27 Oct 04 5:56
The optimum moisture content [OMC], is defined as the moisture content at which the highest dry density was achieved with a specified amount of compaction.
Change the compactive effort , the no. of blows/passess, size of mould, layer thickness, type of compaction, (static, dynamic, vibrating) etc..., will all change the OMC.
Change the material, change the OMC.
The water within a soil aids in the lubrication of the particles to enable closer packing and helps fill the air voids. Compaction is the reduction of air voids, whereas consolidation is the reduction of total voids (mostly driving out water from the pore spaces). Adding water to a dry soil helps to pack the solids closer together but at some point, by adding more water, you begin to fill the voids within the soil matrix with water, and not solids, thus reducing the dry density of the soil. The compaction study of a soil typically forms a bell shaped graph, with the peak indicating the OMC and maximum dry density [MDD].
Care needs to be selected on which method is used to determine the OMC and MDD as an inappropriate choice could result in a soil which although was placed at the OMC for one method, suffers long-term consolidation as the working load of the structure is higher than than this test method is suitable for.
I've just read through the last sentence and I don't think I made it that clear, so please find below a simplified explanation/rough guide on current standard lab methods to determine the OMC in the UK:
1. 2.5kg Rammer (aka: proctor/light/standard) good for cohesive soils, and appropriate to road construction for general fill
2. 4.5kg Rammer (aka: modified proctor/heavy)good for cohesive soils, and appropriate for engineered fill under buildings
3. Vibrating Hammer (light demolition 'kango' hammer with round foot) good for granular soils, and appropriate for road construction and under buildings.