A retaining wall is shown in Figure $4.$ Given: height of the wall is equal to $16ft$ and unit weight of the sand backfill is $114l\frac{b}{f}{t}^3$; calculate the active force, $P_a($Magnitude$,$ point of action and direction$),$ on the wall using Coulomb's equation for $=20$

Sand

Unit weight $=$

$c^{\text{'}\text{'}}=0$

${}^{**}=38$

${}^{\text{'}}($wall friction$)$