Practice Question: Effective Stress at Bottom of a Soil Layer

---

---

Solution

Answer: D) 2634 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:

\[σ_v=(1+0.3)×105 \ pcf×20 \ ft+120 \ pcf×20 \ ft=5130 \ psf\]
Pore Water Pressure:

\[u=62.4 \ pcf × 40 \ ft=2496 \ psf\]
Effective Stress:

\[σ'_v=5130 \ psf-2496 \ psf=2634\ psf\]
 

---