Problem $3.$ Undrained Strength

The soil profile and loading situation from our New Orleans project is shown below. An undisturbed sample was taken in Normally Consolidated fine$-$grained soil before an excavation for the canal is made $($see Problem $4).$

Unconsolidated undrained $($UU$)$ triaxial tests were conducted on three specimens of the soil taken from the undisturbed sample. The following test results were obtained at failure.

a$)$ Classify this soil based on the USCS system. $\backslash (\backslash$mathrm$\{$LL$\}=76,\backslash$mathrm$\{$PL$\}=21.\backslash$mathrm$\{$G$\}\_\{\backslash$mathrm$\{$s$\}\}=2\mathrm{.}74\backslash ).$

b$)$ Plot the three Mohr's circles at failure and interpret these data to obtain the undrained strength of the soil, $\backslash (\backslash$mathrm$\{$S$\}\_\{\backslash$mathrm$\{$u$\}\}\backslash ).$ Use $\backslash (\backslash$sigma$\_\{3\}\backslash )$ and $\backslash (\backslash$sigma$\_\{\backslash$mathrm$\{$df$\}\}\backslash )$ as given in the lab test results provided above.

c$)$ Suppose an areal fill is added to the ground surface and consolidation of the soft to medium clay is allowed to complete. At that time, we take another undisturbed sample at the same elevation as the first sample and conduct UU tests on three specimens. Show qualitatively where you would expect the resulting strength envelope to lie with respect to the first envelope of b$).$ Is $\backslash (\backslash$mathrm$\{$S$\}\_\{$u$\}\backslash )$ in this case higher, lower or the same as in b$)?$