Soil Dynamics – Soils most susceptible to liquefaction during seismic shaking Identify the soil type that is most vulnerable to liquefaction.

Introduction / Context:
Liquefaction is the sudden loss of shear strength and stiffness in saturated soils due to cyclic loading, typically during earthquakes. Identifying susceptible soils is crucial for site selection, ground improvement, and seismic design of foundations and earth structures.

Given Data / Assumptions:

• Fully saturated conditions with shallow groundwater.
• Rapid cyclic shear strains with limited drainage (undrained response).
• Uniformly graded granular soils favor contractive behavior and pore pressure buildup.

Concept / Approach:

Most susceptible materials are clean, uniformly graded fine to medium sands (and non-plastic silty sands) that are loose to medium dense. Under cyclic shear, such soils tend to contract; with undrained conditions, excess pore pressure rises and effective stress drops, potentially reaching zero, which triggers liquefaction.

Step-by-Step Solution:

Verification / Alternative check:

Historical case studies (e.g., Niigata 1964, Christchurch 2011) predominantly report liquefaction in young, loose, clean sands and silty sands with shallow water tables.

Why Other Options Are Wrong:

(a) Dense sands tend to dilate and gain strength under cyclic loading. (c) Plastic clays do not liquefy in the same sense; they may undergo cyclic softening but not classic liquefaction. (d) Gravels/cobbles have high permeability and skeletal stability; classic liquefaction is less likely. (e) High-plasticity silts behave more like clays, with different cyclic response.

Common Pitfalls:

Confusing cyclic softening of clays with sand liquefaction and ignoring the key role of saturation and relative density.

Final Answer:

Saturated fine and medium sands of uniform particle size