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

 

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)

References

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

See also