Void ratio

The void ratio of a mixture or composite is the ratio of the volume of voids to volume of solids. It is a dimensionless quantity in materials science, and is closely related to porosity as follows:

<math>e = \frac{V_V}{V_S} = \frac{V_V}{V_T - V_V} = \frac{\phi}{1 - \phi}</math>

and

<math>\phi = \frac{V_V}{V_T} = \frac{V_V}{V_S + V_V} = \frac{e}{1 + e}</math>

where <math>e</math> is void ratio, <math>\phi</math> is porosity, V_V is the volume of void-space (such as fluids), V_S is the volume of solids, and V_T is the total or bulk volume. This figure is relevant in composites, in mining (particular with regard to the properties of tailings), and in soil science. In geotechnical engineering, it is considered one of the state variables of soils and represented by the symbol e.<ref>Lambe, T. William & Robert V. Whitman. Soil Mechanics. Wiley, 1991; p. 29. ISBN 978-0-471-51192-2</ref><ref>Santamarina, J. Carlos, Katherine A. Klein, & Moheb A. Fam. Soils and Waves: Particulate Materials Behavior, Characterization and Process Monitoring. Wiley, 2001; pp. 35-36 & 51-53. ISBN 978-0-471-49058-6</ref>

Note that in geotechnical engineering, the symbol <math>\phi</math> usually represents the angle of shearing resistance, a shear strength (soil) parameter. Because of this, the equation is usually rewritten using <math>n</math> for porosity:

<math>e = \frac{V_V}{V_S} = \frac{V_V}{V_T - V_V} = \frac{n}{1 - n}</math>

and

<math>n = \frac{V_V}{V_T} = \frac{V_V}{V_S + V_V} = \frac{e}{1 + e}</math>

where <math>e</math> is void ratio, <math>n</math> is porosity, V_V is the volume of void-space (air and water), V_S is the volume of solids, and V_T is the total or bulk volume.<ref>Craig, R. F. Craig's Soil Mechanics. London: Spon, 2004, p.18. ISBN 0-203-49410-5.</ref>

Engineering applications

• Volume change tendency control. If void ratio is high (loose soils) voids in a soil skeleton tend to minimize under loading - adjacent particles contract. The opposite situation, i.e. when void ratio is relatively small (dense soils), indicates that the volume of the soil is vulnerable to increase under loading - particles dilate.
• Fluid conductivity control (ability of water movement through the soil). Loose soils show high conductivity, while dense soils are not so permeable.
• Particles movement. In a loose soil particles can move quite easily, whereas in a dense one finer particles cannot pass through the voids, which leads to clogging.

See also

• Void (composites)

External links

• Relation between void ratio and porosity