NETTCP Soils and Aggregates 5 — Questions and Answers
Question 1: What does the term 'well-graded' mean when describing aggregate gradation?
- All particles are the same size
- Particle sizes are distributed across a wide range with few gaps (Correct answer)
- Most particles pass the No. 200 sieve
- Aggregate is sorted by specific gravity
Correct answer: Particle sizes are distributed across a wide range with few gaps
Well-graded aggregate has a continuous distribution of particle sizes that allows smaller particles to fill voids between larger ones, improving density and stability.
Question 2: In New England transportation projects, frost depth considerations are critical because:
- Frost reduces aggregate abrasion resistance
- Freeze-thaw cycles cause heave and subgrade weakening during spring thaw (Correct answer)
- Cold temperatures increase plasticity index of subgrade
- Frost eliminates the need for drainage layers
Correct answer: Freeze-thaw cycles cause heave and subgrade weakening during spring thaw
In New England's cold climate, frost penetrates deep into pavement structures, and spring thaw weakens saturated subgrades causing pavement heave and failure.
Question 3: Which field test provides a quick assessment of subgrade bearing capacity during construction?
- Hydrometer analysis
- Dynamic Cone Penetrometer (DCP) test (Correct answer)
- Soundness test
- Fineness modulus calculation
Correct answer: Dynamic Cone Penetrometer (DCP) test
The Dynamic Cone Penetrometer provides a rapid in-situ measure of subgrade strength by tracking penetration rate per blow, correlating to CBR values.
Question 4: Organic soils are generally considered poor subgrade materials primarily because:
- They have high unit weights causing settlement
- They compress significantly under load and have low shear strength (Correct answer)
- Their gradation prevents moisture movement
- They have plasticity indices below 5
Correct answer: They compress significantly under load and have low shear strength
Organic soils compress significantly under load due to decomposable material, resulting in excessive long-term settlement and insufficient bearing capacity.
Question 5: The purpose of a geotextile fabric placed between subgrade and base course is to:
- Increase the plasticity index of the subgrade
- Provide separation, filtration, and prevent subgrade intrusion into base (Correct answer)
- Reduce the required base course thickness to zero
- Act as a drainage pipe beneath the pavement
Correct answer: Provide separation, filtration, and prevent subgrade intrusion into base
Geotextile fabric separates weak subgrade from the aggregate base, prevents intermixing, and allows drainage while maintaining structural integrity.
Question 6: When aggregate is described as having 'angular' particles, this shape characteristic typically:
- Reduces internal friction angle and stability
- Increases interlocking and shear strength compared to rounded particles (Correct answer)
- Lowers compaction density
- Improves workability in asphalt mixes by reducing friction
Correct answer: Increases interlocking and shear strength compared to rounded particles
Angular aggregate particles interlock mechanically, increasing internal friction angle and shear strength, which is beneficial for base and surface course stability.
Question 7: A soil with a liquid limit of 55 and a plasticity index of 30 would be classified under AASHTO as:
- A-2-4 (silty sand)
- A-4 (silty soil)
- A-7-6 (plastic clay) (Correct answer)
- A-1-a (gravel)
Correct answer: A-7-6 (plastic clay)
With LL > 41 and PI > 11, and PI exceeding LL minus 30, the soil plots as A-7-6, indicating highly plastic clay.
NETTCP Soils and Aggregates 5
What does the term 'well-graded' mean when describing aggregate gradation?