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The Zero air void line is obtained on a Dry Density v/s Water Content graph. It represents the relationship between water/moisture content and dry density.
The zero air void line doesn’t intersect the curve attained on Dry density v/s Water content graph because soil can never achieve zero void condition with any kind of compaction.
The zero Air Void line is obtained when there is no presence of the air in the pores of a soil mass and totally filled with the water and soil mass get saturated. It is determined by the compaction test of a soil. Draw the graph between Dry Density VS water content, calculate the dry density from the compaction test results, and Zero Air void line is drawn on the graph.
Zero air void line is the line that shows the relationship between the water content and the dry unit weight of the soil with a 100% degree of saturation or at zero void is called the zero air void line or 100% degree of saturation line.
But this does not mean that 15% air void is equal to an 85% degree of saturation.
Basically a soil particle has three layers; the first layer is of air, 2nd water and 3rd is of solids.
Now to build a structure on a soil base there must be minimum air voids(to avoid settlement). We eliminate the air voids through compaction. But no matter how hard we try we can’t eliminate all the air voids and get zero air voids. But theoretically speaking we can calculate the dry density for zero air voids.
Thus draw a graph for dry unit weight and moisture content where the peak represents OMC(optimal moisture content) while there is also a straight line representing soil which is 100% saturated with zero air voids. This straight line is called a ZERO AIR VOID LINE.
Note that zero air void line cant be achieved in real life.
