Graduation of Ines Ben M’hamed: The Strength of Clay in a Test Rig

Ines Ben M’hamed defending her graduation thesis
Ines Ben M’hamed defending her graduation thesis

Last week, Ines Ben M’hamed graduated with good grades on her bachelor thesis. She did a project with us at the Research Department of Damen Dredging Equipment in Nijkerk. The topic was to investigate the strengthening of clay when it is subjected to shear. This deformation is a common phenomenon when cutting clay and as such a contribution to my own PhD project1 and consequently improving our products for these applications. A common problem with clay is clogging up the cutter head, but it is also not completely understood why the clay is behaving as it does and how much power is involved for the various regimes.

Fully covered cutter head in sticky clay

The effects of deformation on the behaviour of clay are much more pronounced than e.g. sand or rock. Rock does not deform, it just breaks. Sand deforms, but as it basically only involves hydraulic and mechanical forces, it is much better understood. Clay particles have wider range of interactions. Next to the hydraulic and mechanical forces, they may experience: adhesion and cohesion, molecular forces, electrostatic charges and chemical bonding in the higher temperature ranges. The general effect is that as the particles in the original situation may have a weak structure, the external disturbance causes the particles to get jostled around and all the mentioned interaction get a chance to hook on to each other.

Shear strengthening due to organising particles
Shear strengthening due to organising particles

The result is, that the particles get oriented and therewith a better opportunity to bond. The effect is a strengthening of the shear stress. As this strengthening is dependent on the strain rate, it is this strain rate, that is of interest for the prediction of the cutting forces. There are many publications available on what the consequences are of the strain rate on the Specific Cutting Energy. A well known model is by Sape Miedema2.

Strain Rate Effect on the Specific Cutting Energy (Credit: SA Miedema)
Strain Rate Effect on the Specific Cutting Energy (Credit: SA Miedema)

The trick with this model is, it depends on this strain rate effect. The sole experimental data available is by Hatamura and Chijiwa3 in 1975. They tested one type of clay on the three governing parameters: static shear strength, dynamic shear strength and the strain rate. There hasn’t been hardly any further experimental investigation into this problem. And as we regularly receive samples and soil reports that we can not test on these properties, it is also hard to predict the performance of our cutter heads. So, we decided to build our own cutting test rig.

Design of Ines’ cutting test rig
Design of Ines’ cutting test rig

This cutting test rig resembles the specifications to the original test rig of Hatamura. This will allow us to verify the parameters in the model ourselves. We also prepared the design with various option to enable us to allow assessment of clay samples that we receive from clients and service engineers. We hope to provide our customers with additional service in this problem. Currently, the parts of the test rig arrived very late and Ines was not able to include the build in her project. Respect for the good grade she received for her thesis. However, the parts are there and provide and excellent opportunity for the next graduation student to do their project with our company. Who dares?

Available parts for the cutting test rig
Available parts for the cutting test rig

References

  1. Personal Announcement: Going Back To School To Cut Some Clay, Discover Dredging
  2. The Delft Sand Clay & Rock Cutting Model, SA Miedema
  3. Analysis Of the Mechanism of Soil : 1st Report. Cutting Patterns of Soils, Hatamura & Chijiwa

See also

 

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

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