2013-09-19_PERMIT FILE - C1981019A ‎(2)‎

• Permit No: C1981019A
• IBM Index Class Name: Permit File
• Doc Date: 9/19/2013
• Doc Name: Geotechnical Report South Taylor Excess Fills
• Section_Exhibit Name: Exhibit 21 Item 1 -ST
• Media Type: D
• Archive: Yes

Colowyo Coal Company, L.P.
Attn.: Mr. Juan Garcia
January 4, 2013
Page 3 of 7
SHANNON 6WILSON, INC.
Spoil. In our previous analysis of South Taylor spoil fills (Shannon & Wilson, 2006) we
used a shear strength of 30 degrees friction and no cohesion for the spoil. This is a low
bound strength that the Division of Mining, Reclamation, and Safety has requested to be
considered in previous spoil stability analyses for permanent excess spoil at the site.
Subsequently, large scale, direct shear testing was performed on five samples of spoil;
results are summarized on Figure 10. The results of the testing indicated a mean value of
35 degrees and cohesion of 750 pounds per square foot (psf). The spoil exhibits an angle
of repose of 38 degrees under low confining pressures. The low bound is considered
conservative when compared to the test results.
■ Clay. A consolidated - undrained (CU) triaxial test with pore pressure measurement was
performed on a sample of valley clay soil from the Collom Temporary Spoil Fill area
(Figure 11). In our opinion, this test result represents the best estimate of long term
drained strength for typical clayey soils expected below the South Taylor fills. This
effective stress shear strength of 31 degrees friction and no cohesion was used as the base
case.
We also considered a lower bound shear strength of 23 degrees, cohesion of 370 psf
(Figure 12) based on the results of five direct shear laboratory test results. In our opinion,
this value represents a reasonable lower bound for the clayey sand (SC) and low plasticity
sandy clay (CL) soil types found in the South Taylor area.
Rock. A rock mass shear strength estimate of 30 degrees friction and 5,220 psf cohesion
was used. The rock bedding in this area is favorable in that the bedding dip is into the
rock face towards the spoil fill. The shear strength is based on back - calculation of rock
slopes that were analyzed in the Collom area as described in our Feasibility Level
Geotechnical Study that analyzed the South Taylor Pit Slopes (Shannon & Wilson,
2006a). This shear strength estimate is intermediate between favorable dipping rock (10
percent into the slope) and adverse dipping rock (10 percent out of the slope). This is a
conservative assumption as the rock bedding has a slightly favorable dip at about 7
percent.
■ Groundwater. The permanent excess fill underdrains extend under this fill (Figure 1).
Our experience indicates that the base of the fill will consist of larger rock particles due
to sorting that occurs as the fill is end dumped in approximate 50- foot -tall lifts. This
sorting results in the base of the fill being relatively permeable. Considering these two
factors, significant water buildup in the base of the fill is not anticipated. .
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