Soil Classification – Key characteristics of silt soils (size range and plasticity) Which of the following statements about silt soils is/are correct in standard geotechnical usage?

Introduction / Context:
Correctly identifying silt is fundamental in soil classification and design. Silt lies between clay and sand in particle size and typically shows low plasticity. Misclassification can lead to poor predictions of compressibility, permeability, and strength, directly affecting foundation design and earthworks.

Given Data / Assumptions:

• Particle size limits follow common systems used in practice (e.g., about 0.002–0.06 mm for silt).
• “Inorganic silt” is considered when discussing plasticity characteristics.
• Quartz is a common mineral in silt-size fractions.

Concept / Approach:
Silt grains are larger than clay minerals yet finer than sand. Because inorganic silts have limited surface activity compared to clays, they display low plasticity indices. Equi-dimensional, quartz-rich silts are typically the least plastic, with behaviour dominated by grain-to-grain contact rather than electrochemical effects common in clays.

Step-by-Step Solution:
Check size range: adopt ~0.002–0.06 mm as the practical band for silt.Assess plasticity: inorganic silts commonly show little or no plasticity (non-plastic to low PI).Relate mineralogy/shape to plasticity: quartz-rich, equi-dimensional grains make silts the least plastic within fines.

Verification / Alternative check:
Atterberg limit tests on silts often return non-plastic or low plasticity values, confirming their distinct behaviour from clayey fines.

Why Other Options Are Wrong:

• “Only (a) and (b)” ignores the well-known connection between grain shape/mineralogy and plasticity captured in (c).

Common Pitfalls:
Assuming all fines are “clay”; silt can appear similar in the field yet behaves very differently under loading and wetting. Always verify with particle size analysis and Atterberg limits.

Final Answer:
All of the above