Atterberg limits (soil consistency): The minimum water content at which a soil thread (rolled to 3 mm diameter) just begins to crumble is termed what?

Introduction / Context:

Atterberg limits are fundamental indices for fine-grained soils that classify consistency states and correlate with engineering behavior such as compressibility and shear strength. Among these limits, the plastic limit captures the transition between the plastic and semi-solid states, identified by a simple yet standardized rolling test.

Given Data / Assumptions:

• A soil thread is rolled by hand on a glass plate.
• Thread diameter at the point of crumbling is 3 mm.
• Standard test conditions per typical geotechnical practice.

Concept / Approach:

The plastic limit is defined as the lowest water content at which the soil remains plastic. Operationally, the plastic limit is the water content at which a thread of soil just crumbles when rolled to about 3 mm diameter. This provides a repeatable criterion for comparing soils and computing the plasticity index PI = LL − PL.

Step-by-Step Solution:

Verification / Alternative check:

Multiple rollings and recombinations of the sample are used to stabilize the determination; the average moisture content corresponds to PL.

Why Other Options Are Wrong:

• Liquid limit is determined via Casagrande device or cone penetrometer, not by 3 mm rolling.
• Shrinkage limit is obtained from volume change on drying, unrelated to the rolling thread criterion.
• “Permeability limit” and “flow limit” are not standard Atterberg terms used in classification.

Common Pitfalls:

• Rolling too fast or with excessive pressure, which overheats and artificially dries the thread.
• Ignoring fines content; coarse soils do not exhibit meaningful plasticity limits.

Final Answer:

plastic limit