  A purpose-built permeameter was used to explore the transient evolution of
porosity during the mixing process in filtration experiments. The experiments
considered upward seepage flow and explored the influence of base and filter
particle sizes, along with different hydraulic conditions. The permeameter
acted as a coaxial transmission line enabling electromagnetic measurements
based on spatial time domain reflectometry, from which the porosity profile was
obtained using an inversion technique. Quantitative characteristics of the
onset and progression of the mixing process were extracted from a porosity
field map. The limiting onset condition was influenced by geometric and
hydraulic factors, with the critical flow rate exhibiting a strong dependence
on the base particle size, while the critical hydraulic gradient exhibited a
stronger dependence on filter particle size. The progression of the mixing
process was characterised by both the transport of base particles into the
filter layer, as well as the settlement of the filter particles into the base
layer due to the reduction of the effective stress at the base-filter interface
leading to partial bearing failure. The rate of development of the mixture zone
was strongly dependent on the hydraulic loading condition and the base particle
size, but the final height of the sample after complete mixing was independent
of the hydraulic loading path.
