Plasticity index – Discover Dredging

Fully covered cutter head in sticky clay

Here at the WODCON 2025 in San Diego1, the theme is ‘Dredging Towards a More Resilient Future’. One of the challenges we encounter, is that even the resources of good construction sand run out. We either have to repurpose sand already dredged or find and alternative construction material. One such an overlooked material is clay. A lot of effort is put into understanding the behaviour of clay in infrastructure applications. The Dutch Centre for Legislation and Infrastructure (CROW)2 has provided recommendations on the applicability of clay for various types of construction. However, the clay has to be dredged and for the adherence potential of clay, there is another recommendation issued by World Association for Waterborne Transport Infrastructure (PIANC)3. Both do use the Plasticity Index and the Consistency Index as criteria to classify the clay. Interestingly, the clay type that is regarded as suitable for construction by the CROW, is also classified by the PIANC as to have the highest adherence potential and thus gives the most problems in dredging.

Another problem with the criteria by PIANC is that they tend to be unreliable. Something they already acknowledge in the supplied explanation to the diagram. When following the literature that led to the recommendation, it turns out the original application was not dredging but tunnel boring4. Where the problem was not so much the clogging of the cutter shield, but the collection of clay in the suction chamber. And even plotting the data used for this assessment shows a large variability. Apparently there is more to the problem of adherence than just the PI and CI. Which might be obvious when considering the original Atterberg Limits. The PI and CI are related to the plastic range of clay, whereas Atterberg already defined a range where adhesion is more relevant.

Relation of PI and CI to the adhesion range according to Atterberg

A situation similar to the cutter clogging is the covering of a drill bit in the oil industry. There, they encounter a phenomenon called ‘Bit Balling’5. It is extremely difficult to assess the bit balling potential from a physical model related to the soil parameters alone. As a solution they developed the ‘Rolling Bar Test’6. A defined amount of clay sample is put into a cylinder with the needed amount of water. Finally a rod is inserted in the sample cylinder. The whole contraption is placed onto a roller set and turned for a set of times. Each time the amount of clay sticking to the rod is measured and plotted in a graph. Eventually, most clay types will loosen their grip on the rod. But some are sticking to the rod indefinitely. Those are the clays that are also likely to show bit balling in the actual process.

Bit balling and procedure of a rolling bar test (data: Mettah, 2011)

As we know that we can’t fight the adhesion of clay, we may as well improvise, adapt and overcome the problem. Since already my graduation, I am working with clay. In that case, it was an auger, that needs the adhesion to the back shield to propagate the clay in the auger. When we were asked by a contractor to improvise a tool that could tackle this sticky clay, we developed a disc bottom cutter head that used the adhesion to move the clay over the blade to a scoop behind the blades. This worked so smoothly, that the satisfied customer bought a second. Eventually he finished to job in time and in budget7.

DOP pumps with special clay cutter head at the ‘Markthallen’ project in Rotterdam

References

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.

Tag: Plasticity index

WODCON 2025: Rolling Out A New Clay Test

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See also

The Origin of Clay, When Dredging Becomes Sticky

References

See also