Example $20\mathrm{.}17.$ A retaining wall $4m$ high, with vertical back supports a backfill with horizontal

ground surface. The soil has unit weight of $18k\frac{N}{m^3}$ and angle of internal friction of $35.$ The angle of

wall friction may be taken as $20.$ A footing, running parallel to the retaining wall and carrying a load

intensity of $18k\frac{N}{m},$ is to be constructed. Find the safe distance of the footing from the face of the wall

so that there is no increase in lateral pressure on the wall due to the load of footing.