Define zero air void

The zero air void (ZAV) line (or saturation line) is the curve showing the relation between dry unit weight and water content when the degree of saturation is 100% ($V_a = 0$):

$$\gamma_{d} = \frac{G\gamma_w}{1 + wG}$$

It is the theoretical maximum dry unit weight for each water content. The compaction curve lies to its left and never crosses it, because air cannot be fully expelled by compaction.