The purpose of compactive effort is to produce a soil mass of maximum dry density. Dear reader in our last post we have discussed about water content and dry density relation. Generally coarse-grained soils under compaction can be reached higher dry density than fine grained soil.
In fig below maximum dry density of well graded sand, low plastic silt, low plasticity clay and high plasticity clay are shown.
When a small amount of fine particles is added to coarse-grained soil, it gain a much better dry density for equal compactive effort. The voids of coarse-grains are filled with the fines resulting less void and high dry density.
When fines are reached such amount that additional fines are available after filling all voids of coarse grained soil, obviously the dry density decreases. So well graded sand obtains higher dry density than gap graded or uniform graded sand; (in one word poorly graded sand).
There have higher air void in cohesive soil relative to cohesionless soil. So cohesive soils have smaller maximum dry density than that of cohesionless soils. More water is required in cohesive soil as compared to cohesionless soils. So optimum moisture content (OMC) is also higher for this soil.
Heavy clays having much higher plasticity definitely have very little dry density and also have much higher optimum moisture content.