PERMFILE100540

• Permit No: C1980007
• IBM Index Class Name: Permit File
• Doc Date: 6/13/2005
• Doc Name: 2.05.5 Post - Mining Land Uses
• Type & Sequence: PR10
• Media Type: D
• Archive: No

West Elk Mine
Neaz-Surface Zone
The neaz-surface zone, which typically consists of weathered bedrock, colluvium, and soil
ranging in depth from a few feet to a few tens of feet, may deform differently than the underlying
bedrock. Field studies by Mr. Dunrud indicate that near-surface colluvium and alluvium,
which consist of predominantly clay and silt, can undergo significantly more extension
without rupturing than can the underlying material. In both the Somerset, Colorado and
Sheridan, Wyoming areas colluvium and alluvium 5 to 10 feet thick were observed to cover
cracks as much as 10 to 14 inches wide so that there was no indication of the underlying
ruptures. Mr. Dunrud's observations in the Bear Creek area in 1976 are discussed in the
Final Environmental Impact Statement for the Iron Point Coal Lease Tract and Elk Creek
Coal Lease Tract (2000).
The zone of continuous deformation, which is transitional to the overlying near-surface zone
and to the underlying zone of fracturing, undergoes differential vertical lowering and
flexure as laterally-constrained plates (in three dimensions) or beams (in two dimensions).
With flexure, shear occurs at the boundaries of rock units with different strength and
stiffness, characteristics, such as sandstones and shales. Zones of tension above the neutral
surfaces of a rock unit, for example, become compressive above the boundary with another
rock unit and below its neutral surface (Figure 2, Enlargement 2 of Exhibit 60B). Any
cracks, therefore, which occur in the tension zone of a rock unit, terminate at the neutral
surface, because the unit is in compression below this point.
Maximum Vertical Disp[acemeny Tilt, Horizontal Straln, and Depth of Surface Cracks
Differential vertical lowering of the continuous deformation and near surface zones causes
vertical displacement (S), horizontal displacement (Sh), tilt (lVi), and horizontal strain (E). Each of
these parameters is graphically illustrated in Figure 2, Exhibit 60B. In flat or gently sloping terrain
(slopes less than about 30 percent), surface profiles of subsidence depressions are similar to flexure
of fixed-end, laterally constrained beams. Tensile stresses are present in azeas of positive curvature
decreasing to zero at the neutral surface before which they reverse to become compressive stresses
(see Figure 1, Exhibit 60B).
In flat or gently sloping terrain, vertical displacement typically increases inward from the
limit of the subsidence depression, is half the maximum value at the point of inflection, and
is at its maximum in the middle of the depression (also called subsidence basin or
subsidence trough). Horizontal displacement and tilt increase inward from the margin of
the depression to a maximum at the point of inflection and become zero again at the point
of maximum vertical displacement (Exhibit 60B, Figure 3). Maximum values of tilt,
curvature, and strain, discussed herein, apply only to slopes less than about 30 percent;
values may be greater on slopes steeper than 30 percent.
Positive curvature (convex upward) and horizontal tensile strain increase inward from the
margin of the depression to a maximum about midway between the depression margin and
the point of inflection and decrease to zero again at the point of inflection. Negative
curvature (concave upward) and compressive horizontal strain increase inward from the
point of inflection to a maximum about midway between the point of inflection and the
1.05-112 RevisedJ~me 2005 PR/0