$tw,t2,t3$ are thicknesses of the strata $1,2,3$ are total unit weights of the strata

GWT is the depth of the groundwater table from the ground surface

$t1=8ft$

$t2=10ft$

$t3=9ft$

$g1=110l\frac{b}{f}{t}^3$

$g2=130l\frac{b}{f}{t}^3$

$g3=119l\frac{b}{f}{t}^3$

$GWT=28ft$

Find: Effective stress in the middle of the SM strata. Answer in $l\frac{b}{f}{t}^2.$