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: FE Civil practice problem 8
For the Clear Lake trunk sewer crossing, a design checker preparing a field-change evaluation classifies fine-grained soil with liquid limit LL = 55 and plasticity index PI = 29. Using A-line PI = 0.73(LL - 20), what USCS group is indicated?
- AMH
- BCL
- CCH
- DML
Show the answer and worked solution
Answer: C — CH
Worked solution
- Step 1. Compute A-line value
PI_A = 0.73(LL - 20)
PI_A = 0.73(55 - 20) = 25.55
- Step 2. Compare to A-line
PI = 29 is above the A-line, so the soil is clayey.
- Step 3. Apply liquid-limit boundary
LL = 55 gives CH.
Why the other choices are wrong
- Choice A
- Dropping the -20 offset and computing the boundary as 0.73 times the liquid limit alone gives 0.73 * 55 = 40.15, a value that sits above the actual plasticity index of 29 and wrongly pushes the point below the line onto the silt side of the chart.
- Choice B
- This label follows only if the LL = 50 divide is read backward, treating a liquid limit of 55 as the low-plasticity side instead of the high-plasticity side it actually falls on; the A-line check itself still holds because PI = 29 exceeds PI_A = 25.55.
- Choice D
- Reaching this group needs the LL = 50 boundary and the A-line comparison to both be misread in the same step, which is not one defensible slip; recomputed directly, PI_A = 0.73(55 - 20) = 25.55 against PI = 29 places this soil elsewhere entirely.