Civil Engineering PE Soil Mechanics and Lab Testing 5 — Questions and Answers
Question 1: A soil has a shrinkage limit (SL) of 12%, plastic limit (PL) of 22%, and liquid limit (LL) of 48%. What is the shrinkage ratio if the soil volume at the shrinkage limit is 24.5 cm³ and dry mass is 36 g, using γw = 1 g/cm³?
- 1.47 (Correct answer)
- 1.22
- 1.84
- 0.94
Correct answer: 1.47
Shrinkage ratio SR = Ms/(Vs × γw) = 36/(24.5 × 1.0) = 1.47 g/cm³.
Question 2: In geotechnical engineering, what is the significance of the preconsolidation pressure (σ'c) determined from a consolidation test?
- It marks the stress boundary between recompression and virgin compression behavior (Correct answer)
- It equals the current effective overburden stress for all soils
- It defines the maximum shear stress the soil can resist
- It is the stress at which soil permeability becomes zero
Correct answer: It marks the stress boundary between recompression and virgin compression behavior
Preconsolidation pressure separates the lightly (elastic) recompression range from the normally consolidated (plastic) compression range on the e-log p curve.
Question 3: A saturated soil specimen in an unconsolidated-undrained (UU) triaxial test shows zero friction angle (φ = 0). If the unconfined compressive strength (qu) is 80 kPa, what is the undrained shear strength (su)?
- 40 kPa (Correct answer)
- 80 kPa
- 160 kPa
- 20 kPa
Correct answer: 40 kPa
For φu = 0, su = cu = qu/2 = 80/2 = 40 kPa; the Mohr circle radius equals the shear strength.
Question 4: A sand has a maximum void ratio of 0.82 and minimum void ratio of 0.45. A field sample has a void ratio of 0.60. What is the relative density (Dr)?
- 59.5% (Correct answer)
- 40.5%
- 73.2%
- 26.8%
Correct answer: 59.5%
Dr = (emax − e)/(emax − emin) × 100 = (0.82 − 0.60)/(0.82 − 0.45) × 100 = 0.22/0.37 × 100 ≈ 59.5%.
Question 5: Which index property best distinguishes between a silty soil (ML or MH) and a clayey soil (CL or CH) in the USCS classification system?
- Position relative to the A-line on the plasticity chart (Correct answer)
- Liquid limit value alone
- Grain size D50 value
- Specific gravity of solids
Correct answer: Position relative to the A-line on the plasticity chart
The A-line (U = 0.73(LL − 20)) on the Casagrande plasticity chart separates clays (above) from silts and organic soils (below).
Question 6: In a sieve analysis, what percentage of soil passes the No. 200 sieve if the retained amounts are: No. 4 = 15 g, No. 10 = 25 g, No. 40 = 35 g, No. 200 = 45 g, and pan = 30 g from a total dry mass of 150 g?
- 20% (Correct answer)
- 30%
- 50%
- 80%
Correct answer: 20%
Mass passing No. 200 = pan fraction = 30 g; percent passing = 30/150 × 100 = 20%.
Question 7: A compaction curve shows optimum moisture content (OMC) at 15%. A contractor compacts soil at 12% moisture. Compared to the dry side of optimum, what is the most significant engineering implication?
- Higher strength and stiffness but more brittle and susceptible to collapse upon wetting (Correct answer)
- Lower permeability and higher swell potential
- Higher plasticity index and more uniform structure
- Lower dry density and lower friction angle
Correct answer: Higher strength and stiffness but more brittle and susceptible to collapse upon wetting
Compaction dry of optimum produces a flocculated structure with higher strength but more collapse potential when saturated, compared to the dispersed, more ductile wet-side structure.
Civil Engineering PE Soil Mechanics and Lab Testing 5
A soil has a shrinkage limit (SL) of 12%, plastic limit (PL) of 22%, and liquid limit (LL) of 48%.
What is the shrinkage ratio if the soil volume at the shrinkage limit is 24.5 cm³ and dry mass is 36 g, using γw = 1 g/cm³?