PERMFILE72922

• Permit No: C1982056
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: Exhibit 26a Geotechnical Investigation Refuse Disposal Area ‎(Revised)‎
• Media Type: D
• Archive: No

• Case 1 B represents a low water condition (water level 5 feet
above the clay) and a factor of safety of 1.6 was calculated (Fig.
C-2).
Case 1 C represents a high water condition (water to about one-
half the coal waste height) and a factor of safety of 1.5 was
calculated (Fig. C-3). A toe drain was provided to drain the fill
and we do not believe saturated conditions to 1/2 the fill height
are likely. For both the low and high water conditions the model
assumes that the clay and bedrock layers are saturated and
buoyant unit weights are used. If a water table were to develop
in the pile we do not believe the underlying clay and bedrock will
be fully saturated and consider this to be a very conservative
model.
Group 2 This analysis used the lower bound strength for both the coal
waste and underlying clay. The unit weights were varied
downward and no significant effect on the factor of safety
occurred. Factors of safety using the lower bound strength and
low unit weights were 1.6 to 1.7 for all three cases. Lowering the
unit weights reduces both the calculated driving force and
resisting force and for the condtions modelled did not affect the
• factor of safety.
Group 3 This analysis evaluated the stability if cohesionless surface
mining spoil exists below the waste pile. For both cases 3A and
3B infinite slope conditions control and factors of safety of 2.0
and 1.6 were calculated for normal and lower bound strengths;
respectively. Case 3C calculated a factor of safety of 1.5 with the
high water condition.
Group 4 Apseudo-static analysis was performed to evaluate the potential
effects of earthquake ground motions on pile stabilky. The well
graded and compacted coal waste, and the underlying clay or
claystone are not considered susceptible to liquefaction and a
pseudo-static analysis is considered appropriate. Apseudo-static
coefficient of 0.1g horizontal acceleration was used in the
earthquake models. The total stress condition is appropriate and
lower bound and normal strength estimates for the clay and coal
waste were evaluated.
The calculated factors of safety ranged from 1.3 to 1.6. As
previously discussed, the low bound total stress strength for the
clay is considered very conservative. Using this strength with the
normal strength for the coal waste resulted in a factor of safety
of 1.3 which is considered a worst case model. When the higher
• clay strength is used the critical circles are shallower and the
coal waste properties control the stability. We believe this is the
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