Water retention characteristics of soils with double porosity

A soil with double porosity is modelled as a collection of aggregated particles, in which a single aggregate is made up of discrete particles bonded together. Separate fractal distributions for pore sizes around and within aggregates are defined. The particle size distribution of the double porosity soil is also modelled using a fractal distribution, which may have a fractal dimension very different to those defining the pore sizes. The surface areas of the particles and the pores within the aggregates are assumed to be equal, enabling an expression linking two fractal dimensions to be defined. It is necessary to introduce ratios between maximum and minimum particle and pore sizes into the expression. A theoretical soil-water characteristic curve is then derived for a double porosity soil. The curve, and the underlying assumptions regarding the distributions of pore and particle sizes, showed good agreement with experimental data for a range of soils having double porosity. A discontinuity is observed in the soil-water characteristic curve at a second air entry value related to the maximum pore size within the aggregates, a feature also observed in experimentally obtained soil-water characteristic curves for double porosity soils.