PIC Subbase Compaction Verification 3 — Questions and Answers
Question 1: What does the 'D' value represent in a Dynamic Cone Penetrometer (DCP) test?
- Penetration rate in mm per blow (Correct answer)
- Depth to groundwater in meters
- Diameter of the cone tip in mm
- Dry density in kg/m³
Correct answer: Penetration rate in mm per blow
The DCP penetration index (mm/blow) indicates the rate of penetration per hammer blow, correlating inversely with soil strength.
Question 2: A Proctor curve shows the relationship between:
- Dry density and moisture content at a fixed compaction energy (Correct answer)
- Wet density and compaction passes
- CBR value and lift thickness
- Roller speed and compaction efficiency
Correct answer: Dry density and moisture content at a fixed compaction energy
The Proctor curve plots dry density versus moisture content at a constant compactive effort, identifying the optimum moisture content for maximum density.
Question 3: Which factor most significantly reduces compaction efficiency in granular subbase material?
- Excessive moisture content well above optimum (Correct answer)
- Slightly cooler ambient temperatures
- Minor variation in gradation
- Using a slightly heavier roller than specified
Correct answer: Excessive moisture content well above optimum
Excess moisture creates pore pressure that reduces effective stress, causing the material to pump rather than compact efficiently.
Question 4: The AASHTO T 310 standard governs:
- In-place density of soil and soil-aggregate by nuclear methods (Correct answer)
- Sand cone density testing procedure
- Proctor compaction test methodology
- CBR test for subbase material
Correct answer: In-place density of soil and soil-aggregate by nuclear methods
AASHTO T 310 covers the nuclear gauge method for determining in-place density and moisture content of compacted materials.
Question 5: When testing compaction with a nuclear gauge in backscatter mode versus direct transmission mode, which statement is correct?
- Direct transmission provides greater depth of measurement and more accurate density readings (Correct answer)
- Backscatter mode penetrates deeper into the material
- Both modes provide identical accuracy at all depths
- Backscatter mode is required for granular materials only
Correct answer: Direct transmission provides greater depth of measurement and more accurate density readings
Direct transmission mode inserts a probe rod into the material for a more accurate, deeper density measurement compared to surface backscatter.
Question 6: A paving inspector observes 'pumping' of subbase material under roller passes. This most likely indicates:
- Excessive moisture content causing pore pressure buildup (Correct answer)
- Material is too dry and needs water added
- The roller is too light for the material type
- Lift thickness is too thin
Correct answer: Excessive moisture content causing pore pressure buildup
Pumping — where material moves or flows under roller passes — is a classic sign of excess moisture generating pore pressures that prevent proper compaction.
Question 7: What is the primary purpose of proof rolling a prepared subbase?
- To identify soft, unstable areas that deflect excessively under load (Correct answer)
- To achieve final compaction density requirements
- To smooth the surface before base course placement
- To test the performance of the roller equipment
Correct answer: To identify soft, unstable areas that deflect excessively under load
Proof rolling with a loaded tandem-axle truck reveals soft spots by visible deflection or rutting, identifying areas needing remediation before paving.
PIC Subbase Compaction Verification 3
What does the 'D' value represent in a Dynamic Cone Penetrometer (DCP) test?