Soil test – Discover Dredging

WODCON 2025: Rolling Out A New Clay Test

Fully covered cutter head in sticky clay

Here at the WODCON 2025 in San Diego1, the theme is ‘Dredging Towards a More Resilient Future’. One of the challenges we encounter, is that even the resources of good construction sand run out. We either have to repurpose sand already dredged or find and alternative construction material. One such an overlooked material is clay. A lot of effort is put into understanding the behaviour of clay in infrastructure applications. The Dutch Centre for Legislation and Infrastructure (CROW)2 has provided recommendations on the applicability of clay for various types of construction. However, the clay has to be dredged and for the adherence potential of clay, there is another recommendation issued by World Association for Waterborne Transport Infrastructure (PIANC)3. Both do use the Plasticity Index and the Consistency Index as criteria to classify the clay. Interestingly, the clay type that is regarded as suitable for construction by the CROW, is also classified by the PIANC as to have the highest adherence potential and thus gives the most problems in dredging.

Another problem with the criteria by PIANC is that they tend to be unreliable. Something they already acknowledge in the supplied explanation to the diagram. When following the literature that led to the recommendation, it turns out the original application was not dredging but tunnel boring4. Where the problem was not so much the clogging of the cutter shield, but the collection of clay in the suction chamber. And even plotting the data used for this assessment shows a large variability. Apparently there is more to the problem of adherence than just the PI and CI. Which might be obvious when considering the original Atterberg Limits. The PI and CI are related to the plastic range of clay, whereas Atterberg already defined a range where adhesion is more relevant.

Relation of PI and CI to the adhesion range according to Atterberg

A situation similar to the cutter clogging is the covering of a drill bit in the oil industry. There, they encounter a phenomenon called ‘Bit Balling’5. It is extremely difficult to assess the bit balling potential from a physical model related to the soil parameters alone. As a solution they developed the ‘Rolling Bar Test’6. A defined amount of clay sample is put into a cylinder with the needed amount of water. Finally a rod is inserted in the sample cylinder. The whole contraption is placed onto a roller set and turned for a set of times. Each time the amount of clay sticking to the rod is measured and plotted in a graph. Eventually, most clay types will loosen their grip on the rod. But some are sticking to the rod indefinitely. Those are the clays that are also likely to show bit balling in the actual process.

Bit balling and procedure of a rolling bar test (data: Mettah, 2011)

As we know that we can’t fight the adhesion of clay, we may as well improvise, adapt and overcome the problem. Since already my graduation, I am working with clay. In that case, it was an auger, that needs the adhesion to the back shield to propagate the clay in the auger. When we were asked by a contractor to improvise a tool that could tackle this sticky clay, we developed a disc bottom cutter head that used the adhesion to move the clay over the blade to a scoop behind the blades. This worked so smoothly, that the satisfied customer bought a second. Eventually he finished to job in time and in budget7.

DOP pumps with special clay cutter head at the ‘Markthallen’ project in Rotterdam

References

Graduation Fabian Kruis: Modelling Friction In Clay

Fabian Kruis presenting his graduation research

Fabian Kruis graduated on his master thesis at the Delft University of Technology on a project for my PhD research1. He investigated the cutting behaviour of plastic clay. As it was the first time we are now actually using the test rig designed by Ines2, he first had to do was a lot of trouble shooting for commissioning the test rig. Spoiler alert: the cutting forces were much higher than expected and the linear drive was not strong enough to cover the whole range of experiments we’ve wanted to do.

The cutting forces involved with cutting of clay are acting on all four sides of the simplified chip. On the outside, there is the barometric pressure of the surrounding water. On the far end, there is an unknown and hard to determine force from the rest of the chip that is not in contact with the blade anymore. At the shear plane, there are the normal force, the internal friction and the cohesion. At the blade, there are the normal force, the external friction force and the adhesion. The sum of these last three forces will give the cutting force we are looking for, as they make up the required cutting power on the drive. But they can only be calculated, once the other forces are known.

