## 3 Experimental Set-Up for Investigation of Soil Water Retention Behavior
### Moisture Profile Logging System Set-Up  
As previously mentioned, the spatial TDR technique was applied to soil moisture profile measurement; four flat ribbon cable sensors were individually inserted into the centers of four empty acrylic columns (diameter = 14.35 cm and length = 240 cm). The bottom terminal of spatial TDR is fixed with an artificial gravel filter designed based on published criteria [49]. The falling head test was conducted on the gravel filter to confirm that the hydraulic conductivity of the filter (6.5 \(\times\) 10\({}^{-3}\)-9.5 \(\times\) 10\({}^{-3}\) cm/s) was larger than the  
Figure 4: (**a**) The overview of the physical experimental set-up for gravitational drainage and spontaneous imbibition experiment; (**b**) the specification of sensor configuration and hydraulic boundary condition for gravitational drainage experiment. hydraulic conductivity of the soil specimen in the column (2.9 \(\times\) 10\({}^{-3}\)-5.6 \(\times\) 10\({}^{-3}\) cm/s). It does not only indicate the end of spatial TDR but is also a highly pervious porous media under the specimen being tested. Moreover, there was no sand or fine gravel flushed out during the soil water drainage process. Each flat ribbon cable sensor had two terminals, demanding two channels on the multiplexer. Due to the need for four sensors for four columns, a TDR trace logging system consisting of a Campbell Scientific SDM X50 multiplexer(r) (8-channels) combined with both a TDR100r and a CR1000 data logger was built to connect to eight connectors of four sensors. The data logging program coded in CRBasic(r) was then applied for automatically logging eight TDR traces from 8 multiplexer channels to complete one/two-way TDR tracing in each soil column. An overview of the experimental set-up and illustration are separately shown in Figure 4a,b. This technique and logging system have been successfully applied to soil column tests and embankment water content dynamics measurement [17; 20]. According to previous performance measurements [17], spatial TDR can achieve a spatial resolution of 3 cm with an average deviation of around \(\pm\)2% for moisture content. The TDR100r can only impulse into the single sensor by changing the single switch along eight multiplexer channels. Therefore, for two-way TDR tracing on each sensor, there has to be at least a one-second delay between the two occurrences of TDR pulsing. However, this is much smaller than the moisture redistribution speed in natural sand. Hence, this delay is neglected, and two-way TDR tracing could be seen as simultaneously implemented. As a gravel filter designed based on filter design criteria [49] guarantees the lowest hydraulic resistivity impacts from the column bottom without internal erosion, the transient flow conditions in the measuring range will not encounter any artificial alleviation of flow velocity. Similar soil column experiments have been implemented by many previous studies [8; 9; 16; 20; 50; 51; 52; 53]. With the exception of the 2-m soil column test [51], most of them, conducted in shorter soil columns of less than 1.2 m, required a hanging column method with a water table below the bottom of the specimen. In this case, there has to be a high air entry (HAE) porous media under the specimen to build up the hydraulic connectivity between the specimen and water reservoir for suction control. To avoid air penetrating through the HAE disk to subsequently disconnect the suction-applying reservoir from the system, such disks or membranes must possess a smaller pore size distribution (PSD), which is much smaller than the specimen. Usually, the HAE disks are ceramic disks made of clay particles sintered together. Thus, the hydraulic conductivity of the HAE material is much lower than the sandy soil specimen. It is somehow inevitable to generate a quasi-steady-state flow condition using a shorter soil column test with the adoption of an HAE disk. Additionally, such smaller column tests only allow the measurement of half of the suction and moisture profile.