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Soil piping (concentrated leak erosion) is a major contributor to soil erosion in many parts of the world, and collapse of eroded pipes can result in the formation of gullies and sinkholes or trigger slope instability. Despite these significant impacts, there is little understanding of factors controlling pipe collapse, and how water within the pipe influences moisture levels within a slope. In this study, physical models were employed on unsaturated model slopes with pre-formed macropores to investigate how soil properties, pipe characteristics, and hydraulic conditions govern internal erosion processes and slope stability. Experiments simulated shallow field conditions (0.45 m overburden) using 4 mm and 12 mm pipes to establish preferential flow paths, while varying model parameters including initial compaction moisture content and density, pipe condition (absent, closed, or open), slope angle, and model width. Volumetric water content sensors monitored moisture evolution, while cameras captured slope responses to subsurface flow. Results demonstrate that initial compaction conditions (water content and density), pipe size, hydraulic connectivity, and pipe condition control internal erosion processes and slope stability. 

A centrifuge test (ODA01) was used as a proof-of-concept test to investigate the effect of vertical differential settlement on crack formation in a model levee. The Yolo loam embankment levee and its foundation were 250 mm and 62.5 mm high in prototype scale, respectively. Viscous pore fluid was used to simulate water behind the levee at a height of 225 mm and the test was conducted at 40g. The foundation of the levee included a moving part (a hydraulic table) and a non-moving part (a jointed wood table). The hydraulic actuators were extended to a maximum height of 25 mm before the start of the test. In the centrifuge, the hydraulic table was lowered to a maximum settlement of 25 mm to simulate the differential settlement of the levee. Hairline, transverse and longitudinal cracks were effectively induced in the levee through this vertical differential settlement. Furthermore, seepage flow was initiated through the cracks. The seepage flow stopped after some time without significant erosion, likely due to swelling of the soil around the crack and lowering of the upstream water level. 

Soil piping is the gradual and progressive erosion of soil grains, causing a void (open pipe) to form as water flows through the soil. In dam engineering, this type of internal erosion is often referred to as concentrated leak erosion and has been a cause of failure at multiple dams. Soil piping has also been observed in many landslides and contributes significantly to soil degradation in hillslopes and agricultural areas. Despite these many important impacts, there is still limited understanding of how soil pipes develop and progress and what factors control pipe stability. One of the significant challenges with analyzing soil piping, or concentrated leak erosion, is that it typically occurs in the vadose zone, where unsaturated conditions are present. However, most studies examining internal erosion have focused on saturated conditions, and few studies have examined the role unsaturated hydraulic properties (i.e., air entry value, matric suction, etc.) may play in the likelihood of internal erosion. Consequently, this study aims to explore the mechanisms controlling the erosion rate within soil pipes from the perspective of unsaturated soil mechanics. Bench-scale experiments were performed to examine the formation and progression of an eroded pipe in a small slope constructed at different water contents. Soil samples were also tested to measure its unsaturated hydraulic properties. The results show that the likelihood of pipe formation varies with the moisture content and, therefore, suction in the soil, as does the potential for pipe collapse. This demonstrates that unsaturated soil properties are key to understanding the formation and progression of piping in slopes.