Zero Air Void Line: If the soil is assumed to be 100% saturated and different dry densities are calculated for 100% saturation, then the resulting line on the compaction curve is called the 100% saturation line or zero air void line.
OR
The line which shows the relation between water content – dry density for the compacted soil having a constant percentage of air voids is knows as zero air void line.
2) The zero air void line is drawn across compaction curve and gives direct indication of percentage air voids or degree of saturation existing at different points of curve.
Significance of Zero Air Void Line:- The actual dry density with respect to water content cannot reach its theoretical value even after applying heavy compaction.