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

Ines Ben M’hamed defending her graduation thesis
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
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)
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.

Design of Ines’ cutting test rig
Design of Ines’ 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
Available parts for the cutting test rig

References

  1. Personal Announcement: Going Back To School To Cut Some Clay, Discover Dredging
  2. The Delft Sand Clay & Rock Cutting Model, SA Miedema
  3. Analysis Of the Mechanism of Soil : 1st Report. Cutting Patterns of Soils, Hatamura & Chijiwa

See also

 

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
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.

Fully covered cutter head in sticky clay

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
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
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
Fresh supply of stroopwafels for practice and celebration

References

  1. Presenting Pump Power Peculiarities, Playing With Pumps And Pipes, Discover Dredging
  2. The Cutting of Sand, Clay and Rock – Soil Mechanics (6041), TU Delft
  3. The Origin of Clay, When Dredging Becomes Sticky, Discover Dredging
  4. New Developments Of Cutting Theories With Respect To Dredging The Cutting Of Clay, SA Miedema
  5. Ludwig Boltzmann, Wikipedia
  6. Experience the Dredging Experience
  7. Stroopwafel, Wikipedia

See also

The Origin of Clay, When Dredging Becomes Sticky

Clay forming Fountain Paint Pot, Yellowstone National Park, Wyoming, USA

This mud pot gave me a revelation on the origin of clay. I was aware, that clay is a completely different mineral than sand. For starters, sand is based on silicon dioxide and clay on some complex aluminium compound. Sand is mechanically worn down rock, usually quartz. But I never got around to understand where clay came from. Here, a small sign at the side of the mud pot revealed a complete different mechanism: chemical alteration of rock by hydrothermal action.

Sign at the Fountain Paint Pot, Yellowstone National Park, USA

Now, it became clear to me, how all the funny properties of clay arise from this generating process. Unlike weathered sand, clay grains are nice symmetric hexagonal crystals. And these crystals grow under changing conditions for temperature, chemistry and pressure. Exactly the environment in these mud pots. The sulphuric acid leaches the chemicals from the rock matrix, in Yellowstone usually Feldspar, the water bubbles to higher levels, transporting and mixing the ions and cooling down along the way. Just like salt crystallises in brine, the clay shakes out like tiny particles, about 2µm. These flakes coalesce into a new sediment: clay1.

Hexagonal sheets of a clay mineral (kaolinite) (SEM image, ×1340 magnification) (Credit: Wikipedia)

The specific mineral of clay, e.g. kaolinite, is a hydrated oxide. And the hydrate allows the charge of these semi-ions to be moved around. As same charges repel and drive themselves apart, the edges and corners of the little crystal will become negatively charged. Now, there are a bunch of discs that have a preference to stick to each other like masonry. Between the discs, there is not much space making the water content low. But, one can add water and the sediment will swell, but there will still be contact between the ends and centres of the disc. Even with this spongy structure, there is still some consistency. It behaves like a plastic substance, you can deform it and it will stay like that.

The plasticity of clay can be measured by rolling the clay in a sausage and measure the water content at which it crumbles. That is a lower limit. An upper limit of plasticity has to be determined by testing the effect of shaking a bowl with clay. Both methods are described2 in ASTM D4318. The difference of water content between the lower plastic limit and the higher liquid limit is the plasticity index. The higher the plasticity index, the more difficult it is to cut this material. It is like cutting warm butter, material is moved around, but you are not severing chunks of the bulk.

Synthesis of clay and the relevant properties for dredging

Whenever you hear dredge people boast about difficulties in dredging, usually it involves clay also. The cutting itself, it is very hard to cut the material out of the sediment. When the chunks come loose, the chunks will stick to the cutter head and the will get completely smeared over and no new material can be cut or sucked up. After that, the clay chunks will tumble down the discharge pipeline. Under certain conditions, the chunks will snowball and form bigger balls. Finally, the clay gets at the reclamation area and will cause problems with the drainage. Remember, fines clog the pores between the grains and prevent the flow of drain water. And clay particles are very fine and they glue the bigger grains together.

Knowing the properties of clay, it is obvious, that normal cutting tools for sand dredging, do not work in a clay environment. Based on the special properties of clay, we once developed a special clay tool for a specific project3. And it worked4. It was fun. And it will be another story.

DOP pumps with special clay cutter head at the ‘Markthallen’ project in Rotterdam

References

  1. Metasomatism, Wikipedia
  2. Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils, ASTM D4318
  3. DOP150 creates underground car park, DopDredgePumps.com
  4. Prestigieus project Markthal Rotterdam vraagt om innovatieve oplossingen, Autograaf 42-p.8, MvO Groep

See also