nimrod2:

A good reference for earthwork operations and the use of optimum moisture content and maximum dry density would be Chapter 4 of Das' book Principles of Geotechnical Engineering on Soil Compaction.  The optimum moisture content is the water content for a given level of compactive effort that yields the maximum dry density.  It is an important concept for field monitoring of soil compaction because it is necessary to have the moisture content close to the optimum moisture content to get a specified high degree of compaction.

Good luck

Good reference.

Keep in mind that the optimum moisture data in Das' book are guidelines; the best way to determine the optimum moisture is to run the appropriate compaction test.

Please see FAQ731-376  by VPL for tips on how to make the best use of Eng-Tips Fora.

I'd suggest Rollings/Rollings book (Can't remember the title presently) but it is McGraw-Hill about 1994; also Monahan's book.  Check out some of the other threads.  Compaction and optimum moisture contents have been extensively covered before.

The optimum moisture content could be determinate by the PROCTOR test. the protocol is lightly different if it's a soil or a gravel. The test gives a graph of the dry volumic mass versus the moisture content.

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.

The optimum moisture content could be given by the proctor test. In france, we have two tests. The first is the normal test and the second is the modified test. The basic process is to compact five or six molds with the same energy but with differrents moistures content. You calculate the dry density for every mold. And you determine the optimum moisture content which give the maximum dry density