In soil mechanics, the void ratio describes the relationship between the volume of voids, or spaces, in the soil or aggregate to the volume of solid constituents, or grains. Algebraically, e = Vv /Vs, where e represents the void ratio, Vv represents the volume of voids and Vs represent the volume of solid grains.
The void ratio of common sand and gravel varies according to how loosely or tightly its grains are packed together. Similarly, the void ratio is proportional to the grain size.
The void ratio of sand varies according to its composition and density. Poorly graded, low density sand typically has a void ratio of about 0.8, while high density sand with angular particles typically has a void ratio about 0.4.
Gravel typically has a void ratio about 0.4, regardless of whether it is well or poorly graded, although the void ratio can be affected by the presence of impurities, such as clay or silt. Gravel with clay has a void ratio of about 0.25, while gravel with silt can have a void ratio of 0.2 or less.