Example: $($a$)$ Calculate the total active thrust on a vertical wall $5$ m high retaining a sand of unit weight $17k\frac{N}{m^3}$ for which ${}^{\text{'}}=35$; the surface of the sand is horizontal and the water table is below the bottom of the wall. $($b$)$ Determine the thrust on the wall if the water table rises to a level $\frac{?}{{?}_{?}}$ below the surface of the sand. The saturated unit weight of the sand is $20k\frac{N}{m^3}.$