Shallow and Deep Foundations Flashcards
7 cards from real Civil Engineering PE practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Shallow and Deep Foundations flashcards as text
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:
Answer: 660 psf
Unit skin friction = α × Su = 0.55 × 1,200 = 660 psf using the alpha (adhesion factor) method.
The primary advantage of a belled (underreamed) drilled shaft over a straight-shaft drilled pier is:
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.
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:
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.
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:
Answer: 0.65
AASHTO assigns φ ≈ 0.65 for driven piles where capacity is verified with high-strain dynamic testing and CAPWAP signal matching.
The lateral capacity of a single pile in soft clay (short rigid pile) under horizontal load is governed by:
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.
For a pile group in clay, the block failure mode should be checked when:
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.
The tolerable differential settlement for a typical steel frame building with brick cladding, per standard practice, is approximately:
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.