Soil Consistency (Cohesive Soils) – Primary Factor Affecting Consistency in Saturated State For a saturated cohesive soil, which parameter most directly controls its consistency (soft, medium, stiff) under usual geotechnical classification?

Introduction / Context:
Consistency of cohesive soils describes their relative firmness or resistance to deformation and penetration. In classification and site description, terms such as soft, medium, stiff, and hard are used. For saturated clays and silts, consistency is closely tied to their moisture state relative to Atterberg limits, which ultimately govern undrained shear strength at field water contents.

Given Data / Assumptions:

• Soil type: cohesive (clayey/silty) and saturated.
• Objective: identify the dominant control on consistency in practice.
• Framework: Atterberg limits and undrained strength correlations.

Concept / Approach:

For saturated clays, undrained shear strength su and penetration resistance vary systematically with liquidity index LI = (w − PL) / (LL − PL), where w is water content, LL is liquid limit, and PL is plastic limit. As water content increases (higher LI), soils become softer; as it decreases, soils become stiffer. Thus, water content is the primary direct factor affecting consistency for saturated cohesive soils.

Step-by-Step Solution:

Verification / Alternative check:

Field tests such as hand penetrometer, vane shear, or SPT N-values in cohesive strata show clear sensitivity to water content; drying increases strength (stiffer), while wetting decreases strength (softer), holding fabric and mineralogy fixed.

Why Other Options Are Wrong:

• Particle size distribution: Influences plasticity limits but does not control short-term consistency as directly as water content variations.
• Density index: Applicable to cohesionless soils (relative density), not cohesive soil consistency.
• Coefficient of permeability: A hydraulic property; not a direct indicator of consistency descriptors.

Common Pitfalls:

Confusing consistency with compaction state; interpreting particle size as the immediate cause rather than a background influence via plasticity.

Final Answer:

Water content