How does excess water makes soil compaction more difficult?

[JavierLG]

I understand how water content helps compaction, by being kind of a lubricant between the soil particles, that helps them slide into position. But I don´t really understand how it starts affecting negatively after surpassing the optimum water content, but not surpassing the void volume, meaning is not occupying space that soil could. Is it because when there is too much water in the voids, that space occupied by water could be used by fine particles, therefore increasing dry density? Also I'm confused with it decreasing after optimum water content, since what I've seen is that the optimum water content also is dependent of the compaction energy.

Braja Das, Principles of Geo-technical Engineering.

 

[Ron]

JavierLG....Notice the long curve on the right side of Das's illustration. That is the "zero air voids (ZAV) curve specific to a material with a specific gravity of 2.7 in this example. That curve is another way of saying that this is the line of saturation for this material.

Once the moisture content passes optimum, you have moisture that is in excess of that needed for lubrication and particle manipulation during the compaction process. In fact, the closer the moisture content gets to the ZAV curve, the less compactive energy there is for compacting the soil.....it gets attenuated by the build-up of pore pressure within the sample. At moisture contents just beyond optimum, the pore pressure can still dissipate quickly, thus some compaction still occurs; however, the closer the moisture content gets to the ZAV curve, the less the pore pressure can dissipate and at a point, further compactive effort is futile because the compactive energy is consumed in the build-up of pore pressure. In the field, the corresponding phenomenon is called "pumping".