Soil mechanics – Discover Dredging

Working Group ‘Sand’ Visits Damen: Perspectives On Sand From Micro to Macro Scale

Working Group ‘Sand’ visiting the Dredging Experience at Damen Dredging Equipment

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.

Effective grain density due to adherent fluid

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.

Measuring the effective volume of sand grains

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.

Effect of apparent density on dredge performance

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.

Picture of various sand grains in an interesting mixed sample

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.

Fully covered cutter head in sticky clay

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

References

Personal Announcement: Going Back To School To Cut Some Clay

Learning early or later in life, studying is always a joy when you make it practical

‘Never too old to learn’ is my motto. Everyday I look around me and I wonder how this beautiful world fits together. Whether it be the stars in the sky, the waves at sea or life as we know it, there is always something to be learned about it. At school, I was not a great pupil, but I was always curious to learn more. For my master thesis at the Delft University of Technology, I investigated the performance of a dredge and made recommendations to improve its operation1. As the project was more focussed on mixture forming (and turbidity) and the redesign of the auger head, there was no attention for the soil mechanics involved in the cutting process.

Now is the time to get that straight. In my daily business, I came across several projects where the clay cutting was a real problem. This was one of the triggers that sparked my interest in sticky clay and made me pursue a more detailed investigation into this nasty stuff. I am very grateful my management was willing to grant me time to go back to the university and start a PhD project with professor Cees van Rhee to learn more about clay.

Synthesis of clay and the relevant properties for dredging

Clay is a completely different material than sand or rock. Those are either plastic and non-cohesive or elastic and cohesive. Clay is the worst of both worlds: plastic and cohesive. It can be described with certain soil parameters as e.g. undrained shear strength and internal friction angle. The failure model is based on Mohr’s circle etc. But those are all continuum approaches2. When you zoom in to the particle level of clay, a whole new world opens up. I already wrote about the interesting particle interaction in a previous post3.

Boltzmann strain rate function in clay cutting

It appears, that the consistency, deformation and failure of clay is related to the tiny electric charges distributed over the platelet crystals. The movement along the charges needs energy. The model to describe dislocation energies along electric charges has been studied by Ludwig Boltzmann4,5. His model governs a wide range of applications, ranging from cosmology to particle physics. I really plunged into the deep end of science with just simple clay. It already took some time to get my head around the concepts involved. Slowly it dawns on my what possibilities there are to improve our understanding of the cutting of clay and possibly to improve our products eventually.

Gallery of my dredging professors (l) prof. de Koning, (m) prof. Vlasblom, (r) prof. van Rhee

My ‘old professor’ de Koning was a proponent of ‘thinking with your hands’6. Professor Vlasbom encouraged me to graduate on a practical problem and also my current professor van Rhee suggested to do some preliminary experiments with sticky stuff to get some feeling about what I am going to study. Of course I took some clay home to play with it. But the best suggestion was by my colleagues, who thoughtfully gave me stroopwafels7. The ultimate representation of sticky non-Newtonian stuff between layers of latticed disks.

Fresh supply of stroopwafels for practice and celebration

Category: Soil mechanics

Working Group ‘Sand’ Visits Damen: Perspectives On Sand From Micro to Macro Scale

References

See also

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

References

See also

Personal Announcement: Going Back To School To Cut Some Clay

References

See also