ASCE Geotechnical and Foundation Engineering 5 — Questions and Answers
Question 1: In the alpha (α) method for driven piles in clay, unit skin friction is calculated as:
- fs = α × σ'v
- fs = α × cu (Correct answer)
- fs = α × N60
- fs = α × qc × tan δ
Correct answer: fs = α × cu
The α method is a total stress approach where fs = α·cu, with α decreasing as cu increases.
Question 2: Negative skin friction (downdrag) on a pile is most likely to develop when:
- The pile is driven into dense sand with no fill
- Recently placed fill causes the surrounding soft clay to consolidate and settle more than the pile (Correct answer)
- The pile is subjected to uplift loads
- The pile tip rests on soft clay
Correct answer: Recently placed fill causes the surrounding soft clay to consolidate and settle more than the pile
Downdrag occurs when surrounding soil settles relative to the pile, dragging it downward.
Question 3: For an infinite slope of dry cohesionless soil with friction angle φ inclined at angle β, the factor of safety is:
- tan φ / tan β (Correct answer)
- tan β / tan φ
- sin φ / sin β
- cos β / tan φ
Correct answer: tan φ / tan β
For dry cohesionless infinite slopes, FS = tan φ / tan β, independent of depth.
Question 4: A field SPT blow count N = 20 was recorded with a hammer having 75% energy efficiency. What is N60?
- 15
- 20
- 25 (Correct answer)
- 30
Correct answer: 25
N60 = N × ER/60 = 20 × 75/60 = 25.
Question 5: A 60-in rock core run recovers pieces of 6, 3, 8, 2, 5, and 12 in. What is the Rock Quality Designation (RQD)?
- 52% (Correct answer)
- 60%
- 68%
- 90%
Correct answer: 52%
Only pieces ≥ 4 in count: (6 + 8 + 5 + 12)/60 = 31/60 ≈ 52%.
Question 6: Prefabricated vertical drains (wick drains) are installed in soft clay primarily to:
- Increase the ultimate magnitude of consolidation settlement
- Shorten the drainage path and accelerate primary consolidation (Correct answer)
- Prevent secondary compression entirely
- Increase the clay's liquid limit
Correct answer: Shorten the drainage path and accelerate primary consolidation
Wick drains create short radial drainage paths, greatly reducing consolidation time, usually combined with preloading.
Question 7: Compared with the standard Proctor test, the modified Proctor test (ASTM D1557) for the same soil typically gives:
- Higher maximum dry unit weight and lower optimum moisture content (Correct answer)
- Lower maximum dry unit weight and higher optimum moisture content
- The same maximum dry unit weight at a higher optimum moisture content
- Identical results because only the mold size changes
Correct answer: Higher maximum dry unit weight and lower optimum moisture content
Modified Proctor uses about 56,000 ft-lbf/ft³ versus 12,400, so the curve shifts up and to the left.
ASCE Geotechnical and Foundation Engineering 5
In the alpha (α) method for driven piles in clay, unit skin friction is calculated as: