Bearing Capacity of Soil MCQ with answer and solution

Concept

The ultimate bearing capacity of shallow foundations depends on the effective unit weight of the soil. When the water table rises to the ground level, the submergence reduces the effective unit weight of the soil by roughly 50%, thereby halving the bearing capacity.

Formula

q_u = c N_c + q N_q + 0.5 B γ N_γ

Step-by-step solution

1. Terzaghi's bearing capacity equation for a strip footing is given by q_u = c N_c + q N_q + 0.5 B γ N_γ. 2. For pure sandy soil, cohesion c = 0, so the first term is zero. 3. When the water table is at a great depth, the full bulk unit weight (γ) is used. 4. When the water table rises to the ground level, the submerged or buoyant unit weight (γ_sub ≈ 0.5 γ) is used in the third term, and the surcharge term (q) also accounts for submerged unit weight if applicable. 5. Consequently, the ultimate bearing capacity reduces by approximately 50% of its original value.

Explanation

According to Terzaghi's bearing capacity theory, when the water table rises from a great depth to the ground surface, the effective unit weight of the soil below the water table is reduced to approximately half (buoyant unit weight). Since the bearing capacity of a strip footing in pure sandy soil is directly proportional to the effective unit weight of the soil, the ultimate bearing capacity is reduced by approximately 50%.

Why the other options are wrong

A: 30% reduction does not match the standard ~50% reduction from buoyant unit weight. C: 70% is too high and not supported by standard water table correction factors. D: 20% underestimates the significant impact of water table submergence on sandy soils.

Shortcut

In standard textbook problems for pure sandy soils with the water table rising from a deep location to the ground surface, the reduction factor for bearing capacity is taken as 0.5 (or 50%).