Soil Compaction and Field Testing
Why compaction matters
Compaction reduces porosity and permeability, which can cause ponding, surfacing, and poor treatment. Construction traffic, stockpiling, and smearing pit faces all compact soil. The inspector's job is to detect it and document it before the system is installed.
Field tests to know
- Penetrometer: measures resistance; sharp increases indicate a restrictive layer or compacted zone.
- Percolation / infiltration testing: presoak, then measure fall rate under the code's prescribed procedure. Presoak time and water depth are common exam details.
- Bulk density and moisture content: compare field density to a reference; high density with low permeability confirms compaction.
- Smearing and rutting observation: visual evidence of construction damage.
Traps in questions
Questions often describe a test performed without presoak, at the wrong depth, or in a smeared hole - all invalidate results. Another trap: confusing a slow percolation rate caused by a high water table with one caused by compaction. Check for saturation evidence before blaming compaction. Watch for unit conversions between minutes per inch and inches per hour.
Worked example / rule of thumb
A hole shows water dropping 2.5 cm in 10 minutes after a proper presoak. That is 4 minutes per centimeter, or about 10 minutes per inch - a moderately slow rate that may still be acceptable depending on code thresholds. Rule of thumb: convert to a single unit before comparing to the standard, and confirm presoak and depth first.