Problem $5$: The maintenance facility shown is located next to a reservoir. The water level in the reservior is proposed to be raised by $10ft$ above the existing water level which is currently located at $22ft$ below the ground surface under maintenance facility. Test boring records indicate that the existing subgrade soils consists of $15ft$ of compacted clayey sand underlain by silty clay. The clayey sand has an average moisture content of $6\mathrm{.}6\%$ and a porosity of $0\mathrm{.}28.$ When dry, $1f{t}^3$ of the clayey sand weighs $108lb.$

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Once the reservoir water level is raised, find the change in effective stress at the base of the clayey sand layer.

$($A$)160l\frac{b}{f}{t}^2$

$($B$)190l\frac{b}{f}{t}^2$

$($C$)1600l\frac{b}{f}{t}^2$

$($D$)1800l\frac{b}{f}{t}^2$