Bryant A. Robbins

Databases for Backward Erosion Piping Laboratory Experiments and Field Observations

Backward erosion piping is a failure mechanism which involves the formation of shallow pipes in a sandy foundation layer and is considered to be a major risk for levees. For understanding this mechanism and the development of prediction models, laboratory experiments are essential. In addition, due to scale effects and heterogeneity in field conditions, field observations and case histories are indispensable for validation of models and delineation of piping sensitive conditions. However, both experiments and field observations are often not easily utilized for this purpose. Piping experiments have been conducted in various research programmes, countries, and in a variety of configurations making the experiments difficult to compare due to inconsistent observations and differing configurations. Case histories are often poorly documented and like experiments, described in different sources and different levels of detail, due to which their full potential is often not reached. Given the importance of experimental and field data for the prediction of backward erosion piping, a need exists for a centralized organization of data. Two different databases are presented here, for laboratory experiments and field observations respectively, each combined with a web application for viewing and exporting the data. The laboratory experiment database is populated with 332 experiments. The field observation database is currently populated with 3 failure cases and 2840 sand boils located in the Netherlands and the United States. Future work will focus on a more complete population of the databases, user-friendliness of the web viewer, and analysis of the gathered data for improvement of prediction models.

Experimental Evaluation of Kovács’ Equations for Estimating Critical Gradients

Kovács (1981) presented two equations for the critical hydraulic gradient required to cause particle movement on slopes of cohesionless soils. One assumes a sliding failure mechanism due to movements parallel to the slope; the other assumes a heave mechanism due to grain movements normal to the slope. The use of these equations has been proposed for the prediction of piping initiation on slopes. The objective of this investigation was to evaluate through laboratory experiments and numerical analysis the reliability of using Kovács’ equations to predict the gradients required for initiation of backwards erosion piping (BEP). Two sands at three slope angles were tested in a custom-made flume. Results showed that BEP initiation occurs over an incredibly small length. Kovács’ formulation based on sliding was found to be overly sensitive to slope angle and did not match experimental observations. Kovács’ heave-based formulation closely matched the experimental observations, indicating that initiation of BEP is a heave-type failure. While the heave-based equation aligned with the experiments, the length over which the gradient must be examined is too small to be of practical use in field applications and should not be relied upon as a predictive tool for BEP initiation. DISCLAIMER: The contents of this report are not to be used for advertising, publication, or promotional purposes. Citation of trade names does not constitute an official endorsement or approval of the use of such commercial products. All product names and trademarks cited are the property of their respective owners. The findings of this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents. DESTROY THIS REPORT WHEN NO LONGER NEEDED. DO NOT RETURN IT TO THE ORIGINATOR. ERDC/GSL TR-18-16 iii

Discussion of “Erosion Charts for Selected Geomaterials” by Jean-Louis Briaud, Anand V. Govindasamy, and Iman Shafii

The authors have compiled and presented a substantial database of erosion properties (critical shear stress and erosion rate) of soils based on laboratory tests performed using an erosion function apparatus (EFA). Given the significance of erosion in determining the resilience of infrastructure to large meteorological events, it is critical that methods be developed for establishing first-order estimates of erosion properties. The authors are to be commended for their attempts to relate the Unified Soil Classification System (USCS) to erosion properties. The discussers hope to add to the considerable value of the authors’ paper by (1) re-evaluating the proposed ranges of critical shear stress in light of additional data and (2) exploring the similarities between critical shear stress (τ c) measurements obtained from the EFA and the jet erosion test (JET) (Hanson and Cook 2004). The authors proposed ranges of τ c by USCS soil classification in their Table 5. The discussers’ Table 1 compares these suggested ranges of τ c to the respective range of τ c values measured for each soil type as presented in the authors’ Table 2. The authors’ suggested ranges do not encompass all of the measured data the authors presented for any of the four soil types. Additionally, ranges of τ c for low-plasticity silt (ML) and high-plasticity silt (MH) were not suggested by the authors due to limited data. These two observations prompted the discussers to compile additional measurements of τ c to further investigate the range of τ c exhibited by differing soil types. The discussers’ Table 2 presents additional measurements of τ c from research described herein made by EFA tests or similar apparatuses. Tucker-Kulesza et al. (2017) presented 2 EFA test results on near-surface clayey soils. Rahimnejad and Ooi (2016) presented 33 EFA test results for predominantly silt soils sampled from water channels in Hawaii. Texas A&M University conducted EFA tests for the Corps of Engineers on Shelby tube soil samples from the American River in Sacramento, CA (unpublished EFA test data from 2011). As the authors were aware of these tests and did not include them in their Table 2, there may be issues with the data the discussers are unaware of. Ghelardi (2004), Straub and Over (2010), and Anderson et al. (2015) conducted EFA tests on Shelby tube soil samples as part of scour investigations. Navarro (2004) measured τ c of Shelby tube soil samples with an open channel flume capable of producing a boundary shear stress of 21 Pa. The soil sample was pushed into the flow from the floor of the flume similarly to the EFA. Because the flow conditions in this scenario are similar to those of an EFA, the data are included in Table 2. Shan et al. (2015) developed an ex situ erosion-testing device (ESTD) that consists essentially of an EFA with a moving upper wall boundary. The moving wall was designed to allow the velocity profile to closely mimic the velocity profile of deep, open channel flows. Particle image velocimetry was used to

