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


  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,
  4. Prestigieus project Markthal Rotterdam vraagt om innovatieve oplossingen, Autograaf 42-p.8, MvO Groep

See also


Hiking Through the Norris Geyser Basin and the Risks for Your Dredge Production

Our Norris Geyser Basin hike in the Yellowstone National Park, Wyoming, USA

Smell is said to have the strongest memories. And usually it happens, that one smells a certain whiff and your mind is instantly transferred to the happy days of childhood where your grandma makes your favourite pie. So, what does it say about me, when we were hiking trails over the Norris Geyser Basin in the Yellowstone National Park, the fumes from the geysers brought me back to the soil laboratory where we bake the soils for analysing and sieve tests?

Overview of our cute little soil mechanics lab

Just like the baking process to dry soil in an oven, the sediment in the Norris Geyser Basin is heated by the hot ground water underneath. The hot water in the basin or in the sample leach silica and calcium from the grains and evaporation transfers those scents to your nose. Also, as the silica and calcium reach the surface, they cool down and get deposited on the outside. In the oven, the calcium will form some white spots and there is a thin crust of just a few grains thick. In the Norris Geyser Basin everything turns white and the crust is much thicker. Still, the crust is relatively brittle and accidents do happen when people stray from the indicated trail and sink through the crust and get cooked in the underground steam1.

Warning: Dangerous Ground (Credit: US National Parks Service)

Calcium cemented sand can sometimes be found in a dredging project too. There it is of some nuisance, as it makes soil reports unreliable and causes some unpredicted difficulties for the operation. The calcium glues the grains together and the grain size appears to be bigger. As smaller grains are more effected, the real particle size distribution might be much wider than anticipated. So, thorough shaking and pounding of the sample is important before sieving.

Effect of calcium bonds on apparent and actual particle size distribution

If you only had a survey for the actual or relative density, you may have estimated, that there is rather course material in an open (loose) structure. During dredging, you might find the bank is not free flowing, but comes down in chunks. You might even run into problems of a bank collapse. On the other side of the pipe line, the bonds will have been broken up by the cutter and the dredge pump. The reclamation area is surprisingly filled with lots of fines in the Particle Size Distribution. And as the fines clog the pores between the bigger particles, they hinder the drainage of the reclaimed land2, you may have problems getting the required relative density and bearing capacity. Bank collapses and an insubordinate reclamation area are better averted. Check the local geology and be vigilant on the soil samples for calcium cementation.

The Norris Geyser Basin in Yellowstone is a very special geological place, with cementation due to hydrothermal activity. However, cemented sand and its descendent, sandstone can be found anywhere. Normally we would encounter cemented sand from a marine and biological origin. e.g. Deltas, Beach and shore face sands, Tidal flats, Offshore bars and sand waves, Storm deposits, Submarine channels and fans3. Pretty much everywhere, where there is dredging. You have been warned…

Example of cemented sand forming sediment normally encountered in dredging (Credit: Wikipedia)


  1. Hydrothermal Safety, Yellowstone NPS
  2. Hydraulic conductivity: estimation from grain size, Wikipedia
  3. Sandstone, Wikipedia

See also


Lessons in Camping: Basic Soil Investigation

Pitching our tent at Bad Bear Campground, Idaho, USA

Oh the horror! An old salt like me had to go camping during our summer holiday. Our daughter had her birthday during our road trip in the USA and she wanted to celebrate it by camping in the woods. Complying to her wishes we pitched a tent and roasted marshmallows. Meanwhile my mind was frantically searching for familiar clues to connect to my maritime heritage. Hammering down the tent pegs, it dawned to me: putting up a tent is basically a simple Standard Penetration Test.

Standard Penetration Test explanation infographic

Standard Penetration Test is one of the easiest soil investigations you could do1. All you need is a pipe and a hammer. You count the number of blows to hammer the pipe down and you have an indication of the effort it takes to cut the soil. This method completely ignores sophisticated parameters as e.g. undrained shear strength, porosity or internal friction angle. It is very crude in its results. On the other hand, the basic principle of driving the pipe into the ground is very similar to the cutting action of the pick points on a cutter. As such, it is a very good indicator for the performance of a cutter head. This is also the reason, why for initial discussions about the performance of a CSD, the SPT is a good starting point to ask the client. He might have a report like this already available, or he can easily perform the tests. Also ‘Sandy’ accepts SPT values for an indication of the soil quality2.

Sandy’s soil parameter input page

Be aware, that SPT’s are often not very deep. Of course, a full soil investigation report with a Cone Penetration Test is much more valuable. We can always translate the results from a CPT report into a SPT value. But the SPT information is not covering all the parameters to translate this to a CPT. Sometimes even an SPT report fails. And then it might be useful to discuss with the client on a qualitative level about the soil condition. Usually people have actually touched the soil, or at least can paint a mental picture of the soil conditions and these criteria might help to use the same descriptive language.

Standardised qualitative description

Hammering a rod into the soil is a relative cheap and quick method to collect the soil consistency. It can be performed everywhere, anytime, under most conditions. That is why it was also selected for the soil investigation on one of the most remote locations imaginable. Although it is still on my wish list of dream destinations, the prohibitive price tag of the ticket will prevent it for me to pitch up my tent over there. I just have to revel in the camping adventures of Neil and Buzz.

Astronaut Edwin Aldrin takes a core-tube sample3 (Credit: NASA)


  1. ASTM: Standard Test Method for Standard Penetration Test
  2. Sandy, Dredge Finder
  3. Astronaut Edwin Aldrin takes a core-tube sample

See also