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 Mine
Attn: Mr. Tay Tonozzi
April 15, 2010
Page 3
Selection of Model Parameters
Laboratory test results, which form the basis for our selection of strength parameters, are
presented on Figures 12 through 15. Additional laboratory testing has been performed at the
site on spoil and clay soil since our initial analysis of the South Taylor (ST) fills in 2006. We
used this information to update our models for this analysis. The parameters for unit weight
and shear strength used in the models are shown in Tables D -1 through D -4 in Appendix D.
■ Spoil. In our previous (2006) analysis of ST spoil fills we used a shear strength
of 30 degrees friction and no cohesion for the spoil. Since then, large scale
direct shear testing was performed on five samples of spoil (Figure 12). The
results of the testing indicated a mean value of 35 degrees and cohesion of 750
pounds per square foot (psf).
■ Clay. Since our 2006 analysis a consolidated - undrained (CU) triaxial test with
• pore pressure measurement was performed on a sample of valley bottom clay
soil from the Collom Temporary Spoil Fill area (Figure 13). In our opinion, this
test result represents the best estimate of long term drained strength for the
valley soils expected below the ST fills. This strength value of 31 degrees
friction and no cohesion was used as the base case, long term strength
(designated CU in Appendix D). These values compare well with the results of
previous direct shear testing of clay soils from valley deposits at other locations
•
across the site (Figure 14).
We selected the lower bound value for clay (23 degrees, cohesion of 370 psf)
shown on Figure 14 based on the results of the five laboratory test results shown
on this figure. 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 valleys.
■ Carbonaceous Mudstone (CMS). This material was presumed to be at the base
of the spoil below previously mined areas in Sections A -A' and C -C' and also
within the dipping bedrock wedge in section D -D' (refer to Figure 11). The
results of shear testing from previous investigations on this material are shown
on Figure 15. Note these values represent residual strength. The mean strength
Exhibit 21, Item 1 Revision Date: 12/06/10
Addendum No. 3- 3 Revision No.: '`R -Al