Atterberg Limits and the A-Line practice problems for the FE Civil exam

Atterberg limits describe the water contents at which a fine-grained soil changes behaviour, and the plasticity chart turns two of those limits into a classification. The liquid limit is the water content at which the soil begins to flow, the plastic limit is where it stops behaving plastically and crumbles, and the difference between them is the plasticity index. That index, plotted against the liquid limit, places the soil above or below the A-line, and that position is what separates a clay from a silt in the Unified Soil Classification System. The FE Civil exam asks this in a very consistent shape. You are given a liquid limit and a plastic limit, you compute the plasticity index, you compute the A-line value at that liquid limit, and you compare the two. A point above the A-line is a clay; on or below it is a silt or an organic soil. A second boundary at a liquid limit of 50 divides low plasticity from high plasticity and supplies the second letter of the group symbol. Keep the two subtractions straight: the plasticity index is LL minus PL, while the A-line equation subtracts 20 from the liquid limit before scaling by 0.73. Interchanging them is the mistake the distractors are built around.

Key formulas

Plasticity index: PI = LL - PL
A-line on the plasticity chart: PI_A-line = 0.73 * (LL - 20)
U-line, the upper bound of observed soils: PI_U-line = 0.9 * (LL - 8)
Classification test: PI > PI_A-line => clay (C); PI <= PI_A-line => silt (M)
Plasticity split: LL < 50 => low plasticity (L); LL >= 50 => high plasticity (H)

Common mistakes

Computing the A-line as LL - 20 and forgetting to multiply by 0.73.
Confusing the plasticity index with the plastic limit when reading the given data.
Missing the liquid limit of 50 boundary and reporting CL where CH is correct.
Comparing the plasticity index against the liquid limit itself rather than against the A-line value at that liquid limit.

8 worked practice problems

Commit to an answer before opening the solution. Each problem shows the governing model, every substitution, a unit check, and why each wrong choice is wrong.

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Background reading

Original question written by FE Exam AI Prep from the public FE Civil exam specification. Before release, a solver model re-derived the answer twice from the question alone, without its answer key, and a critic model checked the result. No licensed engineer reviews these questions. It is not an NCEES question and does not come from any NCEES practice exam. FE Exam AI Prep is an independent study tool and is not affiliated with, endorsed by, or sponsored by NCEES.

Atterberg Limits and the A-Line practice problems for the FE Civil exam

Key formulas

Plasticity index: PI = LL - PL
A-line on the plasticity chart: PI_A-line = 0.73 * (LL - 20)
U-line, the upper bound of observed soils: PI_U-line = 0.9 * (LL - 8)
Classification test: PI > PI_A-line => clay (C); PI <= PI_A-line => silt (M)
Plasticity split: LL < 50 => low plasticity (L); LL >= 50 => high plasticity (H)

Common mistakes

Computing the A-line as LL - 20 and forgetting to multiply by 0.73.
Confusing the plasticity index with the plastic limit when reading the given data.
Missing the liquid limit of 50 boundary and reporting CL where CH is correct.
Comparing the plasticity index against the liquid limit itself rather than against the A-line value at that liquid limit.

8 worked practice problems

Commit to an answer before opening the solution. Each problem shows the governing model, every substitution, a unit check, and why each wrong choice is wrong.

Keep going in Geotechnical Engineering

All Geotechnical Engineering practice problems