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Concentrated leak erosion (CLE) occurs when subsurface flow is concentrated at an unfiltered exit (e.g., crack, void, macropore), leading to the formation of an eroded pipe (void) within the soil. This process primarily occurs above the water table, where unsaturated conditions control both the progression of erosion and the stability of the void. In dams and levees, the erosion can continue until the void reaches the impounded water, at which point a breach is likely to develop. A similar process is also observed in hillslopes, where the term soil piping is more commonly used, and the collapse of these pipes can lead to landslide initiation. Current methods to evaluate CLE do not explicitly consider the unsaturated properties of the soil and rely primarily on empirical models to determine the likelihood of erosion occurring and progressing. This study employed an integrated program of large-scale experiments and numerical modeling to understand how CLE develops in unsaturated slopes and to identify the impact of changes in soil properties and suction on void stability. The experiments were performed by constructing model slopes with small pre-existing voids to initiate the erosion process. Instrumentation was used to monitor changes in water content and suction at different points within the slope. The experiments demonstrated that an open pipe can act as a drain, leading to higher suction in the slope, whereas collapsed pipes could result in a rapid loss of suction and slope failure. Erosion simulations were used to understand the observations of void enlargement and collapse. The results demonstrate that it is critical to consider both erodibility and collapse due to loss of suction to understand the growth of voids in unsaturated slopes. The results of this study are now being utilized to develop new approaches for evaluating CLE in unsaturated soils. 

Soil piping occurs as concentrated subsurface flow leads to the removal of soil particles and the formation of subsurface voids (pipes). Soil pipes have been observed within the scarps of many landslides and previous studies have hypothesized that the presence of pipes can influence the initiation of shallow landslides. Eroded pipes may act as natural drains and increase stability if they remain open, but they may decrease stability if they collapse or become blocked, thereby increasing pore pressures in the slope. Despite the prevalence of soil piping in many parts of the world, there are still no accepted approaches to account for the influence of soil pipes on landslide initiation. In this study, we used physical models to explore the effects of pipe condition (no pipe, partial or closed pipe, and full or open pipe), bed slope angle, and compaction on the hydromechanical response of an unsaturated slope. We used volumetric water content sensors to measure the evolution of water content in the slope, and cameras to record the slope's response to subsurface flow. The results confirm that the open pipes can act as drains, decreasing the water content and enhancing the stability of the slope, while blocked (closed) pipes can trigger rapid increase in water content and slope failure. These experimental results demonstrate that slope angle, pipe condition, and moisture conditions in the slope affect both the timing of landslide initiation and the severity of the sliding. These findings advance the understanding of subsurface erosion as a slope-forming process and highlight the need to incorporate internal pipe evolution into slope stability assessment. 

Soil piping is the process by which subsurface water creates and enlarges channels, or “pipes,” within soil, enabling rapid and preferential flow beneath the surface. The collapse of these eroded pipes can lead to land degradation, gully formation, and potential damage to overlying infrastructure. While the structural consequences of pipe collapse are well recognized, there is limited understanding of the factors controlling pipe collapse and how water within the pipe influences moisture levels within a slope. This study used physical models of unsaturated slopes to examine how compaction conditions, pipe characteristics, and hydraulic conditions affect the progression of internal erosion. Models were created with different initial pipe sizes, moisture contents, densities at compaction and levels of pipe connectivity. Volumetric water content (VWC) sensors and cameras were used to monitor the slope response to subsurface flow, and measurements of pipe geometry were collected after the tests. Results showed that lower initial soil water content was more susceptible to pipe collapse, while higher water content showed improved pipe stability and sustained preferential flow. Fully connected pipes grew through erosion due to the pipe flow, while disconnected pipes grew mainly through local pipe collapse. Hydraulic equilibrium and soil erodibility affected the final pipe morphology more than the initial pipe size. These experimental results demonstrate that soil fabric and hydraulic connectivity of the pipe control the progression of piping, likelihood of collapse, and movement of water within the soil matrix. 

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.