Figure $2\mathrm{.}64$ represents the cross$-$section of a retaining wall supporting an earth fill. The earth pressure per meier of length of wall can be replaced by its resultant $H=58\mathrm{.}4k\frac{N}{m}$ acting as shown in the figure. Determine the minimum thickness $b$ of the wall required to prevent overturning about the front edge $A$ if $h=4\mathrm{.}57m$ and the specific gravity of the wall is $2\frac{1}{2}$