Overview of all the forces involved with the cutting of clay, acting on the chip

Fabian’s assignment was to have our own experience with the cutting of clay and check whether the models used in the dredging industry have any reliability in predicting the cutting forces. checking whether all assumptions and simplifications were justified. e.g. Plastic clay does have similar properties and behaviour as a fluid. And a fluid does not have an internal friction. Consequently, clay should not have an internal friction also. Right? When there is no internal friction, there can’t be an external friction either. Right, right? Fabian tested these assumptions by actually performing shear tests on internal and external planes.3

Explanation of internal friction for solids, fluids and clay

At least for the clay we used in this research, he already found that the assumption for ‘no friction in clay’ is not valid. Consequently, this had knock on effects on the rest of the cutting force calculation. We did find a different behaviour, the shear plane was off and the cutting forces were indeed much higher than expected. It is now up to me to use Fabians results and model modifications to implement into my own research. As a matter of fact, I used part of his thesis to write an article and hope to present this soon. I’ll keep you update on those developments.
As we are very satisfied with Fabian’s work and him as a person himself, we offered him a position in our team at Damen Dredging Equipment in Nijkerk, which he happily accepted. So, next to progress for my research, we have a new colleague. Welcome Fabian, thank you!

Fabian signing his MSc. certificate. — Fabian signing his MSc. certificate sitting in the ‘dredging chair

References

Graduation of Ines Ben M’hamed: The Strength of Clay in a Test Rig

Ines Ben M’hamed defending her graduation thesis

Last week, Ines Ben M’hamed graduated with good grades on her bachelor thesis. She did a project with us at the Research Department of Damen Dredging Equipment in Nijkerk. The topic was to investigate the strengthening of clay when it is subjected to shear. This deformation is a common phenomenon when cutting clay and as such a contribution to my own PhD project1 and consequently improving our products for these applications. A common problem with clay is clogging up the cutter head, but it is also not completely understood why the clay is behaving as it does and how much power is involved for the various regimes.

The effects of deformation on the behaviour of clay are much more pronounced than e.g. sand or rock. Rock does not deform, it just breaks. Sand deforms, but as it basically only involves hydraulic and mechanical forces, it is much better understood. Clay particles have wider range of interactions. Next to the hydraulic and mechanical forces, they may experience: adhesion and cohesion, molecular forces, electrostatic charges and chemical bonding in the higher temperature ranges. The general effect is that as the particles in the original situation may have a weak structure, the external disturbance causes the particles to get jostled around and all the mentioned interaction get a chance to hook on to each other.

Shear strengthening due to organising particles

The result is, that the particles get oriented and therewith a better opportunity to bond. The effect is a strengthening of the shear stress. As this strengthening is dependent on the strain rate, it is this strain rate, that is of interest for the prediction of the cutting forces. There are many publications available on what the consequences are of the strain rate on the Specific Cutting Energy. A well known model is by Sape Miedema2.

Strain Rate Effect on the Specific Cutting Energy (Credit: SA Miedema)

The trick with this model is, it depends on this strain rate effect. The sole experimental data available is by Hatamura and Chijiwa3 in 1975. They tested one type of clay on the three governing parameters: static shear strength, dynamic shear strength and the strain rate. There hasn’t been hardly any further experimental investigation into this problem. And as we regularly receive samples and soil reports that we can not test on these properties, it is also hard to predict the performance of our cutter heads. So, we decided to build our own cutting test rig.

This cutting test rig resembles the specifications to the original test rig of Hatamura. This will allow us to verify the parameters in the model ourselves. We also prepared the design with various option to enable us to allow assessment of clay samples that we receive from clients and service engineers. We hope to provide our customers with additional service in this problem. Currently, the parts of the test rig arrived very late and Ines was not able to include the build in her project. Respect for the good grade she received for her thesis. However, the parts are there and provide and excellent opportunity for the next graduation student to do their project with our company. Who dares?

Available parts for the cutting test rig

Tag: Soil test

WODCON 2025: Rolling Out A New Clay Test

References

See also

Graduation Fabian Kruis: Modelling Friction In Clay

References

See also

Graduation of Ines Ben M’hamed: The Strength of Clay in a Test Rig

References

See also