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 1
For work package 569 on the culvert scour repair, the review engineer performs a contractor change evaluation and classifies fine-grained soil with LL = 59 and PI = 33. Using A-line PI = 0.73(LL - 20), what USCS group is indicated?
- ACH
- BML
- CMH
- DCL
Show the answer and worked solution
Answer: A — CH
Worked solution
- Step 1. Compute A-line
PI_A = 0.73(LL - 20)
PI_A = 0.73(59 - 20) = 28.47
- Step 2. Compare plasticity
PI = 33 is above the A-line, so the soil is clayey.
- Step 3. Use liquid-limit boundary
LL = 59 gives CH.
Why the other choices are wrong
- Choice B
- This choice reverses both checks at once: it treats PI = 33 as falling below the A-line, though PI_A = 0.73(59 - 20) = 28.47 is actually less than PI so the soil plots as clay, and it also drops the liquid limit below the 50 threshold when LL = 59 sits at or above it.
- Choice C
- This choice puts the soil below the A-line as a silt-range material, but PI = 33 exceeds PI_A = 0.73(59 - 20) = 28.47, which plots the soil above the line in the clay range instead.
- Choice D
- This choice keeps the clay classification but drops the liquid limit into the low-plasticity range, even though LL = 59 sits at or above the 50 threshold that separates the H suffix from the L suffix.