Subbase Compaction Verification Flashcards

When should a paving inspector require a new Proctor test (reference standard) for subbase material?

Answer: When the source material or borrow pit changes significantly

A new Proctor is required when source material changes because different material compositions yield different optimum moisture and maximum density values.

According to typical specifications, what is the maximum allowable lift thickness for granular subbase compaction?

Answer: 6 to 8 inches loose measurement

Most highway specifications limit granular subbase lifts to 6–8 inches loose (compacting to roughly 5–6 inches) to ensure uniform compaction through the full depth.

A nuclear density gauge must be calibrated against known standards. How often should field standardization checks be performed?

Answer: At the beginning of each work day

AASHTO and manufacturer guidelines require daily standardization checks against reference blocks to detect drift in gauge readings.

Which roller type is MOST effective for compacting cohesive (clay) subbase materials?

Answer: Sheepsfoot or padfoot roller

Sheepsfoot/padfoot rollers knead and punch cohesive soils from the bottom up, achieving compaction that smooth drum rollers cannot in plastic materials.

The 'one-point Proctor' method is used to:

Answer: Quickly estimate maximum dry density when material source is consistent

The one-point Proctor uses a correlation with a family of curves to estimate maximum dry density from a single test point, saving time when the source is established.

What action is required if in-place moisture content is found to be BELOW the allowable range during subbase compaction?

Answer: Add water uniformly and re-compact

Material too dry of optimum lacks the lubrication needed for particles to rearrange; adding water and uniformly blending before re-compacting corrects the deficiency.

A test result shows 98% relative compaction but moisture content is 4% above optimum. The inspector should:

Answer: Flag the test and evaluate whether the density standard used accounts for high moisture

High moisture above optimum can produce misleadingly high relative compaction; the inspector must verify whether the correct maximum dry density standard was applied.