Soil compaction: Proctor test, OMC and maximum dry density

Compaction pushes soil particles closer by squeezing out air. It raises strength, lowers settlement and reduces permeability, which is why every embankment and subgrade is compacted in layers.

The compaction curve

Compact samples at different water contents with the same effort and plot dry density against water content. The curve rises, peaks and falls.

• The peak is the maximum dry density (MDD).
• The water content at the peak is the optimum moisture content (OMC).

On the dry side, more water lubricates the particles so they pack better. Past the OMC, water starts to take up space that solids could have filled, so dry density falls.

Standard and modified Proctor

Test (IS 2720)	Rammer	Drop	Layers	Blows per layer
Light (Part 7)	2.6 kg	310 mm	3	25
Heavy (Part 8)	4.9 kg	450 mm	5	25

The heavy test applies about 4.5 times the energy and is used for highways and airfields.

Dry density from the test

γ_d = γ1 + w

Zero air voids line

The highest dry density possible at any water content is reached when all air is removed (S = 1):

γ_d = G γ_w1 + w G

The compaction curve always lies below this line and approaches it on the wet side.

On site

Field density is checked with the sand replacement or core cutter method, and the result is reported as relative compaction: field dry density as a percentage of the laboratory MDD. Road specifications usually ask for 95% to 97% of MDD for embankments and higher for the top layers.