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.
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.
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.
References
- 24th World Dredging Congress & Exhibition
- Materialen in (constructieve) ophogingen en aanvullingen; Richtlijn ter beoordeling van alternatieven voor zand, CROW
- Classification of Soils and Rocks for the Maritime Dredging Process, PIANC
- Adhäsion von Tonböden beim Tunnelvortrieb mit Flüssigkeitsschilden, Thewes
- PAO lubricant inhibits bit balling, speeds drilling, Mensa-Wilmot
- The Prevention and Cure of Bit Balling in Water-Based Drilling Fluids, Mettah
- The Origin of Clay, When Dredging Becomes Sticky, Discover Dredging