Civil Engineering PE Shallow and Deep Foundations 5 — Questions and Answers
Question 1: A drilled shaft is constructed in a deep clay layer. The alpha (α) method is used to estimate skin friction. If Su = 1,200 psf and α = 0.55, the unit skin friction is:
- 660 psf (Correct answer)
- 720 psf
- 540 psf
- 840 psf
Correct answer: 660 psf
Unit skin friction = α × Su = 0.55 × 1,200 = 660 psf using the alpha (adhesion factor) method.
Question 2: The primary advantage of a belled (underreamed) drilled shaft over a straight-shaft drilled pier is:
- Significantly increased tip bearing area, which increases end-bearing capacity without increasing shaft diameter (Correct answer)
- Reduced installation cost due to shorter length
- Better performance in liquefiable soils
- Elimination of casing requirements in cohesionless soils
Correct answer: Significantly increased tip bearing area, which increases end-bearing capacity without increasing shaft diameter
The enlarged bell dramatically increases the bearing area at the tip, allowing higher end-bearing capacity while keeping the shaft diameter manageable.
Question 3: A 20-ft × 20-ft raft foundation on soft clay settles 2 inches at the center. To reduce this settlement by 50%, an engineer could:
- Replace 6 ft of soil with compacted structural fill to reduce stress on the compressible layer (Correct answer)
- Increase the raft thickness by 50%
- Add waterproofing membrane beneath the raft
- Increase column spacing to redistribute load
Correct answer: Replace 6 ft of soil with compacted structural fill to reduce stress on the compressible layer
Partial soil replacement (surcharge reduction) decreases net stress on the compressible stratum, directly reducing consolidation settlement.
Question 4: In the LRFD design of deep foundations per AASHTO, the resistance factor φ for compression in a driven pile with dynamic testing (signal matching) is approximately:
- 0.65 (Correct answer)
- 0.45
- 0.75
- 0.90
Correct answer: 0.65
AASHTO assigns φ ≈ 0.65 for driven piles where capacity is verified with high-strain dynamic testing and CAPWAP signal matching.
Question 5: The lateral capacity of a single pile in soft clay (short rigid pile) under horizontal load is governed by:
- Passive resistance of the surrounding soil and the pile's structural moment capacity (Correct answer)
- Skin friction along the pile shaft only
- The end bearing at the pile tip
- Downdrag forces from consolidating clay
Correct answer: Passive resistance of the surrounding soil and the pile's structural moment capacity
Lateral pile capacity depends on mobilized passive soil resistance (p-y behavior) and the pile's ability to resist bending moments without yielding.
Question 6: For a pile group in clay, the block failure mode should be checked when:
- Pile spacing is less than 3 pile diameters and the group acts as a single block (Correct answer)
- The pile cap is designed as a rigid plate
- Individual piles are longer than 40 ft
- The clay has an Su greater than 2,000 psf
Correct answer: Pile spacing is less than 3 pile diameters and the group acts as a single block
Close pile spacing causes failure along the perimeter of the group block; block capacity = perimeter shear + base bearing of the enclosed clay mass.
Question 7: The tolerable differential settlement for a typical steel frame building with brick cladding, per standard practice, is approximately:
- 1/300 to 1/500 of the span (Correct answer)
- 1/100 of the span
- 1/1000 of the span
- Absolute settlement of 1 inch maximum
Correct answer: 1/300 to 1/500 of the span
Angular distortion (differential settlement / span) should not exceed about 1/300 for steel frames with brick infill to prevent cracking damage.
Civil Engineering PE Shallow and Deep Foundations 5
A drilled shaft is constructed in a deep clay layer.
The alpha (α) method is used to estimate skin friction.
If Su = 1,200 psf and α = 0.55, the unit skin friction is: