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

 

A Simple Soil Sample Show

Demonstration at a workshop ‘Advise a dredging customer’

Recently I had to give a demonstration on one of the aspects of my work: investigate what type of soils the customer will be encountering, what type of dredging equipment is most suitable for that application and what will the production be under those conditions. At this demonstration, the purpose was, to show how I can make something simple very complicated in a short time. The challenge here will be to take you with me on this precipitous path to enlightenment.

Ingredients for a sand sieving demonstration

The example case was to examine two different soil samples. Each was artificially mixed, but representing a Particle Size Distribution of two different borrow areas. The samples are delivered in widely available standardised containers. Yeah, you recognised them: 1.5L soda pop bottles. As the samples were wet, they had to be dried. Normally, this can take two to three hours. Here we applied a magical temporal acceleration by employing a Calvin style cardboard box as an oven. The literal main ‘activity’ for the participants was to manually shake the sieve tower. Rest assured, in our own soil analysis lab we use an industrial automatic sieve tower. This antique specimen comes from our museum. The separated fractions from the sieves, then have to be weighed and finally plotted in a so called particle size diagram. Horizontally you can take a sieve diameter and vertically you can read what mass percentage will pass that mesh diameter.

Resulting Particle Size Distribution and key parameters

Although both samples have the same median diameter d50 (354µm), which is indicative of the general particle size, the distribution is very different. Sample A is almost totally the same diameter, where sample B has a broad distribution. A measure for the distribution is the Uniformity Index (d60/d10). Another important figure to be taken from this graph is the silt fraction. This is the percentage of particles that is smaller than 63µm. More parameters can be taken form this graph, but these are the most important for now.

Influence of particle size distribution on slurry transport

From the equation for the critical velocity, the uniformity index plays an active role. If the uniformity index increases, there is more fine material. Fines tend to increase the density and viscosity of the fluid. Consequently the resulting mixture behaves like a heavier fluid, carrying larger particles. Effectively, the resistance of a non-uniform sand is higher than for a more uniform sand mixture.

Influence of particle size distribution on cutter production

Another aspect of the dredging process is the ability of the CSD to excavate the material from the bottom. Here the uniformity index has the complete opposite influence. A uniform sand distribution will have a lot of voids between the grains. The particles will move easily over each other. When there are lot of smaller particles available, they tend to clog up the voids and bond the bigger particles in a gridlock. These sediments are very hard to excavate. If no geotechnical investigation is available, the PSD can help to estimate a SPT.
With the grains size and the SPT, the audience consulted our lovely looking assistant ‘Sandy’ for a first selection of the required CSD.

Short evaluation of a selected dredge at ‘www.dredgefinder.com’

References

Wilson, Addie & Clift, (1992), “Slurry Transport Using Centrifugal Pumps”, Chapter 5 “Heterogeneous Slurry Flow in Horizontal Pipes”

Youd, (1970), “Densification and Shear of Sand during Vibration”, Journal of the Soil Mechanics and Foundations Division, 1970, Vol. 96, Issue 3, Pg. 863-880

See also

Geotechnical investigation

Dredge Finder