Void ratio


The void ratio of a mixture of solids and fluids, or of a porous composite material such as concrete, is the ratio of the volume of the voids filled by the fluids to the volume of all the solids.
It is a dimensionless quantity in materials science and in soil science, and is closely related to the porosity, the ratio of the volume of voids to the total volume, as follows:
in which, for idealized porous media with a rigid and undeformable skeleton structure when the water content of the sample changes, the total volume of an ideal porous material is the sum of the volume of the solids and the volume of voids :
and
where is the void ratio, is the porosity, VV is the volume of void-space, VS is the volume of solids, and VT is the total volume. This figure is relevant in composites, in mining, and in soil science. In geotechnical engineering, it is considered one of the state variables of soils and represented by the symbol .
Note that in geotechnical engineering, the symbol usually represents the angle of shearing resistance, a shear strength parameter. Because of this, in soil science and geotechnics, these two equations are usually presented using for porosity:
and
where is the void ratio, is the porosity, VV is the volume of void-space, VS is the volume of solids, and VT is the total volume.

Applications in soil sciences and geomechanics

  • Control of the volume change tendency. Suppose the void ratio is high. Under loading, voids in the soil skeleton tend to decrease, increasing the contact between adjacent particles and modifying the soil effective stress. The opposite situation, i. e. when the void ratio is relatively small, indicates that the volume of the soil is vulnerable to increase under unloading – the smectite partially dry clay particles present in an unsaturated soil can swell due to their hydration after contact with water.
  • Control of the fluid hydraulic conductivity. Loose soils show a high hydraulic conductivity, while dense soils are less permeable.
  • Particle movement. Small, unbound particles can move relatively quickly through the larger open voids in loose soil. In contrast, in dense soil, finer particles cannot freely pass the smaller pores, which leads to the clogging of the porosity.