Practice Question: Effective Stress on a Soil Profile

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Solution

Answer: B) 2954 psf

Effective stress is calculated based on the portion of stress that may be resisted by soil contact. To calculate effective stress, first calculate total stress at the bottom of layer 2, then subtract pore water pressure. Note that the layer of water above soil layers is ignored in this calculation.

Total Stress:

$$ \sigma_v=110\ pcf\ \ast\ 20\ ft\ast(1+0.25)+135\ pcf\ast20\ ft=\ \ 5450\ psf $$

Pore Water Pressure:

$$ u = 62.4\ pcf\ \ast\ 40\ ft =\ \ 2496\ psf $$

Effective Stress:

$$ \sigma'_v=\ \sigma_v-u=5450\ psf-2496\ psf=2954\ psf $$
 
 

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