ASCE Geotechnical and Foundation Engineering 3 — Questions and Answers
Question 1: The water table is at the ground surface in a soil with saturated unit weight 20 kN/m³. Using γw = 9.81 kN/m³, what is the vertical effective stress at 5 m depth?
- about 49 kPa
- about 51 kPa (Correct answer)
- about 100 kPa
- about 149 kPa
Correct answer: about 51 kPa
σ' = 5 × (20 - 9.81) ≈ 51 kPa.
Question 2: For a cohesionless backfill with φ' = 30° behind a smooth vertical wall with a horizontal surface, the Rankine active earth pressure coefficient Ka is:
- 0.33 (Correct answer)
- 0.50
- 1.00
- 3.00
Correct answer: 0.33
Ka = tan²(45° - φ/2) = tan²(30°) = 1/3.
Question 3: For the same backfill with φ' = 30°, the Rankine passive earth pressure coefficient Kp is:
- 0.33
- 1.73
- 2.00
- 3.00 (Correct answer)
Correct answer: 3.00
Kp = tan²(45° + φ/2) = tan²(60°) = 3.0.
Question 4: An unconsolidated-undrained (UU) triaxial test on a saturated clay typically yields a failure envelope with:
- φ = 0 and strength equal to the undrained shear strength cu (Correct answer)
- c = 0 and φ equal to the drained friction angle
- Both c and φ equal to zero
- A friction angle greater than the drained value
Correct answer: φ = 0 and strength equal to the undrained shear strength cu
Saturated UU tests give equal deviator stresses regardless of confining pressure, producing a horizontal φ = 0 envelope.
Question 5: A sand has Gs = 2.65 and void ratio e = 0.65. What is the critical hydraulic gradient that would cause a quick condition?
- 0.65
- 0.85
- 1.00 (Correct answer)
- 1.65
Correct answer: 1.00
ic = (Gs - 1)/(1 + e) = 1.65/1.65 = 1.00.
Question 6: Using Jaky's equation, the at-rest earth pressure coefficient K0 for a normally consolidated sand with φ' = 30° is:
- 0.33
- 0.50 (Correct answer)
- 0.87
- 1.00
Correct answer: 0.50
K0 = 1 - sin φ' = 1 - 0.5 = 0.50.
Question 7: During drained shearing, a dense sand typically exhibits which behavior?
- Continuous contraction with no peak strength
- Dilation with a peak strength followed by softening toward critical state (Correct answer)
- Zero volume change at all strains
- Liquefaction at small strains
Correct answer: Dilation with a peak strength followed by softening toward critical state
Dense sand dilates as particles ride over each other, giving a peak followed by strain softening.
ASCE Geotechnical and Foundation Engineering 3
The water table is at the ground surface in a soil with saturated unit weight 20 kN/m³.
Using γw = 9.81 kN/m³, what is the vertical effective stress at 5 m depth?