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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. 

Backward erosion piping (BEP) is a major risk factor for both dams and levees. A significant amount of work has been performed to examine the likelihood of initiation of BEP through examination of critical gradients for sand boil formation and the development of semi-empirical approaches to assess the likelihood of erosion continuing and progressing to a breach. Recently, numerical analyses have been developed to model the BEP process and these tools offer a means to incorporate more complex geometries and soil conditions than can be assessed within the semi-empirical approaches. These simulation methods have been primarily applied to laboratory tests and physical models with uniform properties, but there is a need to validate these models using case histories and explore how variability in stratigraphy and properties influences the BEP process. This study will apply numerical approaches for simulating BEP to the case history of the Bois Brule levee breach that occurred during flooding in July 1993. This paper first describes the Bois Brule levee and observations during flooding. Numerical simulations of piping progression are used to explore the effect of reasonable variations in properties and stratigraphy on the likelihood of failure and the sensitivity of the results to the river level at the time of failure. The paper highlights challenges in modeling BEP and provides guidance on which factors have the largest impact of the results.