a$)$ The $12m$ high retaining wall with galvanized steel strip reinforcing in granular backfill is depicted in Figure Q$4.$ Analyze the internal and external stability checking of this retaining wall with requirement of $F{S}_{(B)}FS(P)=3.$ Assume the galvanized steel corrosion rate is $0\mathrm{.}025m\frac{m}{?}$ year and the structure have a life span of $50$ years.

The properties for Galvanized steel reinforcement:

$w=72mm$

$S_V=0\mathrm{.}6m$ center $to$ center

$S_H=1m$ center $to$ center

$f_y=242M\frac{N}{m^2}$

${}_{}^{\text{'}}=20$

Foundation Soil

${}_2^{\text{'}}=28$

${}_2=18k\frac{N}{m^3}$

Granular Backfill

${}_1^{\text{'}}=20$

${}_1=19k\frac{N}{m^3}$