A smooth gravity wall retains a $12m-$high backfill as shown in the figure below. The top $8m$ is sand, which is underlain by some clay. The soil properties are as follows:

Sand: ${}_m=18\mathrm{.}9k\frac{N}{m^3},{}_{\text{s}\text{a}\text{t}}=19\mathrm{.}8k\frac{N}{m^3}$;${}^{\text{'}}=32$

Clay: ${}_m=20\mathrm{.}1k\frac{N}{m^3}$;${}^{\text{'}}=18,c^{\text{'}}=20$kPa

Assuming that the entire soil is in the active state, find the location and magnitude of the total thrust on the wall.

Figure: Gravity Wall