PERMFILE123352

• Permit No: C1980007
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: Exhibit 60 Subsidence Evaluation for Apache Rocks Mining Area & Box Canyon Lease Tract
• Media Type: D
• Archive: No

Subsidence Evaluation For
Exhibit 60 The Apache Rocks And The Box Canyon Mining Areas Page 6
• The near surface zone, which typically consists of weathered bedrock, colluvium, alluvium, and
soil a few feet to a few tens of feet thick, may deform differently than the underlying bedrock
(Figure 1). Field studies by the author indicate that neaz-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 l0 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.
The zone of continuous deformation, which is transitional to the overlying near-surface zone and
to the underlying zones of caving and fracturing, undergoes differential vertical lowering and
flexure as laterally-constrained plates (in three dimensions) or beams (in two dimensions). With
flexure, sheaz 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 also
below its neutral surface (Figure 1, Enlargement 2). 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.
4.3.1 Vertical and Horizontal Displacement, Tilt, Curvature, and Horizontal Strain
• Differential vertical lowering of the continuous deformation and near surface zones causes
vertical displacement (S), horizontal displacement (Sh), tilt (M), curvature (C), and horizontal
strain (E). In flat or gently sloping terrain (slopes less than about 20 percent), surface profiles of
subsidence depressions are similar to flexure of fixed-end, laterally constrained beams. Tensile
stresses are present in areas of positive curvature, which become zero downward at the neutral
surface, then reverse to compressive stresses below the neutral surface.
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 maximum
in the middle of the depression (also called subsidence basin or subsidence trough). Horizontal
displacement and tilt increase inward from the mazgin of the depression to a maximum at the
point of inflection and become zero again at the point of maximum vertical displacement (Figure
2). 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 [he point of
inflection and decrease to zero again at the point of intection. Negative curvature (concave
upward) and compressive horizontal strain increase inward from the point of inflection to a
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831-032.181 Wright Water Engineers. Inc.