$12\mathrm{.}12$ Consider the clay sample in Problem $12\mathrm{.}11.$ A consolidated$-$undrained triaxial

test was conducted on the same clay with a chamber pressure of $10$l$($b$)/($i$)$n$^(2).$

The pore pressure at failure was $(\backslash$Delta u$\_($d$))\_($f$)=4\mathrm{.}2$l$($b$)/($i$)$n$.^(2).$ What would be the major

effective principal stress, $\backslash$sigma $\_(1)^(\text{'}),$ at failure?