Downstream filter control of flow in cracks in embankment dam cores

The Slot Erosion Test and the Hole Erosion Test have been developed as fast and simple tests for studying the erosion characteristics of soil in cracks in embankment dams. The erosion characteristics are described by the Erosion Rate Index, which measures the increase of erosion rate with respect to an increase in the hydraulic shear stress; and the Initial Shear Stress, which represents the minimum hydraulic shear stress when erosion starts. Knowledge of these erosion characteristics aids in the prediction of the likelihood of embankment dam failure due to piping erosion in a risk assessment process. Values of the Erosion Rate Index span from 0 to 6, indicating that the changes in erosion rates in response to changes in hydraulic shear stress can differ by up to 106 times across different soils. The Erosion Rate Index is apparently influenced by the soil fines and clay-sized content, plasticity, dispersivity, water content, dry density, degree of saturation, clay mineralogy, and possibly the presence of cementing materials such as iron oxides. Coarse-grained, noncohesive soils, in general, erode more rapidly and have lower Initial Shear Stresses than fine-grained soils.

A major hazard to embankment dams is internal erosion and piping arising from concentrated leaks through transverse cracks in the core near the crest due to differential settlement or desiccation. This contribution summarises experiments and a formal boundary layer analysis of flow through such transverse cracks subject to near-horizontal pressure gradients. The critical hydraulic gradients, at which erosion in such cracks may initiate, occur at the downstream end of the crack. This location and the maximum crack width defines the local point most vulnerable to erosion. The hydraulic laboratory measurements are shown to be consistent with equivalent international standards describing depths at free overflows, in spite of the narrow vertical profile of the cracks, provided that the tendency towards laminar flow in very narrow cracks is incorporated within the modelling. A backwater model based on conventional representations of flow in narrow channels is verified qualitatively by hydraulic laboratory data. Flow-depth relationships, maximum crack wall hydraulic stresses, normalised hydraulic wall stress profiles and depth profiles along cracks are presented for the two principal crack geometries present in embankment dams. These characterisations are a significant improvement on the simplified methods previously used to assess internal erosion in transverse cracks in dams.

A major hazard to embankment dams is internal erosion and piping arising from concentrated leaks through cracks in the core due to differential settlement. Previous work (Peirson et al., 2023) quantified the hydraulics of flow through near-surface transverse cracks subject to near-horizontal pressure gradients. This present short note extends the previous work to address pressurised flow through cracks that might occur deeper within the core. Water speeds, hydraulic shear stresses, flows are computed as a function of the applied hydraulic gradient. These results extend previous work addressing the hydraulics of very narrow cracks.

Numerical modelling of transverse cracking in embankment dams

Experimental investigation on hole erosion behaviors of chemical stabilizer treated soil

Continuous internal erosion, commonly manifested as piping, is a major cause of failure in earthen structures. This study employs the hole erosion test to examine the internal erosion resistance of zein biopolymer–treated soil, encompassing three sandy soil types with varying particle sizes. The gelation mechanism of the zein binder is evaluated through rheological and shear wave analyses. Treated and untreated specimens are subjected to hydraulic gradients at constant flow rates. The erosion analysis focuses on changes in axial diameter, particle loss rate, shear stress, and erosion rate. The biopolymer gel demonstrates evolving rheological behaviour, transitioning from shear thickening to shear thinning after a 4-hour curing period. Treated specimens exhibit improved shear stress and erosion rate over time, which vary with particle sizes. Hydraulic shear stress decreases with the curing period, and particle size increases, correlating with erosion rate reduction. Higher consistency index of the biopolymer gel leads to decreased hydraulic shear stress, influenced by gel internal friction. Hydraulic shear stress linearly relates to shear wave velocity of the treated specimen. Zein biopolymer enhances erosion resistance of cohesionless sand through gel internal friction and treated specimen shear stiffness.

The aim of this paper is to study the impact of certain soil treatments on the internal erosion characteristics of treated compacted silt. The experiments measured the internal erosion using the hole erosion test (HET). This study aims to describe the effects of clay, lime and cement soil treatments on the internal erosion, specifically with regard to the amount of treatment used and the curing time. A new enhanced HET was developed to apply a high inlet pressure up to 650 kPa and thus generate a hydraulic shear stress up to 10,000 Pa. The internal erosion resistance was quantified by the coefficient of soil erosion and the critical shear stress. The results demonstrated that clay treatment could reduce the coefficient of soil erosion depending on the nature and percentage of clay added to the soil. The results also showed that lime and cement treatment primarily increased the critical shear stress of the tested silt. This increase was higher with cement treatment and was dependent on the amount o...

