Soil mechanics – Discover Dredging

Internship Prasanna Ramadurai: CFD Modelling Clay as a Fluid

Prasanna presenting his work at Damen Dredging Equipment

Prasanna Ramadurai has been doing an internship with us at Damen Dredging Equipment for my PhD project on the cutting of clay1. As an old fashioned analytical and experimental dinosaur, I have been working in my comfort zone. However, when submitting articles the response from the reviewers has been: ‘How about validating your results with a numerical simulation?’ And that is exactly what Prasanna has been doing for me these months. Now he has presented his work and I can use the results in my own research.

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

Clay is a strange material. It is neither a solid, nor a fluid. Depending on the amount of water in the material, it can behave like concrete or like water. The scale on which this can be described is defined by the Atterberg limits2. Well known soil parameters as Plasticity Index and Consistency Index are derived from those Atterberg limits. Atterberg himself defined the following limits:

ID	Limit name	Criteria
1	Upper liquid limit	Starts to show signs of a viscous fluid
2	Lower liquid limit	Normal Casagrande test or Fall cone test
3	Adhesion limit	When no clay sticks to a nickel spatula
4	Upper plastic limit	Can be moulded
5	Lower plastic limit	Normal rolling test for plastic limit*
6	Cohesion limit	When pieces of clay do not stick to each other anymore
7	Shrink limit	Normal shrink limit, constant volume for water content
*Atterberg proposed to roll on a paper surface, whereas ISO 17892 proposes a glass plate

The consistency limits as originally proposed by Atterberg

The resistance of a material to deformation can be expressed as the resulting stress due to a strain rate. When there is immediate stress for even the slightest movement and constant after reaching a yield stress, this is typical of a solid. On the other hand, when a material starts to move immediately and the resistance to deformation increases with the strain rate, it is a fluid. And clay is just the typical material that exhibits both phenomena.

Shear stress models depending on strain rate

In rheology, the factor which shear stress is related to the increased strain rate is called viscosity3. However, due to the internal friction in clay, the stress follows the vertical axis and consequently, the viscosity becomes infinite. Prasanna squeezed out the capabilities of the CFD program using some clever mathematical tricks of a Herschel-Bulkley fluid model to get the simulation to behave. And the results are promising enough to follow up in a separate study.

Compare CFD simulation and PIV experiments

For supervising Prasanna, I am very grateful for the assistance of Suman Sapkota for his knowledge of computational fluid dynamics. Together with my knowledge of clay, Prasanna gained a very special set of skills in this area. Prasanna will be back at the TU Delft to continue his master’s graduation project. I can recommend him for having him in your team.

References

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.

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)

Category: Soil mechanics

Internship Prasanna Ramadurai: CFD Modelling Clay as a Fluid

References

See also

WODCON 2025: Rolling Out A New Clay Test

References

See also

ISFOG 2025: Commissioning The Test Rig And Reporting To Academia

References

See also