3.4  Void Ratio

William W. Woessner; Eileen P. Poeter

3.4 Void Ratio

Engineering and soil mechanics professionals use the term void ratio, e, when describing the amount of void space in a soil or rock. The void ratio is the volume of voids divided by the volume of solids as shown in Equation 7.

$\displaystyle e=\frac{V_V}{V_S}$

(7)

where:

e	=	void ratio (dimensionless)
V_V	=	volume of voids (L³)
V_S	=	volume of solids (L³)

The void ratio, V_V / V_S, is always larger than the total porosity, V_V / V_T, because the value of V_S (the denominator of the void ratio) is always less than the total sample volume, V_T. Recognizing the total sample volume is equal to V_V + V_S, total porosity can be calculated from void ratio as shown in Equation 8.

$\displaystyle n=\frac{e}{1+e}$

(8)

Both porosity and void ratio are used to characterize the open space of soil, sediments and rocks. Groundwater professionals most commonly use total porosity or effective porosity to compare the open space of earth materials. They use effective porosity to determine the capacity of a material to store groundwater.

License