|MadSci Network: Earth Sciences|
Thixotropy is a characteristic of some solids/fluids mixes (such as a mix of clay and water) by which the mixture becomes more fluid when agitated or vibrated.
For example if you were to take a piece of dry bentonite clay about the size of a golf ball (see http://www.ima-eu.org/en/Bentoni.pdf), hold it in your hand and tap it with a knife, it would just, well, sit there. However, if the bentonite was hydrated (had absorbed water or retained water that it had previously absorbed) and you held it in your hand and tapped it, it would turn into a slippery mud and run through your fingers. That is a classic thixotropic reaction.
So, you can have a dry bentonite clay that won't run (become thixotropic) as long as water is kept away from it, or you can have a hydrated bentonite clay that won't run as long as there is no vibration or other physical agitation.
What might happen if you built a house on ground composed of hydrated bentonite clay? Well, if there were no vibration or extra weight added to the building, probably nothing. But if you added a big highway right next to the house, the vibration from nearby constant heavy traffic might cause the bentonite clay to become fluid (thixotropic reaction) and the house might sink into the ground, or its foundation might crack. This can also happen due to seismic vibration during earthquakes.
On the other hand, what might happen if you built a large structure (say a big concrete dam and hydroelectric power station) on ground made up of dry bentonite clay? Well, over time, if the bottom of the reservoir (the ground surface) were not sealed from water, the water behind the dam might seep slowly into the ground and eventually wet (hydrate) the bentonite clay. Hydrated clay+load(weight)+vibration(power turbines)=thixotropic reaction. This happened in Canada. The foundation became unstable (would not hold up the weight of the dam) and eventually flowed like water, and the dam burst and people were killed.
Also, it doesn't necessarily take a bed of thixotropic material completely underlying a structure to cause a disaster. In Italy it was just a thin bed of clay material that got wet and failed.
Vaiont Dam, Italy
This is the site of the world's worst dam disaster, located in NE Italy in the Alps.
In 1963, an enormous debris slide splashed water out of the reservoir and created a huge destructive wave 100 meters high that killed 2,600 people in < 7 minutes.
The dam was constructed in 1960 as the world's highest thin-arch dam (266 meters, second highest in the world).
The hazardous setting was well documented before construction and development was a calculated risk:
Now, if I understand you question correctly, I have show how cracks can form. With regards to draining, I am not sure I understand what you are after, but let me try.
Bentonite clay is also used as a seal. This is because the clay will absorb water (to a point) and hold it as long as no additional energy (vibration or agitation) is added to the mix. What this means, is that a builder can place a layer of bentonite clay under the floor and around the walls of a basement to intercept and prevent water from entering the basement (draining INTO the basement).
If a house were built on bentonitic clay ground (not a great idea, but OK) then the clay ground surface would be relatively impermeable (will not let much water through). What this means is that when it rains heavily, the water draining from the roof and/or gutters will not infilter (pass through the surface) into the soil very well. This means in turn that the water can pond, rise up and cause flooding. This is very common along the Gulf Coast in Texas, USA, where the ground surface is primarily bentonitic clay. A few years ago, during heavy rains associated with a hurricane, there were more than 10,000 families in the Houston, Texas area who lost their houses to flooding because the ground could not absorb the rainwater fast enough.
OK, we've talked a lot about how bentonite clay reacts in general, but how does the thixotropic reaction actually happen in the clay? Well, think of the bentonite clay as composed of plastic plates coated with a water-soluble glue. The plates have been thrown into a big box at random. Many of the plates would land parallel to each other, but many would also be caught by the glue and would be at angles to each other ( http://www.petrographic.com/gallery/Illite-smectite.jpg). They would all stick together a bit, and would be capable of holding up to someone stepping on them.
Now, add water to the box. The plates and water-soluble glue get wet, and the glue dissolves (you have hydrated the clay), but most of the plates stay where they are. Now, shake the box. What happens? Well, most of the plates line up, the total volume occupied by the plates gets smaller, and all the water drains off the plates and out of the box.
(also see here http://www.arch.nus.edu.sg/SOA/design_studio/rda/group4/neha/Untitled-9.JPG)
Now, do the same thing, but build a house on top of the box, before you shake it. Would you want to live there?
Hope this helps.
Mike Munsil, PG
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