The compaction curve of sandy materials is totally different from that of clayey materials.
For sands or gravels, there are two situations of maximum density, namely the completely dry condition and the complete water saturation.
For moisture content of sands and gravels between these two states, the dry density obtained is lower than that obtained in the above-mentioned states.
The presence of capillary forces account for the difficulty of compaction sand at water contents between virtually dry and saturated state.
They are formed in partially filled water void between soil particles and perform as elastic ties cementing soil particles together.
Reference is made to Lars Forssblad (1981).
The compaction curve for clay is suitable for the majority of soil types except sands and gravels because a small amount of clay in soils is sufficient to make the soils impermeable.