Comparison of Erodibility Testing Methods for Cohesive Soils
* Do the hole erosion test (HET) method and the submerged jet erosion test (JET) method produce comparable determinations of erodibility parameters of cohesive soils?
* What are the advantages and disadvantages of applying particular tests to different soils or different site situations?
* Can the hydraulic conditions of the HET be better understood and controlled to produce the most reliable test results?
Need and Benefit
Characterizing the erodibility of cohesive soils has been a difficult problem for as long as engineers have sought to model erosion and sediment transport. Erosion mechanisms and the resisting characteristics of cohesive materials have both been poorly understood, partly because they can vary widely in different flow situations and with seemingly small changes in soil properties (e.g., gradation, moisture content, dry density)
Three approaches to the determination of cohesive material erodibility have emerged in recent years, each driven somewhat by a desire to create a test akin to the intended application of the data. These are described below:
* Researchers studying headcut erosion and dam breaching have developed submerged JET that consist of an impinging jet that erodes a scour hole into a base material, similar to the manner in which a plunging jet might scour the base of a headcut. Principal development of this class of tests has taken place at the U.S. Department of Agriculture (USDA) Agricultural Research Service (ARS) hydraulic laboratory in Stillwater, Oklahoma. Versions for laboratory and field use are available. In most of the devices, the jet is vertical and the tested soil surface is horizontal, but one version uses an inclined jet for testing soil in streambanks. Results of the test have been used to model spillway headcut erosion, earthen dam breaching, and stream-channel erosion processes.
* Researchers studying seepage erosion and piping of embankment dams and their foundation materials have developed a HET and companion slot erosion test (SET) that simulate erosion that might occur in a developing pipe through an embankment. Reclamation has recently focused its interest on the HET, as the more straightforward and economical test of the two.
* Researchers interested in bridge scour have developed so-called flume-type tests in which flow across the surface of a sample produces erosion. The leading test in this category is a commercial device (originally developed at Texas A&M) called the Erosion Function Apparatus (EFA).
All three of the tests described produce similar outputs (determining a critical shear stress and an erosion rate coefficient), but it is not clear whether the results of each test are comparable. One preliminary set of comparative tests of similar soil samples in all three devices (JET, HET, and EFA) has suggested that the EFA is somewhat less reliable, exhibiting more variability, possibly because of more variation of the actual stresses being applied to the tested sample. These tests also suggested that JET and HET results could be correlated, but the tests were very limited in the range and number of soil samples considered.
There are clearly situations in which it might be more feasible to apply one test than the other, and situations where one test might be more desirable. For example, the JET can be applied in the field to a completely undisturbed sample that is still in place, and the test can easily be carried out with local water (water chemistry has been shown to sometimes have a large effect on erodibility). On the other hand, the HET can be run on smaller samples, such as those that might be recovered from deep boreholes. Modelers will likely need to make use of both tests in some situations, and thus a clear understanding of the correlation between the JET and HET is important.
The benefit of this work will be to improve the ability of Reclamation to model erosion of cohesive materials in several different water management applications, including dam safety (embankment breach, spillway headcutting), stream corridor maintenance, rehabilitation, and restoration, and dam removal. Understanding the relationship between the results of these tests will allow the most economical and appropriate test to be used in each situation.
Bureau of Reclamation Review
The following documents were reviewed by experts in fields relating to this project's study and findings. The results were determined to be achieved using valid means.
Determining Erosion Indices of Cohesive Soils with the Hole Erosion Test and Jet Erosion Test (final, PDF,
By Tony Wahl, Mr. Pierre-Louis Regazzoni and Ms. Zeynep Erdogan
Report completed on October 28, 2008