Hole Erosion Test Research Articles

Predicting initial erosion during the hole erosion test by using turbulent flow CFD simulation

CFD simulation with enhanced modeling of turbulence and near-wall treatment is used to model water–clay mixtures flowing through a cylindrical pipe domain. Effects on the wall-shear stress resulting from varying water clay content and applied hydraulic gradient are analyzed. Various parametric studies were performed and had shown that the two-dimensional modelling introduced in the present study does not yield a uniform wall-shear stress along the pipe wall and that clay concentration affects significantly the wall-shear stress value. This is in contrast with the common hypothesis used in one-dimensional modeling approaches where this stress is assumed constant and which gives rise to uniform erosion along the pipe wall. The obtained results had enabled predicting more realistically erosion amount and had allowed for understanding the irregular eroded hole wall shape as observed experimentally after performing the standard hole erosion test.

Energy-Based Method for Providing Soil Surface Erodibility Rankings

The jet erosion test (JET) and the hole erosion test (HET) are two tests used to determine soil erodibility classification, and results are commonly interpreted by two distinct methods. A new method based on fluid energy dissipation and on measurement of the eroded mass for interpreting the two tests is proposed. Different fine-grained soils, covering a large range of erodibility, are tested. It is shown that, by using common methods, the erosion coefficient and average critical shear stress are different with the JET and with the HET. Moreover, the relative soils classifications yielded by the two erodimeters are not exactly the same. On the basis of the energy method, an erosion resistance index is determined for both apparatuses, and a classification of surface-erosion resistance is proposed. For both apparatuses, values of the erosion resistance index are roughly the same for each soil, and a single classification of soil erodibility is obtained.

A Simplified Analytical Modeling of the Hole Erosion Test

Problem statement: Internal erosion occurs in soils containing fine particles under the action of high pressure gradients that could result from water discharge. This phenomenon can yield in its final stage to the formation of piping which constitutes a real threat for hydraulics infrastructures as it can precipitate their entire rupture in very short time. In order to mitigate this insidious hazard, it is important to characterize piping dynamics. In this context, the Hole Erosion Test was introduced to assess the erosive features of soils by means of two parameters, the erosion rate and the critical shear stress indicating the beginning of erosion. Modeling this test can enable to understand more comprehensibly the piping phenomenology. Approach: A simplified analytical modeling of the Hole Erosion Test was considered in this study. A closed form solution of erosion taking place during piping was derived without resorting to the habitual cumbersome developments that are needed to achieve complete solution of the rational equations describing this highly coupled problem. This was achieved by assuming formal analogy between the erosive shear stress and the friction shear that develops at a cylindrical piping wall under an axial viscous flow. The flow was assumed to be uniform along the tube. Results: A closed form analytical formula describing erosion dynamics associated to piping was derived. Theoretical predictions were compared with experimental results and the simplified model was found to predict accurately the increase of flow rate that results from piping erosion. Conclusion/Recommendations: The one-dimensional modeling that was proposed for the Hole Erosion Test under strong simplifying assumptions was found to yield the same features as those obtained in the literature by using other approaches. It gives furthermore the dynamics as function of the fluid regime existing inside the tube. In order to get further insight regarding the flowing flow action, which could be non uniform during erosion, more advanced modeling is needed.

Effect of Wall Roughness and Concentration of Clay on Erosion in the Hole Erosion Test

Problem statement: Internal soil erosion is a real threat for hydraul ic infrastructures. In its final stage it develops in piping involving the formation and progression of a continuous void inside the soil between the upstream and downstream sides. The hole erosion test was introduced to characterize kinematics of piping in terms of the t ime left to rupture. Actual modeling approaches of this test are essentially one dimensional. The wall shear stress generated by the flow is assumed to b e uniform, so that erosion rate is also uniform along the hole length. Experimental observations show however an irregular profile of the eroded hole. Approach: In this study an axisymmetric extension representation of the hole erosion test was perform ed. The biphasic flow at the origin of surface erosion occurring in the porous soil sample was modeled by means of the renormalization group based k-e turbulence equations. Fluent software package was used to perform the numerical modeling. Results: This had enabled to estimate the wall shear stress which was found to be non uniform along the hole length. Erosion rate was the n estimated by using a classical law. Its variation s as affected by the applied gradient pressure, fluid density as well as the actual fluid/soil interface roughness were analyzed. In particular, wall roughn ess and clay concentration were found to increase noticeably the erosion rate. Conclusion/Recommendations: Predicting erosion rate at the start of piping formation can be done by the propos ed model. Flow features are however very complex in the real HET configuration. In particula r, clay concentration does not vary equally along the hole length. The erosion law coefficients are v ariable. Transport phenomenon of some soils particles that detach is present in the problem. Fu rther investigations including these aspects should be performed in order to render more profoundly the complex physics involved in this experiment.

