Bearing Capacity of Soil MCQ with answer and solution

Concept

Groundwater table influence on soil bearing capacity

Formula

γ_sub = γ_sat - γ_w

Step-by-step solution

1. Soil bearing capacity depends directly on the effective surcharge (q = γ·Df) and soil wedge weight (0.5·γ·B·N_gamma). 2. When the groundwater table rises to the ground surface or footing base, buoyant forces act on the soil particles. 3. The effective unit weight is reduced from bulk unit weight (γ) to submerged unit weight (γ_sub = γ_sat - γ_w). 4. Because γ_sub is roughly half of gamma, ultimate bearing capacity decreases by approximately 50%. Answer: B

Explanation

A rise in the groundwater table reduces the bearing capacity of soil because it replaces the bulk unit weight of the soil with the submerged unit weight (γ_sub = γ_sat - γ_w), which is approximately half the dry or moist unit weight. This reduces the effective confining stresses and effective unit weight.

Why the other options are wrong

A) Effective friction angle and true cohesion are intrinsic soil matrix properties not altered directly by submergence. C) Foundation physical depth Df remains geometrically constant. D) Mineral grain friction angle φ remains unchanged.

Shortcut

Water table rise = Effective unit weight replaced by submerged unit weight (halves bearing capacity).

Common mistake

Thinking water table rise reduces soil cohesion or internal friction angle rather than effective unit weight.