# A Simple Water Retention Model Based on Grain Size Distribution
## 4 Materials and Methods
### Methods  
The laboratory methods used in this study all relate to the measurement of the drying path of the WRC for a material. The WRCs were measured for the purpose of model evaluation and validation using reconstituted samples; it is important to note these samples have a different soil structure to materials placed in-situ at mines, and evaluation of models for in-situ conditions was not performed herein. The WRCs were constructed in terms of volumetric water content vs. suction. Suction measurements were first performed on reconstituted samples undergoing drying to get the suction vs. gravimetric water content (w\({}_{\text{c}}\)) WRC. The relationship of the soil bulk volume with w\({}_{\text{c}}\) was determined and used to present the measured WRC in terms of volumetric water content. These two curves were then combined to get the evolution of volumetric water content with suction during drying. Figure 11: Grain size distributions measured for the toposils, tailings, and the representative coal and mineral particles separated from each tailings. Subfigure (**a**) shows the similarity between the two toposils, (**b**,**c**) show how the representative coal and mineral fractions differ between the tailings, and (**d**) highlights the similarity of the tailings. Particle sizes below 75 \(\upmu\)m were determined by laser diffraction, sizes above 75 \(\upmu\)m was determined by wet sieving. Samples for suction measurement were first prepared by consolidation from a slurry within a stainless steel ring of 45 mm diameter, under a relatively low consolidation pressure of around 4 kPa. The reader is directed to [67] for further experimental details on consolidation. Every slurry was prepared at an initial \(\mathrm{w_{c}}\) of at least 150% of the liquid limit. The consolidation process created a saturated sample that a small circular plastic container was gently pushed into, filling the container with consolidated material, and a wire saw was used to cut the material flush with the rim of the container. The initial \(\mathrm{w_{c}}\) of the sample was measured using some offcuts, which allowed the \(\mathrm{w_{c}}\) of the sample to be tracked during drying by measuring its mass change. Immediately after the \(\mathrm{w_{c}}\) was measured, suction was estimated using either a high capacity tensiometer [68] (up to \(\approx\)2 MPa) or a Decagon WP4C dewpoint potentiometer (greater than 2 MPa). To increase the suction within the sample, it was dried incrementally in a 60 \({}^{\circ}\)C oven, and an equilibration time of at least 2 days was given after a drying step before taking another suction measurement. To avoid drying during equilibration, the samples were double sealed in plastic zip lock bags and stored in a small airtight container. Note, most materials required several drying steps before a suction could be reliably measured, due to sensitivity limitations of the tensiometer used. Samples for bulk volume measurement during shrinkage were first consolidated from a slurry in the same manner as samples used for suction measurements. A 1 cm\({}^{3}\) sample was then cut from the consolidated material with a wire saw, though in some cases minor drying was required before cutting for handling reasons. The coated sample was progressively dried and its volume measured by immersion in silicon oil. The hand spray plaster method was used to ensure that the samples did not absorb silicon oil during immersion. A detailed description of the hand spray plaster procedure can be found in [69] and only a brief description is given in this paper. The hand spray plaster procedure involves coating the soil sample (of size \(\approx 1\) cm\({}^{3}\)) in a waterproof (hand spray plaster) material, which is permeable to water vapor (to allow incremental drying of the sample) but impervious to liquids. The volume of the sample is measured by submersion in silicon oil using Archimedes' principle.