Standard Proctor test or Proctor compaction test establishes the variation of the unit weight of soil and degree of compaction with the moisture content. It also helps us find out the optimum moisture content for the highest practical density.
When we plot the results of the Standard Proctor test, we observe that the unit weight or density rises initially with increasing moisture, reaches a maximum, and then declines quickly. The maximum is called the maximum unit weight of soil and the moisture content corresponding to it the Optimum Moisture Content (OMC).
The unit weight first increases due to the filling up of voids. Then the soil can’t hold more water without expanding, and the volume starts to increase. This decreases the density first quickly and then slowly when the soil is almost saturated.