3.2 Experimental setup and procedure
Figure. 2 (a) shows the schematic diagram of in-house fabricated experimental setup for conducting HETs. As suggested by Wan and Fell (2004a) and Xie et al. (2018), the setup included a water tank, a soil specimen, and two chambers for inflow and outflow. The eroding fluid was supplied by the water tank with a high gravity potential. The fixed height of the water tank determined the constant head between two ends of the soil specimen. Flow rate in experiments can be adjusted by changing the height of the water tank. The soil specimen was compacted in an acrylic (transparent) cylindrical mold to form a small soil column with a diameter of 30 mm and a length of 50 mm. A 7 mm-diameter hole path was drilled at the center of the soil specimen along the axis (Říha and Jandora 2015). The eroding fluid passed through the hole path to connect two chambers. The inflow chamber was filled with some gravels to stabilize the flow. The outflow chamber was connected to the atmosphere and the PVC collectors were used to receive the erosion.
The soil specimen was prepared at the maximum dry density with optimum moisture content. Before the tests, the prepared specimen was kept in a desiccator for 24 hours for moisture equalization. Initially, the inflow chamber and the hole path in the specimen were filled with the fluid slowly without causing any erosion. The head difference between the inlet and outlet of the specimen was adjusted for a flow rate and kept constant in each test. The erosion was continuously collected every 10 seconds to track the temporal variation of erosion. The test was performed until erosion became insignificant. This was judged based on visual inspection. The HETs were conducted under different hydraulic conditions (i.e. the variance of flow rates: 2920, 4200, 5450, 7000, 8400, and 10000 ml/min) to verify the stability of proposed model and also repeatability of tests.