Which of the following consistency limit of soil is indicated by rolling the soil into a thread of 3 mm, and it begins to crumble.
Try it first: pick an option, then read the explanation.
Detailed Explanation
The plastic limit is one of the three named boundaries on the water-content spectrum that engineers use to describe how a fine-grained soil behaves. It marks the transition between the plastic state (mouldable without cracking) and the semi-solid state (crumbles when reshaped), and — unlike the liquid limit, which needs a mechanical grooving apparatus — it has a wonderfully simple, low-tech test.
Why "Plastic limit" Is Correct
By definition, the plastic limit (WP) is the water content at which a soil thread, rolled by hand to approximately 3 mm diameter, just begins to crumble rather than continuing to roll smoothly. If it still rolls without crumbling at 3 mm, the soil is wetter than its plastic limit; if it crumbles before reaching 3 mm, it's already drier than the plastic limit.
| Limit | Boundary Between | How It's Tested |
|---|---|---|
| Liquid Limit (WL) | Liquid ↔ Plastic | Casagrande grooving-tool apparatus (25 blows) |
| Plastic Limit (WP) | Plastic ↔ Semi-solid | Rolling into a 3 mm thread until it crumbles |
| Shrinkage Limit (WS) | Semi-solid ↔ Solid | Mercury displacement volume test |
As water content increases from left to right, soil passes through the solid, semi-solid, plastic, and liquid states, separated by the shrinkage limit (SL), plastic limit (PL), and liquid limit (LL).
Key Point
"Rolled into a 3 mm thread, crumbles" is the exact textbook phrasing for the plastic limit test — memorize this specific number and action, since exam questions often paraphrase it.
Key Concepts for Students
- The plastic limit test (thread-rolling) is entirely manual and low-tech, in contrast to the liquid limit's mechanical grooving-tool apparatus.
- Plasticity index, Ip = WL − WP, is only meaningful once both the liquid and plastic limits are known.
- Casagrande's limit is not a standard, separate consistency limit — it's a distractor name referencing the inventor of the liquid limit apparatus, not a limit in its own right.
- The plastic limit test is subjective (depends on the operator's rolling technique and judgment of "crumbling"), which is one reason it has somewhat more scatter than the liquid limit test.