All the clods undergoing the action of water do not react in the same way. Their strength depends, for example, on the materials they are made of.

We developed the procedure described in the publication of Y. Le Bissonnais(1996). Aggregate stability and assessment of soil crustability and erodibility: theory and methodology. European Journal of Soil Science, 47, 425-437. This method, a French standard (Afnor) since 2005 (NF X31-515), is the international standard (ISO) since 2012 (ISO 10930: 2012  - « Soil quality -- Measurement of the stability of soil aggregates subjected to the action of water »).

The procedure is made of three tests carried out on aggregates calibrated between 3 and 5 mm.

The goal of these three tests is to allow for the prediction of the soil structure behavior for various conditions of soil wetting by water. These conditions simulate the climatic, hydraulic and mechanic conditions that can be found in the field.

Here are our advices and additional information regarding the ISO standard.

1. Sampling
2. Preparation
3. Tests
   1. Fast wetting test
   2. Slow wetting test
   3. Shaking test
4. Granulometric separation of fragments
   1. Sieving in ethanol
   2. Sieving after drying
5. Results – Calculation of the MWD

                    1 - Sampling:

• Location: sample at several points (about five) over a homogeneous surface of about 10 m² so to have a composite sample. The surface should not have undergone stresses (trampling, organic residues…). Sample away from the field limits.
• Sample about 1 kg of soil, between 0 and 10 cm, in conditions of moderate humidity. Use a rigid plastic box as a container. Depending of the type of field work and on the goal of the study, the thickness can be modified. When the soil surface is crusted, or for non-cultivated soils, sample below the crust, the litter or the root mat, between 2 and 10 cm.
• The amount of sampled soil should be adjusted depending of the yield estimate in 3-to-5-mm aggregates (yield = ratio between the weights of 3-to-5-mm aggregates and the total sample weight). Hence, if the soil tends to breakdown in small aggregates, the sampled amount should be increased.

                    2 - Preparation:

 Set the sample to air-dry in a room at about 20°C.

• During the drying, aggregates should be broken down by hand. For this breaking down (« crumbling »), there is a range of optimal humidity. If the breakdown tends to smooth the aggregates, it means the sample is still too humid. If the aggregates are difficult to break down, it means the sample is too dry already.
• During the preparation, all the sample should be passed through a 5-mm sieve. The resulting aggregates should be sieved at 3-mm. This sieving should be gentle but efficient. The 3-5 mm fraction is used for the stability tests.
• At the end of the preparation, the yield in 3-to-5-mm aggregates is calculated (yield = ratio between the weights of 3-to-5-mm aggregates and the total sample weight).
• Before the tests, the aggregates are put in an oven at 40°C for 24h to limit the possible variations in humidity and to make more uniform the test conditions.

                    3 - Tests:

                    3.1 Fast wetting test

Remarks:

• Total duration of the immersion of the aggregates into water: 10 min.
• During the pipetting of water, do not pipet the fragments.
• During the transfer to the sieve, be careful not to head the flow of ethanol directly onto the fragments.

Fast wetting test: click on the picture to play the video

                    3.2 Slow wetting test

Remarks:

• Total duration of the immersion of the aggregates into water: 60 min.
• Pay attention to fully saturate the synthetic foam and to keep the water level at about 5 mm from the top of the foam.
• Be careful to fully saturate the filter paper when putting it on the foam and flip it over several times.
• A filter paper can be used several times. It must be carefully rinsed with water before to be re-used.
• A soft and dry brush is used to spread (quickly) the aggregates on the filter paper to avoid their overlapping. Do not move aggregates that start to be saturated.

Slow wetting test: click on the picture to play the video

                    3.3 Shaking test

Remarks:

• Total duration of the immersion of the aggregates into ethanol: 30 min.
• Be careful to apply the proper amplitude and rhythm to the movements, and the correct number of reversals.
• Duration of settling: 30 min. minimum.

Shaking test: click on the picture to play the video

Shaking stage only : click on the picture to play the video

We recommend that you train yourself at doing the proper moves while watching this video

                    4 – Granulometric separation of fragments:

                    4.1 Sieving in ethanol

This sieving aims at separating the fragments larger than 50 µm from the fragments smaller than 50 µm without causing any additional disaggregation.

Remarks:

• The level of ethanol should be about 5 mm above the sieve mesh.
• Be careful not to trap air bubbles under the sieve mesh.
• Do 5 rotating clockwise moves; raise the sieve; let the ethanol flow; immerse the sieve; do 5 rotating anti-clockwise moves

Sieving in ethanol: click on the picture to play the video

                    4.2 Sieving after drying

After drying, the fragments larger than 50 µm are sieved.

Remarks:

• A soft brush makes easier the transfer of the aggregates onto the sieve.
• A hard brush allows retrieving the fragments blocked in the sieve mesh.
• The sieves should be shaken moderately, to avoid an additional disaggregation.
• During the shaking, be careful to spread the fragments over the whole surface of the mesh.
• The shaking duration is proportional to the amount of fragments to be sieved.

Sieving after drying: click on the picture to play the video

                    5– Results – Calculation of the MWD:

For each test, the result is represented as histograms showing the fragment distribution, and also as an index, the mean weighted diameter (MWD).

Remark about the calculation of the MWD for soils with gravels

Coarse elements (gravels) in the 2-5 mm fraction can affect the results. Hence:

• If the percentage of coarse elements is between 10 and 40 %, the coarse elements should be accounted for by washing the fraction larger than 2 mm resulting from the tests. One MWD is then calculated with the coarse elements, and another one is calculated without the coarse elements.
• If the percentage of coarse elements is larger than 40 %, it is likely that the tests are not relevant.