True Triaxial Piping Test Apparatus for Evaluation of Piping Potential in Earth Structures

Abstract Current methods available for testing the piping potential of soils in dams (pinhole test, hole erosion test, and slot erosion test) are limited to cohesive soils that maintain an open hole within the sample. These tests do not adequately simulate conditions within a zoned embankment, where zones of non-cohesive materials are present under relatively high confining stresses. A new apparatus, called true triaxial piping test apparatus or TTPTA, was developed for testing a wider variety of soils under a wider range of confining stresses, hydraulic gradients, and pore pressures than current tests allow. The TTPTA is capable of applying a range of confining stresses along three mutually perpendicular axes in a true triaxial test apparatus. Pore pressures are also controlled through regulated inlet and outlet pressures. The test determines the critical hydraulic gradient and, more importantly, the critical hydraulic velocity at which piping is initiated in non-cohesive soils. Detailed descriptions of the test apparatus and test method are presented, as are initial test results using TTPTA. Three sets of initial tests were conducted using uniform sand to (1) assess the repeatability of test results, (2) evaluate how the rate of change of inflow impacts the critical discharge rate at which piping is initiated, and (3) evaluate how the angle of seepage affects the critical velocity for piping initiation. These initial tests were conducted to evaluate the method and to help set test parameters for future testing. It is found that the TTPTA is capable of yielding fairly consistent results with 10 % scatter in repeat tests. The seepage angle tests demonstrate that the angle between seepage flow direction and the direction of gravity is an important factor to consider when evaluating piping potential. The rate of change in seepage also has a minor influence on test results, but a change in flow rate of 5 (mL/min)/min could produce reliable results. Based on the results, the hydraulic gradient is found to be a less reliable indicator of piping potential than the hydraulic velocity for non-cohesive soils. The TTPTA is capable of simulating conditions within small to medium sized embankments.

The scaling law in the hole erosion test with a constant pressure drop

AbstractA process called ‘piping’, which often occurs in the soil at dams, levees, and dykes, involves the formation and development of a continuous tunnel between upstream and downstream ends. The hole erosion test is commonly used to quantify the critical stress and the rate of piping erosion progression. The aim of this study is to draw up a model for interpreting the results of this test. A characteristic internal erosion time is defined and expressed as a function of the initial hydraulic gradient and the coefficient of surface erosion. It is established here that the product of the coefficient of erosion and the flow velocity is a significant dimensionless number: when this number is small, the kinetics of erosion are low, and the particle concentration does not have any effect on the flow. This finding applies to most of the available test results. Theoretical and experimental evidence is presented showing that the evolution of the pipe radius during erosion with a constant pressure drop obeys a scaling exponential law. Copyright © 2008 John Wiley & Sons, Ltd.

On the modelling of piping erosion

A phenomenon called ‘piping’ often occurs in hydraulics works, involving the formation and evolution of a continuous tunnel between the upstream and the downstream sides. The hole erosion test is commonly used to quantify the rate of piping erosion. However, few attempts have been made to model these tests. From the equations for diphasic flow with diffusion, and the equations of jump with erosion, a piping model is developed. A characteristic time of internal erosion process is proposed. Comparison with experimental data validates our results. To cite this article: S. Bonelli et al., C. R. Mecanique 334 (2006).

Laboratory Tests on the Rate of Piping Erosion of Soils in Embankment Dams

Abstract 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.

Investigation of Rate of Erosion of Soils in Embankment Dams

The slot erosion test and the hole erosion test have been developed to study the erosion characteristics of soil in cracks in embankment dams. The erosion characteristics are described by the erosion rate index, which measures the rate of erosion, and the critical shear stress, which represents the minimum shear stress when erosion starts. Values of the erosion rate index span from 0 to 6, indicating that soils can differ in their rates of erosion by up to 106 times. The rate of erosion is shown to be dependent on the soil fines and clay sized content, plasticity, and dispersivity; compaction water content, density and degree of saturation; and 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 critical shear stresses than fine-grained soils. Knowledge of the erosion characteristics of the soil in the core of an embankment dam aids in the assessment of the likelihood of dam failure due to piping erosion.