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

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.

Influence of hydraulic and geometric conditions on the early stages of the filtration process in idealised granular soils comprising spherical particles

“…An exception to the correlation between the size ratio and 𝐾 𝐿3 occurred with 𝐾 𝑅8 𝐿3 , which is very similar to 𝐾 𝑅5 𝐿3 until the last stage of the simulation, where a decrease in 𝐾 𝑅8 𝐿3 occurred. By considering Equation (11), 𝐾 is positively related to 𝜖 𝑓 and inversely related to the specific area, which are both influenced by the infiltration of base particles, as shown in Figure 9B,C. The specific area of the base-filter mixture is characterised by the mean specific area of particles by volume, S𝑠,𝐿3 , which is given by: S𝑠,𝐿3…”

“…26 The use of spherical particles has been validated in previous numerical studies which generally agreed with experimental outcomes obtained with macro-scale soil samples. 18,27 Moreover, physical experiments that are capable of probing the early stages of the filtration process at the micro-scale, including PIV using transparent soils 14 and spatial time domain reflectometry, 11 employ spherical glass beads. Nevertheless, it is recognised that the use of spherical particles can lead to differing pore space geometry, as angularity of real granular soils can lead to denser configuration of particles.…”

“…Skempton and Brogan 10 suggested a stress reduction factor to account for the reduced effective stress carried by the finer fraction of a soil, which provided a means to understanding why suffusion initiates at hydraulic gradients less than the critical hydraulic gradient for heave failure. Sufian et al 11 measured the change in porosity during filtration experiments using spatial time domain reflectometry, 12,13 which revealed the influence of the size ratio and hydraulic loading condition on the filtration process. Observations of internal erosion initiation in these laboratory investigations are typically based on when material is washed out or when there are measurable changes in the soil fabric or mechanical properties of the soil.…”

The Technical Challenges for Applying Unsaturated Soil Sensors to Conduct Laboratory-Scale Seepage Experiments

“…The in/outflow of a one-dimensional soil column could be measured using versatile methods: (1) the flow rate logging using flowmeters; (2) the accumulative soil moisture discharge logging by applying conventional TDR analysis for an FRC sensor; (3) the weight logging for the entire soil column or outflow tanks using digital bench scales. The flowmeters have successfully been applied to measure in/outflow rates by many single-phase seepage experiments integrated with the TDR sensor technique in previous studies [ 19 , 20 , 21 , 28 , 83 , 84 ]. All those studies applied a high-precision electromagnetic flowmeter for flow rates logging at higher flow rates, which were generated for a single-phase flow seeping through perfectly spherical glass beads in various sizes under unconsolidated conditions.…”