Features (Gravity-Induced Tension Cracks)
Cracks have been observed on high, steep ridges, near cliffs, and in landslides, in the Box
Canyon and Apache Rocks mining areas- These cracks look very much like subsidence cracks,
but cannot be, because no mining had been done when they were observed. A good example of
a gravity-induced crack is the extensive crack that the author observed on the narrow ridge of
West Flatiron in August 2002. This crack was as much as 3.5 in wide and I50 feet long. This
was not amining-related crack because no mining had occurred in the area- The possibility of
gravity-induced cracking in the rugged country above planned mining activities at the West Elk
mining areas is a good reason to perform baseline studies of the area prior to mining so that these
features can be documented prior to any mining.
Cracks and bulges caused by landslides are other types of gravity-induced features that may
appear to be related to subsidence, particularly in areas that have been, or are being, undermined.
However, landslide-induced features are related to the geometry of the landslide rather than Ute
mine geometry. For example, cracks are most common in the upper area of a landslide, whereas,
bulges are most common in the lower area of the slide. This spatial and geometric relationship
to a landslide footprint on steep, unstable slopes, rather than the mine geometry can usually be
used to differentiate between gravity-induced and mine-induced surface featmres.
L-J
831-032.690 Wright Water Engineers, Inc.