Clay – 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

ISFOG 2025: Commissioning The Test Rig And Reporting To Academia

Fully covered cutter head in sticky clayThis week, I will be presenting my paper1 about the initial experiments on the test rig at the 5th International Symposium on Frontiers in Offshore Geotechnics (ISFOG 2025)2. I will be there in the breaks to explain my poster3 in the lunch breaks. For my audience not present at the symposium, I can highlight the most interesting parts here. I presume, most of you are aware of the operation of a Cutter Suction Dredge and also know about its problems when working in clay. The clay will adhere to the teeth and arms and clog the cutter head. This leads to interruption of the project and in consequence: time and cost overruns. Also, the production itself is difficult to calculate. This is why we at Damen Dredging Equipment started the CHiPS project with the TU Delft4 to investigate the process, improve the estimation model and optimise the design of the cutter head for operation in clay.

Forces involved in the cutting of clay

Forces involved in the cutting of clayFor this purpose, we constructed a linear cutting test rig. Last post about the graduation of Fabian Kruis has more on the results of his thesis5. In the ISFOG article, we wrote about the design and performance of the rig and the opportunities it provides for further research. The design criteria for the rig as was laid down in the assignment for Ines Ben M’hamed were6:

Identifying the main parameters influencing the cutting forces and the cutting regime.
Designing general arrangement for testing linear cutting models.
Capture the signals for force and deformation.

The developed test rig was inspired by the model described by Hatamura and Chijiiwa7. The blade is attached to a linear moving trolley, cutting through a block of clay mounted in a frictionless moving soil bin. The reaction forces on the box are measured. and images of the grid printed on the side of the clay block are captured with a GoPro camera of later evaluation with PIVlab®. A set of 30 experiments was defined according to the Buckingham-PI method as presented at the CEDA Dredging Days last year8.

General arrangement of the linear cutting test rig

Next to the cohesion and adhesion, the tensile strength of the clay had to be measured to obtain a consistent result. We could confirm the linear relation between cutting depth and the cutting force as predicted by existing models from literature. As we were using modern techniques for capturing images, we were able to accurately measure the displacements with the PIVlab® application. The good results are due to the novel printing technique developed by Fabian Kruis, to apply a grid on the side of the clay sample. One remarkable result is, that most models for the calculation of the sliding forces, only take adhesion into account, but measurements indicate that the external friction cannot be neglected. This appeared in the measured shear angle, which was much lower than the shear angle predicted by existing models.

Captured deformations in a vector field. Note the differences in shear angle

The experiments yielded a treasure trove of measurements, we are still analysing them. e.g. We noticed some strange reversal of the vertical cutting forces. And we are interested in the transition from one cutting regime to another. Those results will be presented in my next journal paper. In the mean time I am watching all those captured movies over and over again. To me it’s very inspiring and I like to share an example.

A slow motion movie of a clay cutting experiment (ASMR)

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

Tag: Clay

WODCON 2025: Rolling Out A New Clay Test

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See also

ISFOG 2025: Commissioning The Test Rig And Reporting To Academia

References

See also

Graduation Fabian Kruis: Modelling Friction In Clay

References

See also