Question

For compaction of free-draining sands or gravels, what is optimum moisture content to grab maximum density?

Answer

Compaction curve of dirty materials is completely different from that of clayey materials. For sands, gravels, there are two states of affairs of utmost density, namely completely dry condition and complete water saturation. For humidity content of sand and gravels between two states, dry density obtained is lesser than that obtained in above-mentioned states. Occurrence of capillary forces account for difficulty of compaction sand at water contents between almost dry and saturated state. They are shaped in partially filled water invalid between soil particles and do as elastic ties cementing soil particles jointly.
Compaction curve for clay is appropriate for majority of soil types apart from sands and gravels since a little amount of clay in soils is enough to create soils impermeable.