Problem $12\mathrm{.}2$ in textbook $($page $527).$ Take the critical state friction angle $($cs$)$ of this soil as $25$ degrees $($this is the answer to Problem $12\mathrm{.}1$ in homework # $10).$ Solve the problem by sketching Mohr$-$Coulomb failure criteria for this problem with appropriate values on it $($in versus $$ space$).-->$ Problem $12\mathrm{.}2$: Consider the specimen in Problem $12\mathrm{.}1.$ a$.$ If a direct shear test is conducted with a normal force of $675$ N$,$ what would be the principal stresses at failure? b$.$ What would be the inclination of the major principal plane with the horizontal?