If the water table reaches the ground level, the unit weight of soil for bearing capacity calculation is taken as:
- Adry unit weight
- Bbulk unit weight
- Csaturated unit weight
- Dsubmerged unit weightCorrect
Solution
Concept
When soil is below the water table, buoyancy acts, so the effective (submerged) unit weight is used.
Formula
γ' = γsat - γw
Step-by-step solution
1. The water table is at ground level, so all soil below the footing is fully submerged. 2. Water gives an uplift (buoyancy) on soil particles. 3. So the effective unit weight is γ' = γsat - γw. 4. Use γ' in both the q (γDf) term and the 0.5γBN_γ term.
Explanation
Bearing capacity depends on effective stress. When soil is fully under water, buoyancy reduces its weight. So we use the submerged unit weight γ', which is about half of the saturated unit weight.
Why the other options are wrong
A: Dry unit weight is used only when soil is dry and the water table is deep. B: Bulk unit weight is used for moist soil above the water table. C: Saturated unit weight ignores buoyancy, so it gives too much effective stress.
Shortcut
Water table at ground level means use γ' everywhere. It is about γsat/2, so bearing capacity is roughly halved for the γ terms.
Common mistake
Choosing saturated unit weight because the soil is saturated, and forgetting the buoyancy effect.
Correct answer: D) submerged unit weight