Laboratory Jet Erosion Tests on the Lower American River Soil Samples, Sacramento, CA- Phase 2

Abstract : This report summarizes the results of 42 laboratory Jet Erosion Tests performed on Plexiglas tube samples obtained from the Lower American River (LAR) between River Mile (RM) 6.0 and RM 10.0. The results from these tests will be used by the U.S. Army Corps of Engineers, Sacramento District, in assessments of the erosion resistance of the LAR from increases in discharge from 115,000 cfs to 160,000 cfs from Folsom Dam. The test specimens were obtained from 22, 4 in.-diam Plexiglas tube samples. The variations in values of the measured erosion parameters may have been caused by variations in the materials for some of the tested samples (i.e., when the material changed from silt/sand to clay). However, the variations in results for many of the samples were due to changes in the quality of samples. The resulting values of Erodibility Coefficient, Kd, and Critical Stress, c, are very useful information in assessing the erodibility of riverbanks as well as the river bed itself. Because of the observed natural variability of the materials, combining the erosion parameters presented in this report with the drilling logs and local geology will provide beneficial results for assessing the stability of the LAR.

SWCC Prediction: Seep/W Add-In Functions

Abstract : The soil water characteristic curve (SWCC) defines a constitutive relationship between the negative pressure that develops when a soils saturation level is less than fully saturated, and the corresponding volume of water held in the pore space of the soil matrix. As this relationship is not commonly measured in geotechnical laboratories, practitioners often attempt to predict this relationship based on other commonly measured material properties using empirical prediction methods. The performance of five SWCC empirical predictors was evaluated through comparisons to independently measured SWCC data for four soils. SWCC prediction methods were selected for this investigation if they incorporated commonly measured soil properties to predict the SWCC. The error in the SWCC prediction was assessed in terms of both the mean squared error on the SWCC prediction and the impact of the error on a numerical analysis of the Green and Ampt infiltration problem. The results of the numerical analysis were assessed in terms of a normalized saturation coefficient. The normalized saturation coefficient provided a clear means of monitoring a transient seepage analysis through a single measure. Results indicate that the SWCC prediction methods yielding the lowest mean squared error did not necessarily yield the smallest error in the transient seepage analysis. Further, only the Rawls method consistently yielded conservative analysis results for all soil types investigated.

The consistency of laboratory jet erosion tests performed on undisturbed samples

The U.S. Army Engineer Research and Development Center performed 75 laboratory Jet Erosion Tests (JETs) on undisturbed samples collected from the riverbank along the Lower American River (LAR) near the city of Sacramento, California, to identify the erosion resistance of soil along the riverbanks and riverbed of the LAR through the erosion parameters of erodibility coefficient, Kd, and critical stress, τc. Thirty-nine of the samples were Shelby tube samples and the other forty-two were 4 in.-diameter Plexiglas tube samples. The soil type varied from weak sandy silt to stiff silt with the JET results varying from Very Erodible (VE) to Very Resistant (VR). For many of the samples, the variations in results were due to changes in the quality of the undisturbed samples; however, the variations may also be caused by the changes in the soil type from silt/sand to clay in some of the tested samples and natural variability of the soils. The method of sampling tube and the process of cutting the tube may also add to the variability in test results. To use the erosion parameters for specific soil types outside of the samples tested, the erosion parameters were related to index properties of the soil: particle diameter, percent clay content, and Plasticity Index. Erosion parameters were found to be most significant correlated to the particle diameter.