Vertical stress below 3m ground level..

when Ground water table(G.W.T) is at 5 m from the ground surface::

there is no effect of pore water pressure at 3m..

so,,effective vertical stress at 3m = (unit wt X thickness of soil) =18.5 * 3 = 55.5 KN/m^2...

when Ground water table(G.W.T) is at 2 m from the ground surface::

there is effect of porewater pressure that results in decrease in effective stress...

so,,effective vertical stress at 3m= (unit wt X thickness of soil) - (9.81 * 1) = 45.69 KN/m^2