Cutting – Discover Dredging

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

References

CEDA Dredging Days 2024: My Presentation On Clay Cutting

Initial clay cutting tests for my PhD project CHiPS

Next week is the biannual CEDA Dredging Days event1. This time fully focused on presentations, networking and having a good time together with like minded people. Since a long time, it will not be in conjunction with the distractions of the Europort exhibition. Instead, it will be in the impressive WTC Rotterdam. This is an excellent opportunity to share with you the progress of my PhD project on clay cutting at the TU Delft2. As a teaser, I would like to share with you some observations from my literature study, already3. Starting to search for literature via Google results in this:

When estimating the cutting production of a dredge, the objective is to find the specific cutting energy for that dredge in combination with the soil properties and correctly chosen operational settings. The specific cutting energy is the amount of power needed to excavate a volume of soil from the bottom5. The funny thing is, when you work out the dimensions of the specific cutting energy, the unit is similar to a stress or pressure. So, there should be a direct relation between the specific cutting energy and a soil property. But which one? For cohesive soils as clay, there are: shear strength, cohesion, adhesion, tensile strength, yield strength. The VOUB course6 recommends to use an empirically derived relation between the cohesion and the deformation rate (which in turn is based on the operating settings) for the specific cutting energy.

Specific Cutting Energy Empirical (Bart van der Schrieck, 1996)

In contrast to this empirical model, one could also start at the displacements of the clay particles and model the implications for the larger continuum mathematically. This has been investigated by Sape Miedema, who has published countless articles and an impressive book on the topic7. Following this through, the estimated specific cutting energy is in the same range as the empirical model. However, on closer inspection, there are some variations on the outer limits of the deformation rate.

Specific Cutting Energy Theoretical (Sape Miedema, 1992)

This discrepancy is probably due applying the model under all conditions. Miedema already recommends to check for the validity of the assumptions in the model. At very thin cutting layers, the resulting chip may form a long curl. When cutting thick layers, the blade will cut out chunks. And those cutting types will have different force equilibriums, resulting in different cutting forces. Miedema suggested a three regime map of cutting types, which coincidently resembles the curves found empirically.

It appears, there are many more cutting types possible for a myriad of soil properties and operating conditions. However, the published results and proposed models are not directly applicable for the dredging industry. Either the conditions or assumptions differ (dry earth works for example) or parameters or data has been failed to included in the publication. This leads to some white spots in the knowledge that I hope to colour in with my future models and upcoming experiments8.

References

Welcome to CEDA’s (revamped) Dredging Days 2024
Personal Announcement: Going Back To School To Cut Some Clay
CEDA Announcement of my presentation, LinkedIn
What was the first (known) maths mistake? Matt Parker
Experiencing The Cutting Edge Of Dredging Technology
VOUB Cursus 1998, Deel X, hoofdstuk 12, VBKO
The Delft Sand, Clay & Rock Cutting Model, Sape Miedema
Mechanical excavation of clayey soils, a review of the physical phenomena occurring, Mark Winkelman et al (CEDA Dredging Days 2024)

Tag: Cutting

ISFOG 2025: Commissioning The Test Rig And Reporting To Academia

References

See also

Graduation Fabian Kruis: Modelling Friction In Clay

References

See also

CEDA Dredging Days 2024: My Presentation On Clay Cutting

References

See also