Moisture-Density Relationship:
Since various compactive efforts result in varying degrees of compaction, it is necessary to standardize the compactive effort. The most well-known is the Proctor test, prescribed by IS : 2720-Part VII. This consists of compacting the soil in three layers in a mould 100 cm3 volume, with a rammer 2.6 Kg dropped from a height of 31 cm, giving 25 blows on each layer. The Proctor density so derived is used for low volume roads such as Rural Roads. For high volume highways, a heavy compaction is prescribed. The test known as modified AASHTO compaction that (IS : 2720-Part VIII) uses a heavier rammer 4.5 Kg weight, dropped from a height of 45.7 cm, the soil being compacted in 5 layers, each layer receiving 25 blows.
For controlling the density achieved in the field, a core is cut and its volume measured by pouring sand (sand replacement method). Knowing the weight of the core removed and its volume, the density is calculated. It is customary to specify 95 per cent of the laboratory MDD (modified AASHTO compaction) for embankment and 97 per cent for the subgrade for highways. For low volume Rural Roads, it is customary to specify 97 per cent of the laboratory MDD (Proctor's test) for embankment and 100 per cent for the subgrade.