Category: Dredge process

General dredging process from excavation to discharge

ISFOG 2025: Commissioning The Test Rig And Reporting To Academia

Fully covered cutter head in sticky clay

Fully covered cutter head in sticky clayThis week, I will be presenting my paper1 about the initial experiments on the test rig at the 5th International Symposium on Frontiers in Offshore Geotechnics (ISFOG 2025)2. I will be there in the breaks to explain my poster3 in the lunch breaks. For my audience not present at the symposium, I can highlight the most interesting parts here. I presume, most of you are aware of the operation of a Cutter Suction Dredge and also know about its problems when working in clay. The clay will adhere to the teeth and arms and clog the cutter head. This leads to interruption of the project and in consequence: time and cost overruns. Also, the production itself is difficult to calculate. This is why we at Damen Dredging Equipment started the CHiPS project with the TU Delft4 to investigate the process, improve the estimation model and optimise the design of the cutter head for operation in clay.

Forces involved in the cutting of clay

Forces involved in the cutting of clayFor this purpose, we constructed a linear cutting test rig. Last post about the graduation of Fabian Kruis has more on the results of his thesis5. In the ISFOG article, we wrote about the design and performance of the rig and the opportunities it provides for further research. The design criteria for the rig as was laid down in the assignment for Ines Ben M’hamed were6:

  • Identifying the main parameters influencing the cutting forces and the cutting regime.
  • Designing general arrangement for testing linear cutting models.
  • Capture the signals for force and deformation.

The developed test rig was inspired by the model described by Hatamura and Chijiiwa7. The blade is attached to a linear moving trolley, cutting through a block of clay mounted in a frictionless moving soil bin. The reaction forces on the box are measured. and images of the grid printed on the side of the clay block are captured with a GoPro camera of later evaluation with PIVlab®. A set of 30 experiments was defined according to the Buckingham-PI method as presented at the CEDA Dredging Days last year8.

General arrangement of the linear cutting test rig
General arrangement of the linear cutting test rig

Next to the cohesion and adhesion, the tensile strength of the clay had to be measured to obtain a consistent result. We could confirm the linear relation between cutting depth and the cutting force as predicted by existing models from literature. As we were using modern techniques for capturing images, we were able to accurately measure the displacements with the PIVlab® application. The good results are due to the novel printing technique developed by Fabian Kruis, to apply a grid on the side of the clay sample. One remarkable result is, that most models for the calculation of the sliding forces, only take adhesion into account, but measurements indicate that the external friction cannot be neglected. This appeared in the measured shear angle, which was much lower than the shear angle predicted by existing models.

Captured deformations in a vector field. Note the differences in shear angleCaptured deformations in a vector field. Note the differences in shear angle

The experiments yielded a treasure trove of measurements, we are still analysing them. e.g. We noticed some strange reversal of the vertical cutting forces. And we are interested in the transition from one cutting regime to another. Those results will be presented in my next journal paper. In the mean time I am watching all those captured movies over and over again. To me it’s very inspiring and I like to share an example.

A slow motion movie of a clay cutting experiment (ASMR)

References

  1. Cutting of highly plastic clay: analysis of large rapid deformation processes, Winkelman (paper)
  2. 5th International Symposium on Frontiers in Offshore Geotechnics, ISSMGE
  3. Cutting of highly plastic clay: analysis of large rapid deformation processes, Winkelman (poster)
  4. Personal Announcement: Going Back To School To Cut Some Clay, Discover Dredging
  5. Graduation Fabian Kruis: Modelling Friction In Clay, Discover Dredging
  6. Graduation of Ines Ben M’hamed: The Strength of Clay in a Test Rig, Discover Dredging
  7. Analysis of the mechanism of soil cutting (1st report, Cutting patterns of soils)
  8. CEDA Dredging Days 2024: My Presentation On Clay Cutting, Discover Dredging

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

Dellewal Terschelling: Stunning Example Ancient Building With Nature

Panorama of the port of West Terschelling and constructed mudflat
Panorama of the port of West Terschelling and constructed mudflat

For most of us, the summer holiday is already long gone. So for me also. Still there is an observation I made that I want to share with you. We have been sailing on the Waddenzee1 this summer. Sailing, anchoring, mudflat hiking2, counting seals and other animals. One of the highlights was a visit to the island of Terschelling. A lot has changed since I was there last time with our boat. It must have been at least 30 years ago. At that time, we had to moor against the quay wall wherever one could find a spot. Now, there is a modern marine in the back of the port, specially made for yachts. Although stacking the boats next to each other is still the standard.

