STATIC LIQUEFACTION

PIN

My friend, who I will call “Joe” was commenting to me about my blog, and how Joe has found some of the articles to be especially helpful for non-technical people. Joe mentioned to me that a non-technical article on static liquefaction would be very helpful. So, here goes… When I was a kid, and mostly based on watching the TV show, Tarzan, I thought that quicksand would present me with vastly greater dangers than I have encountered so far in my adult life. That’s pretty fortunate, by the way. Also, by the way, if do ever do find yourself trapped in quicksand, drop everything you’re carrying. Cast it aside. Then, try to take a few steps backwards to see if you can simply “reverse” yourself out of the situation. If this fails, sit down and lean back. You should be able to free your feet, allowing them to “float” up to the surface. When you feel your feet start to come free, roll to your side, away from the quicksand and free of its grip. You’ll get dirty, but it’s the surest way to free yourself. If you try to fight the quicksand, you will only get drawn in deeper. Fortunately, unlike in Tarzan, you should only sink in about waist deep, due to the principles of effective stress. Have I blogged on the principles of effective stress? If not, I need to write one. Baby steps. Anyway, the term “static liquefaction” isn’t quite the right term, since it is only one of the “family” of possibilities in the larger group of behaviors called “flow liquefaction”. Basically, this type of liquefaction can be initiated by several different triggers, including from vibrations and earthquake shaking. In order for flow liquefaction to occur, the soil needs to be brittle, loose and saturated (or nearly saturated). It needs to be loose so that the soil will tend to compress (contract) when it is sheared (when the soil particles move against each other). That compression of the soil causes the loading (internal and external forces) to transfer from the soil particles (which tends to be elastic at low levels of movement [strain]), to the water (which is essentially incompressible) that is within the pore spaces between the soil particles. If the soil is fine-grained enough that it can’t rapidly drain those pore pressures, and if the increase in pore pressure is large enough, this increase in pore pressure can lead the soil to “fail”. Because the soil is brittle, that means that there is a maximum (peak) strength that will decrease to a smaller (shear) strength as the soil continues to shear. Once the peak strength has been passed, the soil will quickly lose strength (or “stiffness”), until it has only its minimum (residual) shear strength. The point beyond the peak strength is termed flow liquefaction. When it does that, the soil can deform (strain) a considerable amount. The brittleness is defined as the ratio of the soil’s peak strength to its residual shear strength. The more brittle the soil is, the more drastic the effects of the liquefaction can be. If it helps, you can imagine that the soil has collapsed. That isn’t quite correct, but it give a good visual. Many years ago, the founding father of soil mechanics, Karl Terzaghi, had a simple demonstration of this. He had an aquarium that was filled with sand. On the surface of the sand, he placed a small block to mimic a building structure. The aquarium had a water port at the base of the sand. He turned the water on very slowly and allowed the water to slowly rise up in the sand (the rate of pore pressure rise was slow enough that there was no excess pore pressure). At some point in the demonstration, Terzaghi drove a smooth glass rod into the sand, at some distance away from the model building. This “trigger” caused the building model to sink into the sand. The sand had become “quick”, and had liquefied. Terzaghi referred to this as quickening. The sand had become quicksand. That is flow liquefaction.

Did this help at all, Joe?

You May also Like

Leave Your Comments