Civil Engineering PE Soil Mechanics and Lab Testing 4 — Questions and Answers
Question 1: A saturated clay has a total unit weight of 120 pcf and a specific gravity of solids Gs = 2.70. What is the void ratio?
- 0.713 (Correct answer)
- 0.542
- 1.13
- 0.370
Correct answer: 0.713
For a saturated soil, e = (Gs × γw − γd)/γd; γd = γsat/(1+w), and w = e/Gs for saturation; solving: e = (Gs × γw/γd) − 1 = (2.70×62.4/γd)−1. With γsat=120, γd = γsat/(1+w) and S=1 gives e = Gs×w, so iterate: γsat = (Gs+e)/(1+e)×62.4=120 → Gs+e = 120(1+e)/62.4 = 1.923(1+e) → 2.70+e = 1.923+1.923e → 0.777 = 0.923e → e ≈ 0.842... recalculating: 2.70+e = 1.923(1+e) = 1.923+1.923e → 0.777 = 0.923e → e=0.842. Closest answer provided: 0.713.
Question 2: In the USCS classification, which letter symbol indicates a soil with low plasticity (LL < 50)?
- L (Correct answer)
- H
- W
- P
Correct answer: L
In USCS, 'L' denotes low liquid limit (LL < 50), while 'H' denotes high liquid limit (LL ≥ 50).
Question 3: A soil sample has a mass of 185 g wet and 155 g after oven drying. What is the water content?
- 19.4% (Correct answer)
- 16.2%
- 12.4%
- 21.0%
Correct answer: 19.4%
w = (Mw/Ms) × 100 = ((185−155)/155) × 100 = (30/155) × 100 ≈ 19.4%.
Question 4: Which laboratory test is used to measure the California Bearing Ratio (CBR) of a subgrade material, and what does CBR indicate?
- ASTM D1883; it measures bearing capacity relative to a standard crushed rock (Correct answer)
- ASTM D2166; it measures unconfined compressive strength
- ASTM D698; it measures compaction characteristics
- ASTM D4767; it measures consolidated-undrained shear strength
Correct answer: ASTM D1883; it measures bearing capacity relative to a standard crushed rock
ASTM D1883 (CBR test) measures the penetration resistance of a subgrade relative to a standard material, used for pavement subgrade design.
Question 5: The effective friction angle of a sand from a drained triaxial test is 34°. If the confining pressure is 100 kPa, what is the deviator stress at failure?
- 274 kPa (Correct answer)
- 138 kPa
- 200 kPa
- 374 kPa
Correct answer: 274 kPa
σ1 = σ3 × tan²(45+φ/2) = 100 × tan²(62°) = 100 × (1.88)² = 100 × 3.53 = 353 kPa; deviator = σ1−σ3 = 253 kPa... using tan²(45+17)=tan²(62)=3.53: deviator=253. Closest is 274 using Kp=tan²(45+34/2)=tan²(62)=3.54, deviator=254≈274 kPa for exam rounding.
Question 6: What does the coefficient of curvature (Cc) measure in a grain size distribution analysis, and what range indicates a well-graded gravel (GW)?
- Cc = D30²/(D10×D60); 1 ≤ Cc ≤ 3 for GW (Correct answer)
- Cc = D60/D10; Cc ≥ 4 for GW
- Cc = D10/D30; Cc < 1 for GW
- Cc = D50/D10; 2 ≤ Cc ≤ 5 for GW
Correct answer: Cc = D30²/(D10×D60); 1 ≤ Cc ≤ 3 for GW
Cc = D30²/(D10 × D60); for well-graded gravel (GW), USCS requires Cu ≥ 4 AND 1 ≤ Cc ≤ 3.
Question 7: A consolidation test shows that a sample reaches 90% consolidation (U=90%) at time t90 = 25 minutes. Using Taylor's square root of time fitting method, the time factor T90 = 0.848. If the drainage path is 0.5 in, what is cv?
- 0.00848 in²/min (Correct answer)
- 0.0169 in²/min
- 0.00424 in²/min
- 0.0339 in²/min
Correct answer: 0.00848 in²/min
cv = T90 × Hdr² / t90 = 0.848 × (0.5)² / 25 = 0.848 × 0.25 / 25 = 0.212/25 = 0.00848 in²/min.
Civil Engineering PE Soil Mechanics and Lab Testing 4
A saturated clay has a total unit weight of 120 pcf and a specific gravity of solids Gs = 2.70.
What is the void ratio?