When the water table rises to the ground surface, the bearing capacity of a soil is reduced to:
Try it first: pick an option, then read the explanation.
Asked in SSC JE 2015
Detailed Explanation
Bearing capacity of a cohesionless soil is directly proportional to the unit weight of the soil in the γDfNq and ½γBNγ terms. Below the water table, buoyancy reduces the effective unit weight.
\[ \gamma' = \gamma_{sat} - \gamma_w \approx 20 - 10 = 10\ \text{kN/m}^3 \approx \tfrac{1}{2}\gamma \]
Why “One-half” Is Correct
When the water table is at the ground surface, γ′ replaces γ in both terms and each term is roughly halved. The bearing capacity therefore becomes about one-half of its dry value.
| Water table position | Reduction factor on the γ terms | Effect on bearing capacity of sand |
|---|---|---|
| At depth ≥ B below the footing base | 1.0 (no reduction) | None |
| At the footing base | Rw2 = 0.5 for the ½BγNγ term | Reduced |
| At the ground surface | 0.5 for both γDfNq and ½BγNγ | About half (50% reduction) |
Rw = 0.5(1 + zw/B or Dw/Df). The submerged unit weight is roughly half the saturated unit weight, which is why the maximum reduction is about 50%.
Why the Other Options Are Wrong
- A. One-fourth: One-fourth would mean γ′ ≈ γ/4, which is far too low.
- C. Two-third: Two-thirds understates the effect of full submergence.
- D. Three-fourth: Three-fourths is closer to the case of a water table well below the base.
Key Point
Full submergence → bearing capacity of sand roughly halved.
Key Concepts for Students
- Reduction factor R_w = 0.5(1 + z_w/B), between 0.5 and 1.
- Seasonal high water table should be used for design.
- Dewatering raises bearing capacity during construction only.
Practice more: this is question 31 in Shallow and Deep Foundation MCQs – Part 3. Try the whole set and check every answer instantly.