Forchheimer and Darcy flow in a transparent water-based soil surrogate

What is it about?

A transparent water-based polymer named Aquabeads has been recently applied to model natural soil because its high transparency facilitates studies of the geotechnical problems using the visual technique. Before physically modelling flow in the hydrated Aquabeads, it is necessary to determine its hydraulic properties. According to many previous studies, the non-crushed Aquabeads were gravel-size particles but had the hydraulic conductivity (k) of clean medium sand under unconsolidated conditions. For investigating this inconsistency, the constant head tests are conducted using a rigid wall permeameter in this study. The preliminary results show that the k calculated using Darcy's law highly agree with prior studies. However, the nonlinearity between the hydraulic gradients and Reynolds numbers (Re > 1) indicates the non-Darcy flow regime. Also, an initial hydraulic gradient was observed, thus raising concerns about local heterogeneity. With multiple manometer readings, the analysis for each layer shows that the k decreases from top to bottom with flow regimes transition. After each test, a layer of crushed samples was found on the porous plate when unloading the specimen. In conclusion, Darcy's law is only robust to calculate the k of partially crushed Aquabeads for Re < 1. Instead, Forchheimer flow should be considered in future physical modelling work for Re > 1. The inconsistency is due to inevitably sample crushing.

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Photo by Sharon McCutcheon on Unsplash

Photo by Sharon McCutcheon on Unsplash

Why is it important?

Prior to physical modelling works, the hydraulic characterization of Aquabeads is always essential. According to previous studies, the grain size distribution (GSD) of hydrated Aquabeads lies in the range above fine gravel. Nevertheless, its hydraulic conductivity (k) under unconsolidated conditions is in the range of medium sand (10-2~10-1cm/s). Such an apparent inconsistency between grain size and k could raise concerns about possible non-Darcy flow for the gravel-sized Aquabeads and crushed sample leading to k reduction. So far, those two issues have never been investigated yet. Therefore, this study experimentally revisits the k of fully hydrated Aquabeads, considering non-Darcy flow and local heterogeneity. This work is finally briefed as a technical note to facilitate future physical modelling using this transparent polymer.

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