Soil mechanics – Discover Dredging

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.

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

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

References

Graduation Omar Karam: Rock Cutting The Egyptian Way

Egypt is a great nation when it comes to ancient engineering. No other country has such a concentration of impressive monuments and such an interesting history as over there. If you are not convinced that modern Egyptians are not capable of great engineering feats you are wrong. Last Monday, Omar Karam graduated at our R&D department of Damen Dredging Equipment1 on his thesis about ‘CSD Rock Cutting.’

Cutting processes have been extensively described by Sape Miedema in ‘The Delft Sand, Clay & Rock Cutting Model’2. Omar has been using the frame work of Miedema to make some useful tools for the estimation of the production of our dredging equipment in rock. In due time, you will find the results of his thesis in the online dredge selection tool ‘Sandy’. Omar’s curiosity and ingenuity does not end here. He will continue studying at a university, but I do hope to meet him again, as he would be a valuable asset for our dredging community. Keep an eye out for him.

Program structure diagram of cutting force calculations

His graduation brings me back to my first lessons in dredging technology at the Delft University of Technology by the illustrious professor de Koning. In a sense he was an old school engineer, who hammered it in to us that thinking is done by doing it with your hands3. Back than the Polytechnic School was just rebranded to University and he was mocking that as a university, we had to set the topics in a broader perspective. So, he started his introduction on cutting technology with some slides of the unfinished obelisk at Aswan4 as every aspect of the cutting process could be illustrated.

The story according to de Koning is: ‘Around the quarry of the obelisk, they have found diorites5. These are some sort of volcanic balls of rock. In combination with the marks and scratches all around the obelisk, archaeologists believe these stones have been used to pound the granite. The impact compresses the bedrock and the resulting stresses fracture the contact surface(1). For every hit a whiff of dust is created. Eventually the dust is collected and scooped away for the next layer. Next, trees would be planted in the trench on one side of the obelisk. The growing root system displaces volume and create shear stress underneath the obelisk that would sever the obelisk from the bed rock(2). At last the trees are removed and dry wooden dowels would have been inserted in the shear cracks. Saturating the wooden dowels will make them grow. The last strands of rock will now be broken due to tensile stresses(3). Repeated insertion of new dry dowels and saturating them will lift the whole obelisk enough to pull some ropes under and carry the obelisk away to the building site.’

Although the diorites and the scratch marks are a smoking gun, current archaeologists argue about the feasibility of this process as experiments yield a very low production and it is doubted that the obelisk could be finished in the lifetime of the client6. Even if disputed, de Koning told a story that conveys the message; I vividly remember it and makes me understand the rock cutting process.

These mysterious monolithic ornamental spires have been an inspiration for many legends and stories. When we have solved the riddle of the rock cutting with diorite balls, it may inspire the development of new rock cutting technology for the dredging community and we can put the story of the obelisks to an end.7

End of the story on the cutting of obelisks (Credit: Uderzo)

References

Modern Uses And Legendary Excuses For Manual Depth Sounding

Never waste a moment to tell a good story. Usually, you’ll find informative or educational stories on this platform. This time, I literally found an opportunity to tell you a fun story. All it took, was this nifty little classic navigational instrument. The crew on the dredge used to calibrate their modern survey system1 or checked the delivered depth with this ancient tool. Ever seen one like this? It is a depth sounding lead2. Well, I doubt this one was made from lead, based on the estimated weight and appearance, but it does have all the other characteristics of a normal depth sounding lead.

Evolving from a stone on a rope, the depth sounding lead was used to sound the depth. The plummet was made from lead. The rope was marked at regular intervals according to the shoe size of the current king. Cast overboard, the lead sank and keeping the rope tight, the depth at that location could be read from the markings on the vertical rope. It involved some nimble dexterity to stand at the lee side of a fast moving vessel in a choppy sea to handle the lead, a bundle of coiling rope and accurately reading the depth at the right moment. Hands down to all those seafarers that explored the world in old times and managed to navigate the globe on this instrument.

Sounding the depth manually with rope and lead (Credit: Wikipedia)

The depth was not the only information gained from this action. When you look closely, there is a hole at the bottom of the lead. On the picture above it is empty, but it ought to be filled with grease or wax. When the lead touched the bottom, some of the dirt was caught in the grease. When the lead was retrieved, the cling-ons were inspected. These could be either: sand, mud, gravel, peat, silt or even shells and other biological detritus. The material was reported on the charts also. This made navigation in charted waters easy: compare the sample with the indicated bottom condition. And that brings me to my fun story.

Before the Dutch reclaimed their land, there was a large water body in the Netherlands, called the ‘Zuiderzee’3. Or, South Sea as opposed to the North Sea, which most of you might know. This Zuiderzee, was extensively used for fishing. The skippers did not have charts, but they relied on oral tradition handed down through the ages of where what kind of soil would be available. Near Urk, you might find rocks. Near Pampus, there will be a lot of mud and around Stavoren, there is the famous ‘Vrouwenzand’ (Sand Bank of the Lady of Stavoren4). So, when the fishermen cast their depth sounding leads out, they knew the location of their vessel and the depth beneath it.

Map of the ‘Zuiderzee’ (Credit: Wikipedia)

One of those skippers boasted he did not even have to see and feel the sample, but just by tasting it, he could pinpoint his location within a hundred yards. Hard to believe, right? The cabin boy on board thought likewise. So, he devised a cunning plan. After lunch, the skipper went down to the cabin for a short nap and instructed the cabin boy to bring him the lead to taste the sample. But, our clever cabin boy sank the lead in the crate with potato’s. The bottom of the crate was covered with clay from the potato’s. Carefully bringing the sample to the skipper, the cabin boy woke him up and awaited his reaction. The skipper woke up groggily and grappled for the lead with half closed eyes. He stuck his finger in the sample hole and tasted the material inside. Suddenly, his eyes went wide open and he exclaimed: Oh, disaster! The dikes have broken again! The land is flooded and we are sailing over farmer John’s potato patch!

You never know what you dredge from the bottom of a potato crate

Category: Soil mechanics

Personal Announcement: Going Back To School To Cut Some Clay

References

See also

Graduation Omar Karam: Rock Cutting The Egyptian Way

References

See also

Modern Uses And Legendary Excuses For Manual Depth Sounding

References

See also