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.

Regarding the second factor of water content, it affects the compaction in the following ways. 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. 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.

It is under conditions not only accepted but also required.  For instance in railway embankments, UIC 719R requires that for the prepared subgrade should be compacted to or above 100% of the maximum dry density as determined from the Standard (NB 95% of the Modified) Proctor Test.  However, for the blanket layer it is expected that compaction equals of exceeds 100% the maximum dry density as determined from the Modified Proctor Test, depending on design speed and local regulations.  In Greece for instance, the prepared subgrade is expected to be equal or above 100% of the standard Proctor, whilst the blanket is expected to equal or exceed 103% of the standard Proctor value, for ballasted track and design speeds above 160 Km/h.   

@ Georgios Tsifoutidis , can you please send me your references to what you have mentioned in your answer? Thanks

@Romel Georgees, the standard UIC 719R and many more can be found in the International Union of Railways (Union Internationale des Chemins de fer) site https://uic.org/

In field compaction, some times the compaction rate exceed 100% can be accepted because the sample used in laboratory compaction may not represent the materials in site (i.e. contains more fine).