PERMFILE58753

• Permit No: C1982056
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: EXHIBIT 31 SPOIL ANALYSIS
• Media Type: D
• Archive: No

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We understand the temporary storage pile will be constructed over spoils
placdd duf•ing grading and backfilling of a portion of the 8050 pit. The
downslope or last highwall for the pit is believed to exist approximately
beneath the toe of the proposed storage pile. With these conditions, the
disposal area would be underlain by 80± feet of spoil then hard sedimentary
bedrock. The spoil was placed by dragline and is, in our opinion, loose
compared to materials placed for construction of the Eckman Park disposal
piles. The in-place spoil is estimated to have angles of internal friction
near the range of 34 dagrees, while compacted spoil has internal friction ,
angles approaching 40 degrees.
A cohesion of 5,000 psf and an angle of internal friction of 20 degrees
was used to model the strength of the bedrock. We used an angle of internal
friction of 37 degrees for the compacted spoil, and an angle of internal
friction of 34 degrees for the loose spoil over the regraded pit. The
above values are consistent with those used during our analysis of stability
of Excess Spoil Disposal Areas I and II within Eckman Park (Job No. 5664)
and Evaluation of Slope Stability far the Twenty Mile Facility.
Stability of the fill was evaluated using circular failure surfaces analyzed
using the simplified Bishop method. We have this analysis method on an
in-house computer. The program "searches" and analyzes numerous possible
failure circ]es until a lower limit factor of safety is computed. The failure
surface of a cohesionless material degenerates to a relatively flat circle
and an infinite slope type failure. We used a cohesion value of 100 psf for
the spoils to avoid degeneration of the problem and to evaluate deep-seated
failures.
Based upon exploratory test pits excavated near the crest and toe of the
existing 8050 spoil bank, it appears that the near-surface spoil is pre-
dominantly sail sizes and contains more than 15 percent clay and silt sizes.
In a compacted state, we believe that the materials will have relatively
low permeability. The near-surface spoils will be removed first from the
pit and will probably be placed in the lower portions of the storage fill.
We anticipate that the spoil will become much larger in size and more rack-
]ike as grading extends into the existing spoil bank. After blasting the
materials removed from the upper portions of the highwall will probably be
soil-like and become more rock-like with increasing depth. The larger
material is estimated to have very high permeability in a compacted state
and will probably be placed in the upper portions of the disposal area.
Based upon the assumed placement procedure, we believe that formation of
relatively impermeable zone near the lower portions of the fill is passible.
Infiltration of water through the more permeable upper spoil could result
in the formation of a "perched" water table within the fill after construc-
tion. The water table condition assumed in our analyses was used to
model this passibility.