For the given L$-$shape retaining wall, determine:

a$)$ Lateral active and passive stress at depth $6\mathrm{.}0$ m $.$

b$)$ Safety against sliding.

c$)$ Referring to $b,$ if the retaining wall isn't safe, determine the key depth so that the wall is safe against sliding.

d$)$ Safety against bearing capacity if you know that the $q_{\text{u}\text{l}\text{t}}$ of soil $400k\frac{N}{m^2}.$

$=20k\frac{N}{m^3}$