Entropy-based Model for Gully Erosion - A combination of probabilistic and deterministic components.

A method is presented for the approximate estimation of the time for progression of internal erosion and piping, and development of a breach leading to failure in embankment dams and their foundations. The method accounts for the nature of the soils in the dam core, the foundation, and the materials in the downstream zone of the dam. Guidance is also provided on the detectability of internal erosion and piping, taking account of the mechanism of initiation, continuation, and progression to form a breach, for internal erosion and piping in the embankment, the foundation and from the embankment to foundation. It is shown that in many dams which have poor internal erosion and seepage control and are constructed mainly of earthfill, the time for potential development of piping is short, and for these dams continuous monitoring of seepage or surveillance would be needed to detect the piping in time to give warning of possible failure, and to give time to attempt intervention to prevent the failure.

In this study, a series of rigid-wall downward erosion tests were conducted to investigate the mitigation effect of microbially induced calcite precipitation (MICP) on soil internal erosion. Sand-kaolin mixture was used in this study, which proves to be internally unstable in terms of particle size distribution. Binary mixture was prepared using dry tamping method. Bacteria and cementation solutions were injected into samples at several phases. For comparison, control samples without MICP treatment were prepared. All samples were subjected to downward seepage flow with incremental flow rates. Evolutions of hydraulic conductivity, effluent kaolin concentration, and total erosion mass were measured against hydraulic gradient and shear stress. Results confirmed that, with MICP treatment, both critical hydraulic gradient and shear stress were significantly increased and erosion mass was reduced under the same hydraulic conditions. © 2014 American Society of Civil Engineers.

Soil detachment by concentrated flow is often predicted using hydraulic shear stress (t) as a predictor variable. The nature of this relationship is uncertain, however, and may depend on many factors that influence soil resistance to detachment. Research was conducted to better understand the relationship between soil detachment from concentrated flow and t and the influence of soil bulk density (rb) and erosion duration on detachment. Overland flow was simulated using a 4.9-m-long hydraulic flume. Soil cores from a Mexico silt loam soil (fine, montmorillonitic, mesic Udollic Ochraqualf), 102-mm in diameter, were prepared at 1.2 and 1.4 Mg m3 bulk density and exposed to overland flow at shear stresses ranging from 1.1 to 38.2 Pa. Soil detachment increased with test duration and t for both rb soils. Within a t range of 1.1 to 15.0 Pa, soil detachment was observed to be 5.5 times greater for the 1.2 versus the 1.4 Mg m3 rb samples with a mean detachment of 11.1 and 2.1 kg m2, respectively (P = 0.01). Soil erodibility (Kr), defined as the detachment rate per unit increase in t, was 1.10 g m2 s1 Pa1 ( 103 s m1) for the 1.2 Mg m3 rb and 0.23 g m2 s1 Pa1 for the 1.4 Mg m3 rb, samples indicating that the 1.4 Mg m3 rb samples were 4.7 times as resistant to detachment compared to samples packed at 1.2 Mg m3 rb soil. Critical shear stress, defined as the shear stress where detachment is not different from zero in a graph of detachment rate versus t, was 1.3 Pa for the 1.2 Mg m3 rb and 4.0 Pa for the 1.4 Mg m3 rb samples. Vane shear strength difference between the two bulk densities was highly significant with mean strength of 3.0 kPa for the 1.2 Mg m3 rb and 14.4 kPa for the 1.4 Mg m3 rb soil.

1. Introduction 2. Key Geological Issues 3. Geotechnical Questions Associated with Various Geological Environments 4. Planning, Conducting and Reporting of Geotechnical Investigations 5. Site Investigation Techniques 6. Shear Strength, Compressibility and Permeability of Embankment Materials and Soil Foundations 7. Clay Mineralogy, Soil Properties, and Dispersive Soils 8. Embankment Dams, their Zoning and Selection 9. Design, Specification and Construction of Filters 10. Control of Seepage, Internal Erosion and Piping for Embankment Dams 11. Analysis of Stability and Deformations 12. Design of Embankment Dams to Withstand Earthquakes 13. Embankment Dam Details 14. Specification and Quality Control of Earthfill and Rockfill 15. Concrete Face Rockfill Dams 16. Concrete Gravity Dams and their Foundations 17. Foundation Preparation and Cleanup for Embankment and Concrete Dams 18. Foundation Grouting 19. Mine and Industrial Dams 20. Monitoring and Surveillance of Embankment Dams.

