Graduation Fabian Kruis: Modelling Friction In Clay

Fabian Kruis presenting his graduation research
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.

Clay cutting test rig at DDE in Nijkerk
Clay cutting test rig at DDE in Nijkerk

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
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
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

  1. Personal Announcement: Going Back To School To Cut Some Clay, Discover Dredging
  2. Graduation of Ines Ben M’hamed: The Strength of Clay in a Test Rig, Discover Dredging
  3. Direct shear test, Wikipedia

See also

CEDA Industry News: Snap Back Story out of the Old Stone Box

Cutter wheel dredge ‘Sylvia’ at work on the TIWAG Langkampfen power dam reservoir
Cutter wheel dredge ‘Sylvia’ at work on the TIWAG Langkampfen power dam reservoir

You might have read the CEDA Industry News1 publication of September and the LinkedIn announcement2. Or, have been present at Session 10, The Story of Safety3 of the CEDA Dredging Days 2024, where I initially recounted my story of misfortune. For the uninitiated, I’ll shortly recount it over here, although the focus will be more on the design problem back than.

Me telling this story at the Safety Session of the CEDA Dredging Days 2024 (Credit: CEDA)
Me telling this story at the Safety Session of the CEDA Dredging Days 2024 (Credit: CEDA)

When I was fresh out of university, working for my previous employer, I was assigned to advise during the commissioning of a bucket wheel dredge in Austria. Here at my website, I’ve already mentioned a very peculiar problem we had with starting the dredge on Monday morning4. The whole dredge was a funny contraption, trying to fulfil the specific requirements set by the customer. The task of the dredge was to clean out the sediment caught in a silt trap5 in front of a power dam to prevent the material flushing through the delicate turbines. Furthermore, the fine sediment should be removed from the system to be sold for beneficial use, but the larger boulders that came along should be rejected from the system. Unless they were very large and stay in place for forming the liner of the silt trap.

General modes of siltation at the usual location in a reservoir

The proposed method was to use a bucket wheel to gobble up the larger stones, but leave the even bigger ones down at the pit floor. Additional advantage of the bucket wheel is that they tend to have a very low resuspension rate which is beneficial for reducing the turbidity in the river and save the dam turbines. However, the larger stones taken by the bucket wheel will not pass the dredge pump. A proven solution to filter out stones for the dredge pump is a so called stone collection box. They come in various executions, but this one was a large cylindrical vessel with a cover on top. Inside was a cage that connected to the inlet and outlet of the vessel. When the cage was filled, the lid was lifted by the crane and the attached cage came out. The cage was swung over to a barge alongside and the stones were unloaded through a trap door. Sounds straightforward, right?

General arrangement of the ladder and suction system on bucket wheel dredge ‘Sylvia’
General arrangement of the ladder and suction system on bucket wheel dredge ‘Sylvia’

The thing is, when you have anything in the suction pipe section, it will create a pressure loss and the pressure in front of the dredge pump becomes lower. Or higher vacuum, as you wish. This is already a problem for an inboard pump when you are working at sea level, even more so when working at altitude. The NPSHr is easily reached. In order to have enough NPSHa, even when the stone collecting box was filling up, it was decided to have a submerged dredge pump for increasing the pre-pressure. Consequently, the stone collecting box would also be placed on the ladder, in front of the dredge pump.

Explanation on NPSHr and NPSHa in mountains and submerged, including stone box
Explanation on NPSHr and NPSHa in mountains and submerged, including stone box

All in itself no problem. The disadvantage is that a submerged stone collecting box should be very strong, big and heavy to withstand the under pressure, although there is very little room around the ladder, the ladder itself becomes quite heavy and bulky and the submerged frontal area increased dramatically, increasing the total drag of the dredge in the fast flowing river current. To mitigate this, the ladder winch, the service crane and the 25 ton forward wire winch were all stacked over each other, so each could do it’s job independently. Although none of us ever did a collision check in the design phase of the dredge. In actual operation, they all were in each others way. When lifting the cover of the stone collecting box, it got caught by the forward wire. When lifting it further, it slipped of the side by the strong drag forces on the wire. I was to first to experience our negligence and got hit on my head when I was to eager to inspect the contents of the box. I blacked out, fell in the frigid water in the ladder well and woke up washed up on the ladder. My colleagues rushed me to the hospital, where I ended up between all sorts of winter sports injuries. Fortunately for me, I am able to recount my story and share it here and through the CEDA Industry News.

Annotated overview of the accident site where the wire slipped from the cover onto my head.
Annotated overview of the accident site where the wire slipped from the cover onto my head.

References

  1. Dredging safety under the spotlight, CEDA
  2. Safety is paramount, CEDA
  3. Welcome to CEDA’s (revamped) Dredging Days 2024
  4. The Dredge That Refused to Work on Monday Morning
  5. A Reservoir of Dredging Opportunities

See also

CEDA Dredging Days 2024: My Presentation On Clay Cutting

Initial clay cutting tests for my PhD project CHiPS
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:

Literature on clay (Kushim, 3400BC)4
Literature on clay (Kushim, 3400BC)4

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)
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)
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.

Cutting types (Sape Miedema, 1992)
Cutting types (Sape Miedema, 1992)

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.

Clay cutting test rig at DDE in Nijkerk
Clay cutting test rig at DDE in Nijkerk

References

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

See also