Practice Question: Effective Stress at Bottom of a Soil Layer

---

---

Solution

Answer: D) 1833 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.

Total Stress:

$$ \sigma_{v} = 115 \ pcf \ast 15 \ ft + (1 + 0.2) \ast 110 \ pcf \ast 15 \ ft = \ \ 3705 \ psf $$

Pore ​​Water Pressure:

$$ u = 62.4 \ pcf \ast 30 \ ft = 1872 \ psf $$

Effective Stress:

$$ {\sigma^ \prime} _{v} = \sigma_{v} - u = 3705 \ psf-1872 \ psf = 1833 \ psf $$
 

---