Geology Reference
In-Depth Information
FIGURE 9.8 Trough or sag subsidence over a coal mine with collapsed pillars. Note the damage on the corner of each house over the
fracture zone. The unit below the coal seam represents a clay paleosol (an old soil).
(Modified from Turney (1985) after DuMontelle and others, 1981, Illinois State Geological Survey; Environ. Geol. Notes, No. 99; used with
permission.)
4. a. Circle the option below that would most likely be the
setting for a pronounced subsidence feature at the surface, if
the size of the void and the rock types were the same in both
cases.
7. In Figure 9.8, a block diagram of a subsidence event, what
is the major type of subsidence feature shown?
Void in a deep mine
8. In Figure 9.8, describe what is occurring:
a. to the land and the buildings (visible damage) on the
margins of the subsidence feature
Void in a shallow mine
b. Explain
b. to the land in the central area of the subsidence feature.
5. Why might your house be at risk even if it were not
directly above a void shown on a mine map? (Hint: Review
Introduction.)
9. a. In Figure 9.8, on the margins of the collapsed area above
a depth of ~35 ft, draw (and identify) a sloping line to show
the increased width of surface subsidence that begins at this
depth.
b. What does the line that you have drawn in this diagram
suggest about the difficulty of determining if your house
is "at risk"?
6. Over a period of several months, what damages, clues, or
processes might you observe that help you to recognize that
your house is subsiding?
c. The level above which the width of subsidence begins
to increase also is where a major change in geologic
materials occurs. Above is unconsolidated glacial till in
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