ASCE Geotechnical and Foundation Engineering 2 — Questions and Answers
Question 1: A clay has a natural water content of 35%, liquid limit of 45%, and plastic limit of 25%. What is its liquidity index?
- 0.25
- 0.50 (Correct answer)
- 0.75
- 1.40
Correct answer: 0.50
LI = (w - PL)/PI = (35 - 25)/(45 - 25) = 0.50.
Question 2: The activity of a clay soil is defined as which ratio?
- Liquid limit divided by plastic limit
- Plasticity index divided by percent clay-size fraction finer than 2 µm (Correct answer)
- Water content divided by liquid limit
- Shrinkage limit divided by plasticity index
Correct answer: Plasticity index divided by percent clay-size fraction finer than 2 µm
Skempton's activity A = PI / (% finer than 2 µm) indicates clay mineral swelling potential.
Question 3: Under USCS, a soil with 70% passing the No. 200 sieve, LL = 62, and Atterberg limits plotting above the A-line is classified as:
- CL
- MH
- CH (Correct answer)
- OH
Correct answer: CH
Fine-grained soil with LL ≥ 50 plotting above the A-line is a high-plasticity clay, CH.
Question 4: The overconsolidation ratio (OCR) of a clay is defined as:
- Current effective vertical stress divided by preconsolidation pressure
- Preconsolidation pressure divided by current effective vertical stress (Correct answer)
- Total stress divided by pore water pressure
- Compression index divided by recompression index
Correct answer: Preconsolidation pressure divided by current effective vertical stress
OCR = σ'p / σ'v0, so normally consolidated clay has OCR = 1.
Question 5: Secondary compression of a clay layer is best described as settlement that:
- Occurs instantly upon load application due to elastic distortion
- Occurs while excess pore pressure dissipates
- Continues after excess pore pressure has essentially dissipated, due to creep of the soil skeleton (Correct answer)
- Results only from lowering of the water table
Correct answer: Continues after excess pore pressure has essentially dissipated, due to creep of the soil skeleton
Secondary compression, governed by Cα, is creep that follows primary consolidation.
Question 6: In Terzaghi's one-dimensional consolidation theory, the time factor Tv corresponding to 90% average degree of consolidation is approximately:
- 0.197
- 0.287
- 0.848 (Correct answer)
- 1.129
Correct answer: 0.848
Tv ≈ 0.848 for U = 90%, while Tv ≈ 0.197 corresponds to U = 50%.
Question 7: A 6-m-thick clay layer is sandwiched between two sand layers. What drainage path length should be used in consolidation time calculations?
- 1.5 m
- 3 m (Correct answer)
- 6 m
- 12 m
Correct answer: 3 m
With double drainage, Hdr equals half the layer thickness, 6/2 = 3 m.
ASCE Geotechnical and Foundation Engineering 2
A clay has a natural water content of 35%, liquid limit of 45%, and plastic limit of 25%.
What is its liquidity index?