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contractive behavior. As a result, it was decided to utilize the correlation between blow counts and <br />undrained shear strength to estimate the undrained shear strength of the soft sediment. <br />The undrained shear strength ratio of the soft sediment was estimated as 0.23, and this is the value <br />that was used to model the soft sediment for the USSA. The undrained shear strength ratio is defined <br />as the ratio of the undrained shear strength su divided by the vertical effective stress d„a. This ratio <br />was based on undrained shear strength from blow counts normalized by the vertical effective stress. <br />The average blow count in the soft sediment is about 4 blows/ft, which corresponds to an s„ of 500 <br />psf (Terzaghi et al., 1996). The typical effective stress is about 2,200 psf, resulting in a ratio of 0.23. <br />Also, 0.23 was further corroborated by a published correlation (Terzaghi et al., 1996) between <br />undrained shear strength ratio and plasticity index, which was used to obtain a value for the more <br />conservative direct simple shear (DSS) failure mode. <br />Figure 9 shows the drained shear strength envelope for the soft sediment based on triaxial and direct <br />shear tests. It can be seen from Figure 9 that the drained failure envelope is non-linear and yields <br />variable shear strength with effective stress. The results from both the triaxial and direct shear tests <br />display fairly consistent results. Therefore, the soft sediment material was modeled with anon-linear <br />failure envelope, as shown in Figure 9, for the ESSA. <br />Due to the low blow counts and granular nature of this soil, its susceptibility to liquefaction was <br />evaluated. Blow counts (assuming raw blow counts N corresponded to an energy of 60% of the <br />theoretical value, or N60) were used with an earthquake acceleration of O.15g and a magnitude of 6.0. <br />Based on these inputs, none of the blow counts were in the liquefiable range. The primary reason for <br />the non-liquefiability of the soil is its relatively high fines content, which is defined as the percentage <br />of material passing the #200 sieve. All tested values were greater than or equal to 23%, with field <br />descriptions indicating fines contents above about 30%. <br />8.2.2.4 Colluvium <br />The undrained shear strength of the colluvium was not measured and thus it was estimated from blow <br />counts (Terzaghi et al., 1996). The blow counts from the SPT in the colluvium (Cross-Section C) are <br />in the range of 5 to 100 blows/ft, with typical values between 10 and 20 blows/ft. For the upper <br />colluvium, the blow count correlation yielded an undrained shear strength s„ of 2,000 psf using <br />average blow counts of 15 blows/ft. The same correlation resulted in an s„ of 4,000 psf in the lower <br />colluvium using average blow counts of 31 blows/ft. <br />The drained shear strength of the colluvium was measured in the direct shear test only. Figure 10 <br />shows the results of the direct shear test on the colluvium. It can be seen from Figure 10 that the <br />failure envelope is non-linear. This failure envelope yields a similar strength as used in previous <br />stability analyses (GEI, 2006). Published correlations (Stark and Eid, 1997) point to afully-softened <br />friction angle closer to 32 degrees for liquid limit values around 40% and clay-size fractions less than <br />P:\Mpls\06 CO\26\0626067\WorkFiles\DesignReport\FINAL\DesignReportFINAL.doc 27 <br />