Soil Liquefaction MCQ with answer and solution

Concept

Soil liquefaction is a phenomenon where saturated loose cohesionless soil loses its shear strength and stiffness due to increased pore water pressure under cyclic loading.

Formula

σ' = σ - u = 0 (at the point of complete liquefaction)

Step-by-step solution

1. Identify the conditions favorable for liquefaction: full saturation (submerged), loose relative density, uniform grading, and lack of cohesion. 2. Dense sand tends to dilate, causing negative pore water pressure (suction) which prevents liquefaction. 3. Clays possess cohesion and plasticity, rendering them far less susceptible to liquefaction. 4. Therefore, submerged loose sandy soil is the most susceptible to earthquake-induced liquefaction.

Explanation

Liquefaction occurs predominantly in saturated, loose cohesionless soils (loose sands and silty sands) subjected to dynamic or earthquake loading. Under cyclic shear, loose sand tends to contract, causing a rapid rise in excess pore water pressure that reduces the effective stress to zero. Dense sands tend to dilate under shear, while cohesive soils (clays) have cohesion that resists liquefaction.

Why the other options are wrong

A: Dense sand dilates upon shearing, generating negative pore pressure and increasing effective stress. B: Cohesive soils like clays generally deform plastically and do not liquefy unless highly sensitive or having very low plasticity. D: High plasticity clay particles impart cohesion, significantly reducing liquefaction susceptibility.

Shortcut

Liquefaction keyword match: loose + saturated + sand / non-cohesive.

Common mistake

Confusing soft clay with loose sand; although soft clay has low bearing capacity, it does not typically liquefy in the classic sense because cohesion resists pore pressure-induced complete loss of strength.