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?
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.
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.
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…
- Hydrothermal Safety, Yellowstone NPS
- Hydraulic conductivity: estimation from grain size, Wikipedia
- Sandstone, Wikipedia