Indices of consistency and strength: Identify the correct statements about flow index, plasticity index, shrinkage index, and toughness index for fine-grained soils.
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AIn soils, the flow index indicates how shear strength varies with water content
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BLiquid limit minus plastic limit is the plasticity index of the soil
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CPlastic limit minus shrinkage limit is called the shrinkage index of the soil
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DThe ratio of plasticity index to flow index is called the toughness index of the soil
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EAll the above
Answer
Correct Answer: All the above
Explanation
Introduction / Context:
Atterberg-related indices quantify how fine-grained soils transition between liquid, plastic, semi-solid, and solid states. These indices not only aid in classification (such as the plasticity chart) but also correlate with engineering properties including compressibility and undrained shear strength.
Given Data / Assumptions:
- Standard definitions for Atterberg limits and derived indices are used.
- Laboratory determinations follow typical procedures (Casagrande/cone for LL and rolling threads for PL).
Concept / Approach:
Plasticity index (PI) = LL − PL. Shrinkage index (SI) = PL − SL. Flow index (If) is the slope of the flow curve in the liquid limit test; greater If implies larger reduction in strength with small reduction in water content. Toughness index (It) = PI / If, reflecting the material’s resistance to deformation near the plastic limit.
Step-by-Step Solution:
State each definition: PI = LL - PL; SI = PL - SL; If from flow curve; It = PI / If.Relate If qualitatively to undrained shear strength decay with water content.Confirm that all four statements follow directly from these standard definitions and interpretations.Verification / Alternative check:
Correlations such as liquidity index vs. undrained shear strength support the interpretation of flow index as reflecting sensitivity of strength to water content near LL.
Why Other Options Are Wrong:
- Options a–d are all correct; therefore “All the above” is the only comprehensive choice.
Common Pitfalls:
- Confusing shrinkage index with shrinkage limit alone; SI is a difference between PL and SL.
- Assuming flow index equals plasticity index; they measure different aspects.
Final Answer:
All the above