LIQUEFACTION AND DUCTILITY
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01-01-2011, 01:17 PM





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Liquefaction
Liquefaction is a physical process that takes place during some earthquakes that may lead to ground failure. As a consequence of liquefaction, soft, young, water-saturated, well sorted, fine grain sands and silts behave as viscous fluids rather than solids.
Liquefaction takes place when seismic shear waves pass through a saturated granular soil layer, distort its granular structure, and cause some of its pore spaces to collapse.
The collapse of the granular structure increases pore space water pressure, and decreases the soil's shear strength. If pore space water pressure increases to the point where the soil's shear strength can no longer support the weight of the overlying soil, buildings, roads, houses, etc., then the soil will flow like a liquid and cause extensive surface damage


Liquefaction Ground Failure
Geologists classify what happens next into several different kinds of ground failure.
Flow failure happens when liquefied sediments are sitting on a slope. The result is some form of landslide.
Lateral spread occurs on flatter terrain. The ground just oozes out sideways, no more than a few meters at the most. But that's enough to completely disrupt things like foundations, pipelines, railroads, and retaining walls. Lateral spread is common near riverbanks, and it's especially harmful to things like bridges, whose feet may sit in young river sediments.

Ground oscillation affects flat ground. The liquefied sediment starts to slosh into waves as shaking continues. Whatever is on top of the sediment gets broken and thrown around. Cracks in the ground open and close, and water or mud may erupt from them.
Settlement happens in all types of terrain as the soil compacts and the ground water dissipates. This can cause a lot of damage to natural features, such as drowning low-lying forests, and it can shift stream courses.

Soil Liquefaction and Cyclic Mobility Evaluation for Level Ground during Earthquakes
It is shown that the design engineer has two basic choices if he considers it appropriate to neglect the possible effects of drainage occurring during the period of cyclic stress applications:

(1)To calculate the stresses induced in the ground by the design earthquake, and to compare these stresses with those required to cause cyclic mobility or liquefaction of representative samples in the laboratory.
The main problem in this approach lies in correctly assessing the characteristics of the in-situ deposit from laboratory tests performed on even good quality undisturbed samples.

(2)to be guided by the known field performance of sand deposits correlated with some measure of in-situ characteristics, such as the standard penetration test. In some cases it is desirable to evaluate the possible effects of pore pressure dissipation in different layers of a deposit during and following earthquake shaking.

Liquefaction occurs in saturated soils, that is, soils in which the space between individual particles is completely filled with water.
This water exerts a pressure on the soil particles that influences how tightly the particles themselves are pressed together.
Prior to an earthquake, the water pressure is relatively low. However, earthquake shaking can cause the water pressure to increase to the point where the soil particles can readily move with respect to each other




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