Understanding In-Situ Dry Density
Checking that field-compacted soil actually met the compaction target
Why field testing is necessary
A laboratory Proctor test establishes the maximum dry density a specific soil can achieve at its optimum moisture content, under controlled compaction effort. In-situ dry density testing checks whether the soil actually compacted in the field reached an acceptable percentage of that laboratory-determined maximum — a direct quality-control check on the compaction work performed.
Common field methods
The sand-cone method (ASTM D1556) excavates a small hole, weighs the removed soil, and fills the hole with a calibrated sand of known density to determine the hole's volume, from which dry density is calculated. The nuclear density gauge (ASTM D6938) measures density non-destructively using radioactive source readings, and is widely used for its speed, though it requires trained, licensed operators due to the radioactive source involved. The drive-cylinder method (ASTM D2937) is another option, pressing a known-volume cylinder into the soil.
Comparing to the compaction target
The field result is expressed as a percentage of the laboratory maximum dry density — commonly called 'percent compaction.' Specifications typically require a minimum percent compaction (often in the range of 90-95%, though the exact figure is set by the project specification, not a universal constant) before fill is accepted or construction proceeds on top of it.
Related Calculators
More From the Blog
Sources
ASTM D1556 / D6938 / D2937
Standard Test Methods for Density of Soil in Place (Sand-Cone Method / Nuclear Methods / Drive-Cylinder Method).