Question

Maximum water contant after that further reduction does not cause a volume decrease

Answer 1

Compaction is a process of increasing soil density and removing air, usually by mechanical means. The size of the individual soil particles does not change, neither is water removed.
Purposeful compaction is intended to improve the strength and stiffness of soil. Consequential (or accidental) compaction, and thus settlement, can occur due to vibration (piling, traffic, etc.) or self-weight of loose fill.