$($c$)$ Determine the effective stress at a depth of $4$ m below the soil surface at a location where the soil profile consists of a $2$ m thick layer of partially saturated fill overlying a deep layer of sand. The water table is level with the top of the sand layer. The soil properties are:

Fill: dry unit weight, ${}_d=16k\frac{N}{m^3},$ moisture content $=12\%,$ specific gravity $G_s=2\mathrm{.}6$

Sand: dry unit weight, ${}_d=18k\frac{N}{m^3},$ moisture content $=16\mathrm{.}0\%,$ specific gravity $G_s=2\mathrm{.}6$