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

Which Teeth Will Survive The Cut? Adapting Your Selection

Me, explaining about our cutter systems
Me, explaining about our cutter systems

After my last post1, I received some comments and questions about the actual products we are applying in our cutter systems for our CSD’s2. Indeed, from a pure physical perspective, last post cuts to the heart of the processes, but does not explain our design of the working tool that makes a cutter suction dredge do its work.

Over the years, there has been a lot of development in this tool. Originally, suction dredges were plain suction dredges, working in non-cohesive sand. When the soil was more cohesive than could be dug with the standard suction dredges, attaching a mechanical device for loosening the ground enabled the suction dredge to work in this environment. From this original concept, the cutter head was already recognisable as a crown with teeth on a back ring and a suction mouth in the centre. From there, a lot of experimentation was done, but ultimately it all came back to this concept. Although modern cutter heads have a vastly improved performance and lifetime.

The cutting process in a modern cutter head is a combination of the rotation of the head and the swing of the dredge. The teeth describe a compound path of translation and rotation and each individual tooth has its own set of cutting parameters for depth and angle varying over time. Moreover, the combination of teeth on the different arms, allow for a staggering of the teeth that each tooth cuts fresh material and optimising the use of the teeth and spreading the wear. This results in a complicated geometry of the arms and a intricate pattern of the teeth.

Teeth system with adapters (left) teeth system direct on arm (right)
Teeth system with adapters (left) teeth system direct on arm (right)

Once a cutter design has been chosen, there is still some tuning possible. Normally, the teeth are fitted on adapters and there are several teeth types available for a certain adapter. Pick points, Chisels and Flares. Most productivity can be expected from the wider teeth. However, the penetration of the wide teeth is less. So, for harder material you want to select narrower teeth.

Adapter system (left), teeth range with adapter (top), teeth range direct on arm (bottom)
Adapter system (left), teeth range with adapter (top), teeth range direct on arm (bottom)

Wear is also an issue3. And as the teeth are in direct contact with the fresh material, the wear rates can be severe. The disadvantage of a teeth and adapter system is that that are quite big. So, less teeth fit on an arm, reducing production on average. As most of our CSD’s are working in more gentle sands we selected a cutter system, that provides more teeth to engage in the action, increasing production. As these teeth are fitted directly on the arms, there are no adapters that wear also. Consequently having no adapters, simpler arms and dirt cheap teeth result in a low investment low OPEX cutter system. Although you might have to check the state of your teeth more often, in the end you spend less money on a cubic meter produced.

Teeth in various stages of degradation
Teeth in various stages of degradation

Teeth can be worn down to the root. Also they are not wearing evenly. Usually, they last longer on the outside, near the back ring. You might consider using different tooth forms over the arm. Experience and practice, will guide you in selecting the best combination. In line with the previous post, the analogy will be on the table. Just as you select different teeth for your fork, you can select different teeth on your cutter depending on the dish being served.

Different teeth selection for tableware
Different teeth selection for tableware

References

  1. Experiencing The Cutting Edge Of Dredging Technology, Discover Dredging
  2. Cutter Suction Dredger, Damen
  3. Wear of Rock Cutting Tools, Peter Verhoef

See also