PIC Subbase Compaction Verification 2 — Questions and Answers
Question 1: A nuclear density gauge reads a wet density of 128.5 pcf and a moisture content of 6.2%. What is the dry density?
- 121.0 pcf (Correct answer)
- 136.4 pcf
- 122.8 pcf
- 134.7 pcf
Correct answer: 121.0 pcf
Dry density = wet density / (1 + moisture fraction) = 128.5 / 1.062 ≈ 121.0 pcf.
Question 2: Which condition would cause a sand cone test to yield an artificially LOW density result?
- Sand spilling into voids adjacent to the test hole (Correct answer)
- Compacting the sand during pouring
- Using a cone that is too large
- Performing the test on a warm day
Correct answer: Sand spilling into voids adjacent to the test hole
If sand escapes into side voids beyond the actual hole volume, the calculated hole volume is overstated, producing a falsely low density.
Question 3: The term 'relative compaction' is best defined as:
- Field dry density divided by maximum laboratory dry density, expressed as a percentage (Correct answer)
- Field wet density divided by optimum moisture content
- Ratio of compaction passes to lift thickness
- Difference between field moisture and optimum moisture
Correct answer: Field dry density divided by maximum laboratory dry density, expressed as a percentage
Relative compaction (%) = (field dry density / max lab dry density) × 100, indicating how well field compaction matches the lab standard.
Question 4: When using a Clegg impact soil tester, a higher Clegg Impact Value (CIV) indicates:
- Greater soil stiffness and compaction (Correct answer)
- Higher moisture content
- Looser, less compacted material
- Thicker lift placement
Correct answer: Greater soil stiffness and compaction
A higher CIV reflects greater resistance to the falling hammer, indicating stiffer, more compact material.
Question 5: A subbase layer fails compaction testing at 91% when 95% is specified. The FIRST corrective action should be:
- Scarify, adjust moisture if needed, and re-compact (Correct answer)
- Remove and replace all material immediately
- Add a thicker layer of base course on top
- Increase roller speed to apply more energy
Correct answer: Scarify, adjust moisture if needed, and re-compact
Scarifying and re-compacting after moisture adjustment is the standard first step before considering material removal.
Question 6: Which laboratory compaction method uses 56 blows per layer and a 10-lb hammer to simulate heavy compaction effort?
- Modified Proctor (AASHTO T 180) (Correct answer)
- Standard Proctor (AASHTO T 99)
- California Bearing Ratio test
- Atterberg limits test
Correct answer: Modified Proctor (AASHTO T 180)
Modified Proctor uses a 10-lb hammer dropped 18 inches with 56 blows per layer in 5 layers, representing higher compaction energy.
Question 7: During roller compaction of a granular subbase, 'bridging' refers to:
- Coarse particles spanning voids without fines filling the gaps below
- The roller drum spanning over a soft spot without compacting it (Correct answer)
- A geotextile layer used to bridge weak subgrade
- The arch effect in a properly compacted aggregate layer
Correct answer: The roller drum spanning over a soft spot without compacting it
Bridging occurs when the roller drum rides over a soft or weak zone without adequately compacting it, leaving a hidden weak spot.
PIC Subbase Compaction Verification 2
A nuclear density gauge reads a wet density of 128.5 pcf and a moisture content of 6.2%.
What is the dry density?