$16\mathrm{.}1$ Figure P$16\mathrm{.}1$ shows a cantilever sheet$-$pile wall penetrating a granular soil. Here, $L_1=3m,L_2=8m,=16k\frac{N}{m^3},{}_{\text{s}\text{a}\text{t}}=18k\frac{N}{m^3},$ and ${}^{\text{'}}=30.$

a$.$ What is the theoretical depth of embedment, $D?$