Physical observations of the transient evolution of the porosity distribution during internal erosion using spatial time domain reflectometry

TL;DR

This study used spatial time domain reflectometry in a custom permeameter to observe how porosity evolves during internal erosion, revealing how particle sizes and hydraulic conditions influence the mixing process.

Contribution

It introduces a novel electromagnetic measurement technique to monitor porosity changes during internal erosion experiments, providing detailed insights into the process dynamics.

Findings

Critical flow rate depends on base particle size.

Hydraulic gradient influences mixing progression.

Final sample height is independent of hydraulic loading path.

Abstract

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.