Soil compaction is the process of increasing the soil density by reducing the volume of air within the soil mass. Soil compaction depends mainly on the degree of compaction and the amount of water present for  lubrication.

Normally 2.5kg rammers and 4.5kg rammers are available for compaction in laboratories  and  the maximum dry densities produced by these rammers cover the range of dry density obtained by in-situ compaction plant.

Second factor of water content affects the compaction in the following ways:

i) In low water content, the soils are difficult to be compacted. When water content is increased  gradually,  water  will  lubricate  the  soils  and  this  facilitates  the  compaction operation. 

ii) However, at high water content, as an increasing proportion of soils is occupied by water, the dry density decreases with an increase in water content.

For soil compaction tests, the dry density obtained from compaction carried out in-situ by vibrating  roller/vibrating plate is compared with the maximum dry density conducted in laboratories using 2.5kg rammer of compaction with similar soils.

In essence, the in-situ compaction is compared with the compacting effort of using 2.5kg (or 4.5kg) rammer in laboratories. In case the compaction test results  indicate values exceeding 100%, it only means that the in-situ compaction is more than that being carried out in laboratories which is treated as the basic criterion for satisfactory degree of soil compaction. Therefore, the soil  results  are  acceptable  in  case  compaction  test  results  are  over  100%.  However, excessive compaction poses a risk of fracturing granular soils resulting in the reduction of soil strength parameters.