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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. 

Abstract. Landslides along transportation corridors pose substantial risks to infrastructure and public safety, necessitating accurate prediction and mitigation strategies. Many early warning systems for landslides are based on rainfall thresholds derived from historical data that distinguish landslide triggering from non-triggering events. However, it is widely recognized that antecedent moisture conditions have a major impact on the likelihood that a rainfall event will trigger a landslide. We aim to improve existing rainfall thresholds for landslides along highways by incorporating antecedent soil moisture conditions. The landslide inventory for this study was compiled from inclinometer data at suspected landslide sites and from landslide reports following major storm events along Alabama highways. This inventory was combined with precipitation data from the National Oceanic and Atmospheric Administration (NOAA) and soil moisture data from NASA's Soil Moisture Active Passive (SMAP) satellite. We evaluated the accuracy of rainfall thresholds from previous studies for forecasting landslides along highways in Alabama. Additionally, we investigated the potential of reducing the number of non-landslide events that exceed the thresholds (false positives) by utilizing soil moisture data derived from SMAP. This study demonstrates that sites with multiple inclinometers in a landslide region produce more robust datasets compared to those with a single inclinometer, enabling more effective differentiation between landslide and non-landslide events. Furthermore, using normalized soil moisture in the development of rainfall thresholds shows potential for reducing false positives, as approximately 75 % of the false positive cases in this study occurred when the soil moisture was at or below average conditions. Our proposed normalized soil moisture-dependent threshold framework will support decision-making systems by enabling users to weigh the trade-offs between potential false alarms and missed alarms, based on the relative costs or risks of each for a given project. The findings will aid transportation authorities and civil engineers in making informed decisions about potential interventions or preventive maintenance. 

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. 

Levees and dams constructed across active faults face a risk of failure due to the formation of transverse cracks induced by a vertical offset in the foundation and the subsequent initiation and progression of concentrated leak erosion (CLE). The present understanding of crack formation, developed from a limited set of case histories and numerical simulations, includes large uncertainties, and the empirical methods used to estimate crack potential and extent are based on simplified geometries and limited data. A research program was undertaken to better understand mechanisms of crack formation and propagation using the 9m radius centrifuge at the University of California at Davis. A model test was performed at 40g to simulate a levee/dam with a height of 10 m and side slopes of 1.5:1. The homogenous embankment was constructed using a low plasticity clay (locally sourced Yolo loam) prepared to a relative compaction of 96%, based on the standard Proctor test, and a water content of 17.2% (dry of optimum). A vertical offset in the foundation was incrementally induced using a hydraulically actuated table with a maximum offset equal to 10% of the levee/dam height. Cameras and digital image tracking algorithms were used to measure deformation through 3D tracking markers embedded on the downslope surface. At low offset displacements, a rigid block rotation mechanism caused tensile cracks in the upthrown side which first initiated at the crest and was followed by a complimentary crack originating at the base of the embankment. At higher offset displacements, a third shear-induced crack was observed adjacent to the downthrown side. Following testing the model was dissected and the geometry of the transverse and shear cracks were mapped. Comparison of the results against the simplified geometries routinely used in engineering practice methods reveal notable differences. 

There is a need in dam engineering to better understand certain failure mechanisms, including embankment cracking and concentrated leak erosion (CLE). This is especially pertinent for Potential Failure Mode Analysis. The present understanding of these mechanisms has been developed from limited case histories, numerical simulations, and bench-scale testing. Centrifuge tests were performed to investigate embankment crack propagation and CLE using the 9 m radius centrifuge at the University of California, Davis. Tests have been performed at 40g to simulate a levee/dam with a height of 10 m and 1.5H:1V slopes. The homogeneous embankments were constructed using a low plasticity clay compacted above 95% standard Proctor maximum dry density. Tests lowered one side of the foundation using a hydraulically actuated table to induce transverse cracking under unsaturated and saturated embankment conditions. Crack propagation was tracked as the hydraulic table was lowered using markers and digital image tracking on the downstream slope. An upstream reservoir was maintained, allowing for flow through the cracks and for CLE to initiate. After testing the final crack geometries were carefully recorded as the model was dissected. These tests provide system-level experimental data under in-situ stress conditions against which current empirically- and numerically-based analysis methods can be evaluated. 

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. 

An inventory of landslides along Alabama highways was created using both inclinometer readings collected by the Alabama Department of Transportation and based on Detailed Damage Inspection Reports (DDIRs) submitted to the Federal Highway Administration (FHWA) for emergency assistance. For the inclinometer-based dataset, we have processed the readings to extract the change in displacement from the previous reading at the top of the slide plane. For the emergency relief landslides, we have extracted key information from the submitted DDIRs. These datasets can be used for landslide susceptibility modeling, testing remote sensing-based monitoring or warning systems, or to better understand landslide patterns in the southeastern United States. 

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. 

Understanding subsurface conditions is critical to creating and maintaining resilient infrastructure systems, such as dams and levees. Seismic geophysical tools can be very effective for site characterization of these structures as they directly measure the elastic moduli and can provide insight into both the soil properties and groundwater conditions. Full waveform inversion (FWI) is one processing option for seismic geophysics that seeks to overcome some of the limitations in the traditional approaches by using the full time-domain recording of the wavefield to develop 2D or 3D profiles of shear wave velocity. In addition to providing characterization data, FWI can also potentially be used as a monitoring tool for dams and levees to assess how elastic moduli are changing with time and to infer how these changes might relate to changes in the hydromechanical properties of the soil. This study seeks to explore the use of seismic FWI as both a characterization and monitoring tool through numerical simulations of seismic surveys on a hypothetical levee with a low velocity anomaly in the foundation. The simulations are used to assess both the spatial resolution and the ability of the simulations to detect changes in properties that might be related to softening of the foundation or development of internal erosion failure modes. The findings from the study will be used to highlight potential benefits and challenges to using seismic FWI for characterization and monitoring of dams and levees. 

Earthquake-related failure modes for embankment dams are commonly evaluated through numerical simulations using finite element or finite difference approaches. This is especially true for liquefaction triggering or cyclic softening of fine-grained materials where advanced constitutive models are used to capture the dynamic response of the dam and the nonlinear behavior of the soil. Both liquefaction and cyclic softening can lead to significant strength loss, which can lead to large deformations within the dam, but these numerical tools often cannot capture these large deformations due to excessive mesh distortion and subsequent numerical errors. This leads to significant uncertainties in estimating potential crest settlement, which is often a critical value for risk assessments of dams. Hybrid numerical methods like the material point method (MPM) offer a promising alternative to model large deformations, but their application to dams is still limited and relatively little validation has been done on using MPM for post-earthquake stability analyses. This study focuses on applying MPM simulations to evaluate the post-earthquake stability of a hypothetical embankment dam and to examine potential deformations of a flowslide that occurred in Palu, Indonesia in 2018. The MPM program Anura3D is used for the analyses with modifications to allow for assigning residual strengths. The results from the Palu flowslide are compared with observations from the field to show that the MPM analyses are able to capture the extent of the slide, but underpredict the measured displacements in the central portion of the flowslide. The analyses for the embankment dam are compared with post-earthquake stability results from finite difference analyses using FLAC. The MPM analyses are able to capture the full deformation of the flowslide, while the FLAC analyses are halted due to excessive mesh deformation. These results demonstrate the potential of MPM to be used as a complement to existing numerical tools for evaluating the seismic response of dams, but additional work is needed to validate this approach using case histories with both large and small deformations.