Let $G$ be the specific gravity of soil solids, $w$ the water content in the soil sample, $\gamma_w$ the unit weight of water, and $\gamma_d$ the dry unit weight of the soil. The equation for the zero air voids line in a compaction test plot is

- $\gamma_d = \frac{G \gamma_w}{1+Gw}$
- $\gamma_d = \frac{G \gamma_w}{Gw}$
- $\gamma_d = \frac{Gw}{1 + \gamma_w}$
- $\gamma_d = \frac{Gw}{1 – \gamma_w}$