Consistency Limits – Definition of Plasticity Index (Atterberg Limits) In soil consistency characterization, the plasticity index (PI) is defined as the numerical difference between which two Atterberg limits?

Introduction / Context:
Atterberg limits quantify water content thresholds at which fine-grained soils transition between states. The plasticity index (PI) is a foundational parameter used in classification systems (e.g., plasticity charts) and in predicting engineering behavior of clays and silty clays.

Given Data / Assumptions:

• Standard Atterberg limits: liquid limit (LL), plastic limit (PL), and shrinkage (semi-solid) limit (SL).
• Water content is reported as percent of dry mass.

Concept / Approach:

The plasticity index (PI) expresses the width of the plastic range: PI = LL − PL. It measures the span of water contents over which soil exhibits plastic behavior (mouldable without cracking and without water bleeding). Higher PI usually indicates higher clay content and greater compressibility and swelling potential.

Step-by-Step Solution:

Verification / Alternative check:

Plasticity charts (Casagrande) plot PI versus LL to classify clays and silts; this usage confirms the definition.

Why Other Options Are Wrong:

Pairs involving the semi-solid or solid limits do not define PI; those limits are used for shrinkage analysis.

Common Pitfalls:

Using different moisture units or mixing up LL and PL procedures (cone penetrometer vs Casagrande device) does not alter the PI definition.

Final Answer:

Liquid limit and plastic limit