A Sample of Soil Samples

Soil sample exhibit at the Damen Dredging Experience

‘Welcome Sir, very nice you would like to buy our dredge. What do you want to dredge with it?’
‘Sand.’
‘Ah Sir, very well. Our dredges are very capable of dredging sand. What kind of sand are you going to dredge?’
‘Grey sand!…’

Sounds familiar? Well to me it does. Sometimes, even our most esteemed customers lack a basic knowledge of their primary process. Often, I’ve been called into a meeting with the customer, to explain about sand and its physical properties. Nowadays, we can show them an exhibit in the Damen Dredging Experience1 to discuss their particular case. Eventually, we can tease out the information we need, to inform the customer the estimated production of their dredge. Educated customers do know about their operation and can make the estimation themselves.

There are several parameters that influence the performance of the dredge. Particle sizes, grain forms, densities, mineral types, cohesion and many more. And we are very happy when the customer already has his own soil report of his particular operation. This would comprise bore logs, particle size distribution and cone penetration tests. If this is not possible, a sample of the concerned soil will do. We do have our own small soil mechanics lab, in which we can measure the required properties.

Elementary soil mechanics laboratory at our company

The most useful property is the particle size distribution or PSD. This can be done in a sieve tower. (red circle) The soil sample is placed in the top sieve and the sieves are vibrated to separate the various fractions. The contents of the sieves are weighed and plotted in a logarithmic graph. This resulting PSD can then be used in production estimations.

Procedure to establish a particle size distribution

For a good measurement, we need a sample the size of a 1.5L coke bottle. About two thirds of soil and one third of water. We need the water to capture the fines in the sample. Do not drain it! And the bottle is a good container for transport and widely available. Stash it in your check-in luggage. Otherwise, you will run into trouble with the airport security about carrying liquids in your hand luggage.

Examples of sample containers encountered for estimation

All these variations in soil properties have a major influence on the performance of the dredge. So, this is why we would like to know exactly what the customer is going to do with our dredge. Otherwise, we might end up in an analogous situation when somebody wants to buy a truck. You can explain all about the installed power and the cargo capacity and then you get this question:
‘Sir, can you tell me how much paper I can transport with this truck?’
‘Ah well, we are not in the transport business ourselves, but as you are a good customer, we can make an estimate for your convenience. What kind of paper? Kite paper? Cardstock? In blocks, boxes or roles?’
‘Oh, I don’t know. Just paper’.

Model dump truck from a customer2, typically used at dredging projects

References

  1. Kommer Damen opening the ‘Damen Dredging Experience’, DredgingToday
  2. 2. 35 ton dumper, Martens en Van Oord

See also

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

References

  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, DopDredgePumps.com
  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)

References

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

See also