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Solution
Answer: A) 1,723 PSF
Effective stress = Total stress – Pore water pressure
\[\sigma'_v= \gamma _1 h_1+ \gamma '_2 h_2+( \gamma _3 'h_3)/2\]
where;
\(\sigma'_v=\) effective vertical stress
\(h_1=\) thickness of Silt layer
\(h_2=\) thickness of Sand layer
\(h_3=\) thickness of Desnse Sand layer
\(\gamma '_2 = \gamma _2- \gamma _w\)
\(\gamma '_3 = \gamma _3- \gamma _w\)
\(\gamma _w=62.4\ lb/ft^3\)
\[=80 \times 8+(120-62.4) \times 10+(130-62.4) \times15/2=1,723\ PSF\]
Alternatively, it can be calculated as:
\[\sigma_v-U= \gamma _1 h_1+ \gamma _2 h_2+( \gamma _3 h_3)/2- \gamma _w (h_2+h_3/2)\]
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