This is an article I didn’t want to write. My professor Cees van Rhee passed away last weekend1. This was devastating news for his family and all those who surrounded him at the TU Delft. Cees was still enjoying life, he died doing his favourite hobby: fishing. Always looking ahead, he was determined to be my promotor for my PhD. project. I know him already so long. When he was doing his PhD. project, I was graduating on my masters in the same lab. It would be fitting to be his last PhD. student. Fate has decided differently. As he was a public figure, you will receive in the news probably some factual biographies about him2. But he was so much a person to me, that I want to highlight some of his thoughts and contemplations for you.
When Cees assumed his position as dredging professor3, he set three goals to work on:
Bring the department of dredging engineering under a single faculty
Improve the quality of research and education
Make the dredging community conscious of designing equipment for performance
He managed to merge the civil engineering branch and the mechanical engineering branch of the dredging engineering department as one of his first feats as new professor. The second goal can be sensed from the enormous increase in publications during his tenure4. And not only in the usual dredging literature, but also in highly regarded scientific journals. A standard he also imposed on my own PhD. project. The final goal is a real dot on the horizon and is therefore harder to achieve. He engaged with the rest of the dredging community through his numerous activities for the CEDA. Bringing his academic perspective to the industry.
In response to the drivers for dredging: population growth, transport demand and sea level rise, he saw the following challenges:
Continuous innovation to stay ahead of the competition
Re-allocation of labour for the manufacturing of equipment
Understanding the dredging processes on a fundamental level
Development of AI for supporting optimal operation
These challenges are still valid and are a sign of his visionary academic approach to dredging. Most of the research are contributing to solve these challenges. As a leader of the department of Dredging Engineering he lead his colleagues into a solid self-supporting team of specialists. I think we are still in a good position to tackle the challenges, but have to be careful to maintain this advantage.
Cees left a legacy of a high academic standard for dredging engineering. Torn away from life and so many projects ongoing, there will be a lot of loose ends to tie. And I am proud, that I can be part of it. When I approached him for my PhD. plans, he suggested to follow certain ideas that have been slumbering around, but were dormant because by now everybody uses finite element applications to study those problems. He was really happy that I came along with my old school analytic competences to pick up that gauntlet. I have some promising preliminary results already. And I am so sorry, he will never be there to enjoy the results that confirm he was onto something good.
You may already know, that I am very interested in this miniscule particle that is the foundation of our business. To learn more about this element, I joint the Working Group ‘Sand’ of the Dutch Association for Geological Activities1. It is a colourful group of enthusiasts that collect, photograph and research sand in all its splendour. During the relaxed Saturday afternoon meetings, the members gradually noticed, I had a slightly different, professional interest in sand. They boldly asked if they could visit our company for their annual excursion. Maybe my presentation, by at least the excellent weather made for a very successful event.
One aspect of the sand grains we wanted to measure was the buoyancy of the particles. This is done by measuring the density of the grains. You have a tube of water with a known volume. You add sand with a known mass. And just as Archimedes2 predicted, the water will rise with the displaced volume of the grains. Dividing the mass by the displaced volume yields the density of the grains. Surprisingly, this method is quite accurate. For a static condition this is perfectly satisfying. However, in most dredging situations, the grains are dynamically jostled around in slurry transport or breaking up sediment at the cutter.
When a solid grain is moving through a fluid, it is usually considered as a perfect sphere. Nothing is perfect in nature and grains do have a range of shapes, that at best are similar to potato’s. A very jagged grain will have lot’s of nooks and crannies filled with the fluid. This fluid is moving with the particle and contributes to the mass and volume of the particle. This adherent fluid is much more reliably assumed to be a sphere. Fluids in a zero gravity situation tends to behave like a sphere. The diameter of the sphere can be taken as the maximum diameter of the grain that can be measured.
Through a microscope, you will only be able to see the lateral area or the cross-section of a grain. Both area and volume have a relation to diameter. So, the measured area is reduced to an equivalent round area with an equivalent diameter and consequently an equivalent volume. The volumes and masses of that equivalent volume of sand and the shell of adherent water will yield an apparent density of the moving particle.
In the end, my objective was to learn through the microscope the effect the shape of the sand had on the performance of our dredges. As seen in a calculation in our production estimating program, the effect can be significant. Certainly an influence we want to know and assist our customer with appropriate advise3. My visits to the meetings of the Working Group ‘Sand’ were a real benefit in understanding sand. But, to my surprise, through the working group I also learned to appreciate the beauty of the all the different sand minerals that can be found.
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.
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.
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?