Civil Engineering PE Geotechnical Engineering and Slope Stability 2 — Questions and Answers
Question 1: A cantilever retaining wall achieves overturning resistance primarily from:
- Passive pressure on the front face
- Weight of retained soil over the heel of the base slab (Correct answer)
- Self-weight of the concrete stem only
- Anchor ties embedded in rock
Correct answer: Weight of retained soil over the heel of the base slab
The heel of the base slab extends under the retained fill so that soil weight over the heel contributes to the overturning resistance.
Question 2: The minimum acceptable factor of safety against sliding for a retaining wall is typically:
- 1.0
- 1.5 (Correct answer)
- 2.0
- 3.0
Correct answer: 1.5
AASHTO and most codes require FS ≥ 1.5 against sliding, computed as total horizontal resisting forces divided by total active horizontal thrust.
Question 3: The modified Proctor compaction test (ASTM D1557) delivers approximately how many times more compactive energy than the standard Proctor (ASTM D698)?
- 1.5×
- 2.5×
- 4.5× (Correct answer)
- 6.0×
Correct answer: 4.5×
The modified Proctor uses a 10-lb hammer with an 18-in drop in 5 layers, delivering roughly 4.5 times the energy of the standard Proctor.
Question 4: Primary consolidation settlement for a normally consolidated clay is calculated using:
- S = Cc/(1+e₀) × H × log(σ′f/σ′i) (Correct answer)
- S = Cs × H × log(σ′f/σ′i)
- S = mv × Δσ′ × H only
- S = Cc × H × log(e₀/ef)
Correct answer: S = Cc/(1+e₀) × H × log(σ′f/σ′i)
For NC clay, S = [Cc/(1+e₀)] × H × log(σ′f/σ′i), where Cc is the compression index and e₀ is the initial void ratio.
Question 5: The dimensionless time factor Tv in Terzaghi's consolidation theory is defined as:
- Tv = cv × t / Hdr² (Correct answer)
- Tv = Hdr² / (cv × t)
- Tv = t / (cv × Hdr)
- Tv = cv / (t × Hdr²)
Correct answer: Tv = cv × t / Hdr²
Tv = cv·t/Hdr², where cv is the coefficient of consolidation and Hdr is the length of the drainage path.
Question 6: Soil liquefaction during an earthquake is most likely to occur in:
- Dense, well-graded gravel
- Dry, compacted clay
- Loose, saturated, fine sand (Correct answer)
- Stiff, overconsolidated clay
Correct answer: Loose, saturated, fine sand
Liquefaction occurs when cyclic shear stresses generate excess pore pressure equal to effective confining stress, most easily triggered in loose saturated sands.
Civil Engineering PE Geotechnical Engineering and Slope Stability 2
A cantilever retaining wall achieves overturning resistance primarily from: