Poorly compacted fill keeps settling over time, cracking floors and pavements and deforming road layers. Good compaction raises the density and bearing capacity of the soil and reduces settlement.
Step one: the Proctor test in the laboratory
A sample of the fill material is compacted with a standard effort at several moisture contents, and the relationship between moisture and dry density is plotted. The test gives the maximum dry density that can be reached and the optimum moisture content at which it occurs.
Step two: field density on site
After each layer is spread and compacted, its density and moisture are measured on site with a nuclear gauge, which gives a result within minutes. The field dry density is then divided by the laboratory maximum to give the degree of compaction.
What compaction is required?
The project specification sets it according to the layer and its use, and it is often 95% of the maximum density or more. If a layer falls short, it is recompacted with its moisture adjusted and retested before the next layer is placed.
Related tests
- Sieve analysis and liquid and plastic limits: to classify the fill and confirm it is suitable.
- California bearing ratio (CBR): to assess road layer materials and the subgrade beneath them.
- Sand equivalent: to estimate the clay content of sand.