2011-09-23_REPORT - C1980007 ‎(8)‎

• Permit No: C1980007
• IBM Index Class Name: REPORT
• Doc Date: 9/23/2011
• Doc Name: Spring 2011 Subsidence Report - Appendix A Thru C ‎(Part 2 of 3)‎
• From: Wright Water Engineers, Inc
• To: DRMS
• Permit Index Doc Type: Subsidence Report
• Email Name: TAK
• Media Type: D
• Archive: No

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Subsidence Prediction Based on Measurements
Appendix A at West Elk Mine Page 3
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 A). Field studies by the author 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 such that there was no indication of the underlying ruptures.
The zone of continuous deformation, which is transitional to the overlying near - surface zone and
also 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 rock units, for
example, become compressive above the boundary with other rock units and below their neutral
surface (Figure A, 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.
Vertical and Horizontal Displacement, Tilt, and Horizontal Strain
Differential vertical lowering (downwarping) of the continuous deformation and near surface
zones causes vertical displacement (S), tilt (M), and horizontal strain (E). 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 areas
of positive curvature, but become zero downward at the neutral surface, then reverse to become
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 (S /2, Figure B),
and is 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 (Figure B). Maximum values of tilt, curvature, and strain, discussed herein, apply
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 point of maximum vertical
displacement and decrease to zero again at the point of maximum vertical displacement.
Maximum Vertical Displacement (Subsidence)
The following range of vertical displacements (subsidence values) are projected for the South of
Divide mining area, based on the baseline data obtained from subsidence measurements above
831 - 032.690 Wright Water Engineers, Inc.