Port of West Terschelling and the new marina
Port of West Terschelling and the new marina

The new marina is more or less in the same area where we used to moor. From there you have an excellent view on a mudflat, that has been there been for ages3. When I was young, I couldn’t understand what its purpose was. Only that old fashioned Dutch flat bottomed boats were still allowed to anchor and stand dry. For us kids, it was an excellent place to muck about with our little dinghy and get dirty walking on the mudflat. We just enjoyed it was there. I still don’t know the exact name of the mudflat. I’ve seen it called ‘Dellewal’, although that seems to be just the quay side over there. Other names that I found on charts are ‘Oostelijk Ras’ and just ‘De Plaat’. Enjoying a nice sundowner and contemplating life and dredging in particular, I suddenly saw the purpose of the area: it works like a ‘Spuikom’4. I really don’t know how to translate this in English. So, let me explain what it is intended for.

Explanation on the working principle of a ‘Spuikom’
Explanation on the working principle of a ‘Spuikom’

Basically it is a part of the mudflat that is located at the south of Terschelling. About 85 ha in surface area, it is separated from the Waddenzee by a low dam. Just high enough to the high water level in the neap tides. The dam has an opening at the back of the harbour, near the marina. I Noticed that the water outside the marina was rushing by and the water inside was practically standing still. I figured out that the rushing tide was used to flush the old port. The huge surface area stores a lot of water that has to pass the quay in the port. Effectively increasing the flow velocity there and reducing sedimentation. This certainly helps in maintaining a navigable depth for marine traffic. Moreover, as each tide the area is filled from all around the dam and mostly emptied through the port, there is a resulting nett transport out of the port.

Hjulström diagram for particle transport in streams (Credit: Wikipedia)
Hjulström diagram for particle transport in streams (Credit: Wikipedia)

Voila, that is why the old islanders build those dams! Any other person would be satisfied with this plausible answer. Have pity on me, I can’t stop solving the riddles of the sands. Wouldn’t this be easier with a dredge? Apparently, near Terschelling, there is a sedimentation rate of 0.5 to 1 mm per year5, or about 600 m³ annually. With the dam, this has to be kept out of the port with the volume behind the dam. The average increase in flow is about 0.5 knots. According to the Hjuström diagram6, this will transport particles smaller than 10 mm out of the harbour. The stored volume has a potential energy as in a power dam of about 6.25 GJ. This is released twice each tide, resulting in a delivered power of 280 kW. Combined, this results in a specific transport power consumption of 4000 kW/m³/h. No contractor in his right mind will ever use a machine with such a performance. BUT: the energy is free and working flawlessly for at least 200 years. I still have to see a machine doing that. OK. We can step up the analysis even further. Drawing the 280kW continuously from the tide is eventually slowing down the rotation of the Earth. Just for those worried: each year, one day will be in the order of 10-19 seconds longer…

The moon decelerating earth’s rotation by tugging on the tides (Credit: NASA)
The moon decelerating earth’s rotation by tugging on the tides (Credit: NASA)

References

  1. Wadden Sea, Wikipedia
  2. Mudflat hiking, Wikipedia
  3. Landschapsvisie Baai Dellewal Terschelling, Feddes Olthof
  4. Spuikom, Wikipedia (Dutch))
  5. Slibsedimentatie in de kwelders van de Waddenzee, Arcadis
  6. Hjulström curve, Wikipedia

See also