This paper presents results from a three-year field study that is being conducted to determine the morphology, processes and migration rates of nine knickpoints in the cohesive clay beds of the Yalobusha River System, Mississippi. The roles of surface erosion by hydraulic shear stress and mass failure during the upstream migration of knickpoints are evaluated. Results of submerged jet tests reveal a wide variation in the erosion resistance of the streambeds. Values of critical shear stress, τc, span almost four orders of magnitude with a median of 31 Pa, while values of the erodibility coefficient, k, span about three orders of magnitude with a median of 0.022 cm 3 /N-s. To relate τc and k-values to the erosion potential of flows, an excess shear stress approach is used to estimate erosion rates using values of average boundary shear stress, τ0. Most flows are competent to erode streambeds composed of the Naheola formation (τc = 2.24 Pa), while only the deepest flows and steepest hydraulic gradients generate boundary-shear stresses great enough to erode the Porters Creek Clay formation (τc = 199 Pa). Repeated thalweg surveys indicate that knickpoints formed in Naheola clay have migrated at an average rate of 7.2 meters per year since 1997, while knickpoints in Porters Creek Clay have migrated at an average rate of around 1.2 meters per year over the same period. Comparison of calculated erosion rates and observed knickpoint retreat rates indicates a discrepancy between migration rate and available hydraulic shear stress. Additional mechanisms have hence been identified as contributing to the migration of cohesive knickpoints: a cyclical mass-failure mechanism, and detachment of aggregates by upward directed pore-water pressure on the falling limb of hydrographs. To assess the role of mass failure in knickpoint retreat a combination of finite-element hydrologic and limit equilibrium slope-stability modeling was carried out. Thalweg survey data were used to construct a series of finite-element meshes based on the geometries of the knickpoints during migration, while head and tail water elevations were taken from stage data logged every 30 minutes above and below the knickpoint. At one example location in Naheola clay, Big Creek, every observed event greater than 0.3 m has been extracted from the stage record and simulated. Where a mass failure was indicated by a factor of safety close to or below one, the predicted failed distance was compared with the observed knickpoint retreat distance at the next survey. Results at this location show a close agreement between modeled and observed retreat rates. Quantification of knickpoint migration rates and processes in the field enables river managers to accurately determine the locus of incision at a particular time, allowing estimates of sediment sources and yields. This study has also shown that knickpoints formed in resistant substrata may act as a form of natural grade control, while knickpoints formed in more erodible substrata may require direct intervention by management agencies.

Internal erosion is one of the most important factors that cause earth structures that retain water, such as embankment dams, to collapse. Concentrated leak erosion, one of the forms of internal erosion, occurs in cracked fine-grained soils and pressurized flow conditions. To evaluate the concentrated leak erosion risk of cracks/voids, it is necessary to ascertain the erosion resistance of these materials. The erosion rate and critical shear stresses determine internal erosion resistance in concentrated leak erosion. This study determined soil’s concentrated leak erosion resistance using test equipment that allowed the flow to pass through a hole with stress-free (no loading), anisotropic-compression stress, anisotropic-expansion stress, and isotropic stress conditions. The stresses that developed in the samples’ hole wall where erosion occurred were determined with numerical modeling as pre-experimental stress conditions. The experiments were performed under a single hydraulic head on four selected cohesive soils with different erosion sensitivity. Time-dependent flow rates obtained from the test system can be used to determine hydraulic parameters, such as energy grade lines, with the help of basic theorems of pipe hydraulics in theoretical hydraulic models. Moreover, the erosion rates were quantitatively determined using the continuity equation, while critical shear stresses were qualitatively compared for concentrated leak erosion developed by the dispersion mechanism. As a result of the experiments, stress conditions influence the concentrated leak erosion resistance in the soil samples with dispersive erosion. Moreover, the shear strength in the Mohr–Coulomb hypothesis can explain the erosion resistance in these soils under stress conditions depending on the sand/clay ratio.

Use of rotational erosion device on cohesive soils : Chapuis, R P Trans Res RecN1089, 1986, P23–28