Civil Engineering PE Shallow and Deep Foundations 5

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:

Accepted Answer

660 psf

Answer

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:

Accepted Answer

Significantly increased tip bearing area, which increases end-bearing capacity without increasing shaft diameter

Answer

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:

Accepted Answer

Replace 6 ft of soil with compacted structural fill to reduce stress on the compressible layer

Answer

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:

Accepted Answer

0.65

Answer

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:

Accepted Answer

Passive resistance of the surrounding soil and the pile's structural moment capacity

Answer

Lateral pile capacity depends on mobilized passive soil resistance (p-y behavior) and the pile's ability to resist bending moments without yielding.

Civil Engineering PE Practice Test

Civil Engineering PE Practice Test

Civil Engineering PE Shallow and Deep Foundations 5