## 3 Experimental Set-Up for Investigation of Soil Water Retention Behavior
### Specim Installation and Operating Procedure  
The density control of sand specimen installation in the column was conducted by tapping on the side of the acrylic column and vertically compacting on top of each layer. The top and bottom of each column were fixed to be exactly vertical to avoid tilting off during compaction and any inclines afterwards. In regard to minimizing the density variation along such a long column, the thickness of each soil layer was controlled to 22 \(\pm\) 1 cm. With the same inner diameter of the column, it was possible to achieve a mean dry density of 1.61 \(\pm\) 0.05 g/cm\({}^{3}\) (porosity 39% \(\pm\) 2%) for beach sand and 1.45 \(\pm\) 0.1 g/cm\({}^{3}\) (porosity 45% \(\pm\) 4%) for loamy sand. The water used for fabricating this saturated specimen was tap water from the Geomechanics Laboratory of the University of Queensland, St. Lucia QLD, Australia, due to a lack of access to a large amount of de-aired water produced close by. However, it is non-saline water for general purpose. Under a low suction range for sand soil (0-22 kPa), soil water temperature 22 \(\pm\) 1 \({}^{\circ}\)C and normal room temperature 23 \(\pm\) 1 \({}^{\circ}\)C, there should be no further issue regarding nucleation, such as cavitation and boiling. The segmentation of each layer was carefully controlled to mismatch each pinhole for tensiometer insertion. A previous soil column test mentioned two approaches for  
\begin{table}
\begin{tabular}{c c} \hline
**Specification** & **Range** \\ \hline Measuring range & +100–85 kPa \\ Precision & \(\pm\)0.5 kPa \\ Shaft diameter & 5 mm \\ Shaft length & 20 cm \\ Output signal & \(-\)100 Mv + 85 mV \\ \hline \end{tabular}
\end{table}
Table 1: Specification of UMS T5 micro-tensiometer [55].  
tensiometer installation: installing a sensor through a pre-drilled hole or installing a sensor during the compaction process [58]. Here, the T5 sensors were all inserted into the column during the compaction process. Compared to the first method, the second ensures perfect contact between each sensor and ambient soil [58]. The gap between each sensor and pinhole was sealed with both a waterproof rubber O-ring and gas leakage-free thread tap. Each T5 sensor was originally filled with deionized water using an automatically de-aired water refilling kit, manufactured by UMS\({}^{\otimes}\). Except for the occurrence of the ceramic tip dried out for soil gas percolation, there will be bubble-free water in the sensor and shaft up to \(-101\) KPa by mechanical vacuuming.  
For the gravitational drainage test, the soil was oven-dried before loading into the column to avoid soil segregation carrying bubbles inside the microscale soil structure. Beach sand is completely cohesionless sand. Once the sample was oven-dried, there was no more segregation, while the loamy sand needed some crushing and mechanical shaking because of slightly cohesion. The water table was always above each layer that was being compacted for the preparation of the saturated specimen. The spatial TDR sensor was located in the center of the column, with good contact between soil and sensor, so there should be no more concerns about the failure of the spatial TDR technique because of gaps between the soil and sensor [37] and inductance variation due to distortion of the sensor geometry [17].