$10\mathrm{.}6.$ A concrete drainpipe has inside diameter $30$ inches and wall thickness $4$ inches. When dry, it is loaded by external soil pressure of $10$ psi and when full, the maximum internal pressure is $30$ psi $.$ Determine the factor of safety against failure if concrete obeys the modified Mohr criterion $(2\mathrm{.}2\mathrm{.}4)$ with $S_t=0\mathrm{.}6$ksi,$S_c=5\mathrm{.}0$ksi. Assume that the axial stress ${}_{zz}=0.$

Anser: S$.$F$.=7\mathrm{.}9$