Atterberg limits — indices and definitions: Choose the correct combined statement about plasticity index, consistency index, liquidity index, and toughness index for fine-grained soils.

Difficulty: Easy

Correct Answer: All the above

Explanation:


Introduction / Context:
Atterberg limits and derived indices describe the consistency and plasticity of cohesive soils. These parameters are widely used for classification, correlation with engineering properties, and specification compliance in earthworks and foundations.


Given Data / Assumptions:

  • Liquid limit (LL), plastic limit (PL), and natural water content (w) are known or measurable.
  • Flow index (If) is the slope of the flow curve in the liquid limit determination.


Concept / Approach:
Definitions:
Plasticity index (PI) = LL − PL.Consistency index (Ic) = (LL − w) / PI.Liquidity index (LI) = (w − PL) / PI.Toughness index (It) = PI / If.These indices help compare soils with different Atterberg limits and evaluate field consistency relative to the plastic range.


Step-by-Step Solution:

State each definition and confirm dimensional consistency.Note that LI + Ic = (LL − PL)/PI = 1, showing their complementarity.Recognize that higher PI often implies higher plasticity and potential swelling.


Verification / Alternative check:
Worked examples with typical values (e.g., LL = 60, PL = 25, w = 35) confirm the indices and their ranges.


Why Other Options Are Wrong:
Each of A–D correctly states standard definitions; therefore the combined correct answer is “All the above”.


Common Pitfalls:
Using percentages without consistent units; confusing consistency index with liquidity index; misreading the flow curve slope units when computing It.


Final Answer:
All the above

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