Example $13\mathrm{.}1$: A retaining wall, $6$ m high, retains dry sand with an angle of friction of $30$ and

unit weight of $16\mathrm{.}2k\frac{N}{m^3}.$ Determine the earth pressure at rest. If the water table rises to the

top of the wall, determine the increase in the thrust on the wall. Assume the submerged unit

weight of sand as $10k\frac{N}{m^3}.$