$13\mathrm{.}9$ Consider the clay specimen in Problem $13\mathrm{.}8.$ A consolidated$-$undrained triaxial test was conducted on the same clay with a chamber pressure of $15l\frac{b}{i}{n}^2.$ The pore pressure at failure is $(u_d{)}_f=4\mathrm{.}8l\frac{b}{i}{n}^2.$ What would be the major principal effective stress, ${}_1$ : at failure?

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Answer:

$31\mathrm{.}86l\frac{b}{i}{n}^2$