$(2)$ A ritaining wall with a vertical back, $8m$ high, upports $$ sand soil with $c^{\text{'}}=0$ and ${}^{\text{'}}=24.$ Neglecting wall friction, calculate the total actine thrust on the wall, i$)($a$)$ s the water table is below the base of the wall $)=(16k\frac{N}{m^3},($b$)$ If the water table rises up to a height of $4m$ above the base of the wall $)=(20\mathrm{.}5k\frac{N}{m^3}$