Erosion tests
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Determining the resistance to erosion of hydraulic embankments
An erosion test aims to characterize the resistance to erosion of a hydraulic embankment by experimentally measuring this resistance on an embankment sample taken in situ, most often from a borehole.

There are several types of erosion processes, and no single test can reproduce all erosion phenomena. Therefore, the choice of test is crucial and depends primarily on the type of erosion likely to be occurring in the structure at the precise location where the sample was taken. It also depends on the technical characteristics of the sample and the practical limitations of the tests.

geophyConsult assists its customers in this selection process to obtain the most appropriate result for the specific problem the project owner is facing.

Given the small size of the samples compared to the overall structure from which they originate, particular care must be taken in their selection beforehand, ensuring that they are truly representative of the investigated strata.
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Proposed tests



Hole Erosion Test
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Jet Erosion Test
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Contact Erosion Test
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Crumb Test
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Custom-made test
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Hole Erosion Test (HET)
When a defect exists in a structure (burrow of a burrowing animal, crack, etc.) and is subjected to pressure, a leak appears. Depending on the soil's resistance to erosion, this leak may be acceptable or lead to the formation of an erosion conduit, or even a breach, sometimes within a few hours.

The HET was developed by INRAE ​​to reproduce this process. The test consists of forcing water to flow through a 6 mm diameter hole drilled into a soil sample brought to the laboratory. Analyzing the enlargement of the hole due to erosion of the walls during the test allows the soil's resistance to this type of erosion to be determined.
The HET marketed by geophyConsult is sold under license from INRAE. It complies with the AFNOR XP P94-065 standard (to which geophyConsult contributed, as a pioneer in the implementation of the test). It allows for obtaining:

  • a positioning of the soil erosion rate in relation to the tests in the literature, via the Fell classification (which ranges from « Extremely fast » to « Extremely slow ») ;
  • the parameter tc which allows us to determine the hydraulic conditions that lead to the initiation of erosion ;
  • the index Ie which allows us to assess the rate at which a newly initiated erosion hole is likely to degenerate into a breach.


This test can be carried out on undisturbed soil, provided that the soil to be tested does not contain particles larger than 5 mm, and provided that it is supplied in a core sample with a diameter greater than 8 cm and a height greater than 30 cm. It can also be carried out on disturbed soil, provided that at least 2.5 kg of soil is supplied (and preferably 5 kg to allow for retesting in case of problems).


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Jet Erosion Test (JET)
During overtopping, water flows over the downstream face of the structure, most often forming a « staircase » composed of a series of mini-waterfalls. Surface erosion of the structure is observed and can lead to breaching.

To represent this process, the JET, derived from the American standard ASTM D5852, was developed. It consists of impacting a soil sample brought to the laboratory, or the soil of the structure in situ, with a jet of water of standardized dimensions. Analyzing the evolution of the depth of the resulting scour allows the soil's resistance to external erosion to be determined.
The JET test will soon be standardized according to AFNOR XP P 94-134 (to which geophyConsult contributed as a pioneer in the test's implementation). It allows for the determination of:

  • soil positioning relative to tests in the literature, using the Hanson classification (ranging from « Highly erodible » to « Highly resistant ») ;
  • the parameter tc which allows us to determine the hydraulic conditions that lead to the initiation of erosion ;
  • the parameter kd which allows us to estimate theopening kinetics of a breach in the structure.


This test allows for the characterization of soil erodibility more generally, without necessarily considering the overtopping process (for example, to compare embankments considered for reinforcement or construction projects). The results are then often used more qualitatively than quantitatively.

It can be performed with 3 jet diameters : Ø = 6 mm (for samples containing no particles larger than 6 mm), Ø = 12 mm (for samples containing no particles larger than 12 mm), and Ø = 20 mm (for samples containing no particles larger than 20 mm).

This test can be performed on undisturbed soil, provided that the soil to be tested does not contain particles larger than the jet diameter, and provided that, for Ø = 6 mm (the most common diameter), it is supplied in a core sample with a diameter greater than 8 cm and a height greater than 30 cm. It can also be performed on disturbed soil, provided that, for Ø = 6 mm, at least 2.5 kg of soil is supplied (and preferably 5 kg to allow for retesting in case of problems).

This test can be carried out in situ, provided that the soil to be tested does not contain gravel larger than the diameter of the jet.


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Contact Erosion Test (CET)
When a layer of « fine soil » (such as a silt or clay core) is in contact with a layer of « coarse soil » (fondation alluviale en graviers…) subject to groundwater flow, « contact erosion » can dévelop. The progression of this erosion depends on i) the difference in particle size between the two layers and ii) the intensity of the flow.

The CET was developed to reproduce this process in the laboratory. The two soil layers are placed in a cell and then subjected to i) a vertical mechanical stress replicating the in-situ stresses and ii) a flow at a controlled rate, increased in successive stages. Analyzing the amount of fine soil eroded and transported out of the sample during the test allows for the evaluation of the erosion resistance of this fine soil/coarse soil pair.
The CET test, which is not standardized, allows us to determine:

  • whether the difference between particle sizes in each layer is small enough to rule out « contact erosion »,
  • the minimum Darcy velocity in the coarse soil layer (as well as the associated hydraulic gradient) at which contact erosion is likely to develop,

  • an estimate of the permeability k (expressed in m/s) of the tested coarse soil.


This test can only be carried out on undisturbed soil, and requires providing at least 10 kg of fine soil and 7 kg of coarse soil (and preferably double that amount so that it can be repeated if there is a problem).


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Crumb Test (CT)
Dispersive soils have the characteristic of dispersing into suspension (deflocculating) when immersed in still water. They are therefore particularly susceptible to erosion, since even without flow or a deflocculating agent, they erode through disintegration.

The CT (ASTM D6572 standard) is a simple and quick test for qualitatively assessing a soil's dispersivity. The test involves immersing a 15 mm cube of soil (undisturbed or disturbed) in a large volume of distilled water and then observing the suspension that forms after 2 min, 1 h and 6 h of immersion.

Depending on the degree of suspension, the soil is classified into one of the following 4 categories  : « Non dispersive », « Intermediate », « Dispersive » ou « Highly dispersive ».
Custom-made test
When conventional tests (HET, JET, CET, etc.) cannot address the issues under investigation, geophyConsult is generally able to offer specific, customized tests, most often derived from its existing equipment and protocols.

This type of situation arises, for example, when a specific geometry/configuration needs to be investigated, when available samples are not compatible with standard test benches, or when precise parameters need to be tested (long timescales, specific hydraulic loads, etc.).
References
geophyConsult has billed for approximately 1,500 commercial erosion tests, primarily HETs and JETs (the remainder being mainly a few dozen CETs, a few dozen CTs, and about ten custom tests). geophyConsult has also conducted several hundred erosion tests for research purposes.
Erosion risk assessments and erosion tests leafleft in the pdf format
Erosion risk assessments and erosion tests leafleft
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