Optimum Moisture Content/ Maximum Dry Density (OMC/MDD)

Optimum Moisture Content/Maximum Dry Density (OMC/MDD)

Optimum moisture content/maximum dry density (OMC/MDD) – when placing soils as fill materials, it is important to achieve suitable compaction, primarily in order to reduce the susceptibility of a soil to settlement. The ability to attain acceptable levels of compaction is reliant on the moisture content of the soil being placed along with the compactive effort achievable from the plant used.  In order to monitor compaction of soils placed on site, in-situ density testing is frequently undertaken.  Results of this in-situ testing are compared to theoretical maxima derived in the laboratory on samples of the material being placed.  Sub-samples of a material are compacted into a mould using a specified compactive effort provided by one of a 2.5kg (light) rammer, a 4.5kg (heavy) rammer or a vibrating hammer.

The dry density is calculated for each moisture content and plotted against this moisture allowing a curve to be drawn through the resultant points.  The peak of this curve provides the theoretical maximum dry density and the optimum moisture content at which to place the soil to obtain this density.   If required air void content as a guide as to the level of compaction that can be expected.