Several studies have reported that piping in sandy layers might destabilize natural slopes. Piping is a complex behavior related to groundwater flow and soil structure. Therefore, the progression of piping in sandy layers is not clear. We conducted a 1D column test with multistep water injection and observed the change in soil structures using a microfocus X-ray CT scanner.
Differential pressure did not change during water injection when the injection rate was low. On the contrary, differential pressure changed significantly when the injection rate was high. It appeared that soil particles might have been transported along the flow direction. In addition, CT images showed the difference between the change in the soil structures of fine sand particles and a mixture of fine sand and gravel.
The geodisasters caused by heavy rain are observed all over Japan every year. There is a requirement to examine the slope failure mechanism and to develop a method of evaluating slope stability during heavy rain. In general, the main factors that make slopes unstable are increase in pore water pressure or decrease in cohesion with increasing water saturation. In addition, the piping phenomenon is an important behavior for slope failure. Several reports in the geodisaster field have shown piping holes. Pierson (1983) and Shindo (2001) reported that soil pipes have significant impact on mountain body groundwater flow and slope stability. In addition, piping failure is an important process for the safety evaluation of river levees.
Piping is the internal erosion caused by the transport of sand particles with groundwater flow. The hydraulic gradient increases when piping holes are created, and piping is expected to accelerate even further. There has been extensive research on piping, primarily through laboratory experiments. Fujikura (2001) conducted a multistep constant head permeability test. They investigated the critical hydraulic gradient for several soil samples. Saito (2015) conducted 2D tank experiments. They examined the influences of soil stratum conditions, water leakage, and piping failure behavior on river levees. These experiments confirmed the occurrence of piping by measuring pressure or through visual inspection from outside. Therefore, the piping behaviors in internal soil samples have not yet been clearly elucidated.
Considering this fact, we investigated piping progression behavior using a microfocus X-ray CT scanner. We conducted 1D column experiments for multiple sand materials and attempted to visualize 3D piping behaviors using the scanner.
Fig. 1 shows the schematic representation of the experimental apparatus. The 1D column was fabricated from acrylic grass. Its internal diameter and height were 60 mm and 200 mm, respectively. Differential pressure was measured between inlet and outlet points using a differential pressure gauge. De-aired water was injected using a double plunger pump (HPLC pump) and a screw pump. The HPLC pump was used at less than 10 mL/min and the screw pump was used at more than 10 mL/min based on pump ability. The injected water percolated through sand materials and flowed into a circulating tank. Seepage water was re-injected to the sand materials again. Gravel filters were placed at the upper and lower parts of the soil sample for preventing the outflow of sand particles from the 1D column.
