Environmental Engineering Reference
In-Depth Information
Detailed study
of conditions requires core borings to obtain cores of the roof, floor, and
pillars, permitting an evaluation of their condition by examination and laboratory testing.
Finite-Element Method Application
General
The prediction of surface distortions beneath a proposed building site caused by the pos-
sible collapse of old mines by use of the finite element method is described by Mabry
(1973). The site is in the northern anthracite region of Pennsylvania near Wilkes-Barre, and
is underlain by four coal beds at depths ranging from 260 to 600 ft, with various percent-
ages of extraction
R
, as illustrated in
Figure 10.13.
Finite Element Model
Pillar analysis revealed low safety factors against crushing and the distinct possibility of
subsidence. To evaluate the potential subsidence magnitude, a finite-element model was
prepared incorporating the geometry of the rock strata and mines to a depth of 700 ft, and
engineering properties including density, strength, and deformation moduli of the inter-
vening rock and coal strata. The finite element mesh is given in
Figure 10.14.
Analysis and Conclusions
Gravity stresses were imposed and the ground surface subsidence due to the initial min-
ing in the veins was determined. Subsequent analysis was made of the future surface dis-
tortions, such as those would be generated by pillar weathering and eventual crushing.
Pillar weathering was simulated by reducing the joint stiffness (see
Section 6.4.4),
and pil-
lar collapse or yield was simulated by setting the joint stiffness in the appropriate intervals
of the coal seams to zero. After pillar weathering in the coal seams was evaluated by
changing joint stiffness values, the intervals of the veins were “collapsed” in ascending
order of computed safety factors for several extraction ratios. The results are summarized
in
Table 10.3.
After an evaluation of all of the available information, it was the judgment of the inves-
tigator that the more realistic case for plant design was Case 2 (Table 10.3), and that the
probability of Cases 3 and 4 developing during the life of the structure was very low.
Site
600
500
400
300
270 ft
George vein
George
vein
R
= 0%
Abbott vein
370 ft
200
Abbott vein
R
= 45%
R
= 85%
R
= 10%
R
= 20%
R
= 5%
After robbing
100
R
= 10%
R
= 40%
Not
mined
R
= 30%,
R
= 65%
500 ft
Mills vein
After robbing
0
R
= 15%
R
= 20%
R
= 20%
R
= 35%
R
= 35%
R
= 45%
R
= 40%
R
= 45%
R
= 45%
Hillman Vein
R
= 20%
R
= 30%
100
R
= 30%
R
= 50%
R
= 50%
R
= 20%
R
= 45%
FIGURE 10.13
Section illustrating coal mines and percent extraction
R
beneath a proposed construction site near Wilkes-Barre,
Pennsylvania. (From Mabry, R.E.,
Proceedings of ASCE, 14th Symposium on Rock Mechanics,
University Park, PA,
1973. With permission.)
