NETTCP Soils and Aggregates 4 — Questions and Answers
Question 1: Which compaction test method uses a 10-lb hammer dropped 18 inches and is commonly specified for highway subgrade?
- Standard Proctor (AASHTO T 99)
- Modified Proctor (AASHTO T 180) (Correct answer)
- Harvard miniature compaction
- Vibratory compaction test
Correct answer: Modified Proctor (AASHTO T 180)
The Modified Proctor test uses a 10-lb (4.54 kg) hammer with an 18-inch drop and higher compactive effort, simulating modern heavy compaction equipment.
Question 2: The plasticity index (PI) is calculated as:
- Liquid limit minus plastic limit (Correct answer)
- Plastic limit minus shrinkage limit
- Liquid limit plus plastic limit
- Liquid limit divided by plastic limit
Correct answer: Liquid limit minus plastic limit
Plasticity index equals liquid limit minus plastic limit and represents the range of moisture content over which a soil behaves plastically.
Question 3: A nuclear density gauge used for field compaction testing measures:
- Aggregate hardness and abrasion
- In-place wet density and moisture content simultaneously (Correct answer)
- Bearing capacity via dynamic cone penetration
- Gradation of base material
Correct answer: In-place wet density and moisture content simultaneously
Nuclear density gauges use gamma radiation and neutron scatter to simultaneously measure in-place wet density and moisture content of compacted material.
Question 4: What is the primary purpose of lime stabilization in subgrade soils?
- Increase aggregate gradation range
- Reduce plasticity and improve strength of high-plasticity clays (Correct answer)
- Eliminate organic matter from topsoil
- Increase drainage coefficient of subgrade
Correct answer: Reduce plasticity and improve strength of high-plasticity clays
Lime reacts with clay particles through pozzolanic and cation exchange reactions to reduce plasticity index and improve strength and stability.
Question 5: During a sieve analysis, the cumulative percent retained on a sieve is used to calculate:
- Fineness modulus and percent passing (Correct answer)
- Specific gravity and unit weight
- Plasticity index and liquid limit
- Los Angeles abrasion loss
Correct answer: Fineness modulus and percent passing
Cumulative percent retained values are used to compute percent passing each sieve and to calculate the fineness modulus for fine aggregates.
Question 6: Which aggregate characteristic is most important for resistance to polishing under traffic in surface course applications?
- Low absorption capacity
- High micro-Deval loss
- High polish stone value or resistance to abrasion (Correct answer)
- Low specific gravity
Correct answer: High polish stone value or resistance to abrasion
Aggregates with high resistance to polishing maintain surface texture and skid resistance under repeated traffic loading in pavement surface courses.
Question 7: A relative compaction of 95% means the field dry density is:
- 5% below the maximum dry density from Proctor test
- 95% of the maximum dry density from Proctor test (Correct answer)
- 95 pcf regardless of Proctor test results
- Equal to optimum moisture content times 0.95
Correct answer: 95% of the maximum dry density from Proctor test
Relative compaction is expressed as the ratio of field dry density to maximum dry density from the Proctor test, multiplied by 100%.
NETTCP Soils and Aggregates 4
Which compaction test method uses a 10-lb hammer dropped 18 inches and is commonly specified for highway subgrade?