Wednesday, February 11, 2009

What is Liquefaction ?
Liquefaction is a phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking or other rapid loading. Liquefaction and related phenomena have been responsible for tremendous amounts of damage in historical earthquakes around the world. 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.
When has liquefaction occurred in the past ?
Liquefaction has been observed in earthquakes for many years. In fact, written records dating back hundreds and even thousands of years describe earthquake effects that are now known to be associated with liquefaction. Nevertheless, liquefaction has been so widespread in a number of recent earthquakes that it is often associated with them. Examples of earthquake :

Niigata, Japan, 1964 Kobe, Japan, 1995
Where does liquefaction often occur ?

Because liquefaction only occurs in saturated soil, its effects are most commonly observed in low-lying areas near bodies of water such as rivers, lakes, bays, and oceans. The effects of liquefaction may include major sliding of soil toward the body slumping and of water . Liquefaction also frequently causes damage to bridges that cross rivers and other bodies of water. Such damage can have drastic consequences, impeding emergency response and rescue operations in the short term and causing significant economic loss from business disruption in the longer term. Liquefaction-induced soil movements can push foundations out of place .

Why does liquefaction occur?
To understand liquefaction, it is important to recognize the conditions that exist in a soil deposit before an earthquake. A soil deposit consists of an assemblage of individual soil particles. If we look closely at these particles, we can see that each particle is in contact with a number of neighboring particles. The weight of the overlying soil particles produce contact forces between the particles - these forces hold individual particles in place and give the soil its strength.

Soil grains in a soil deposit.
The height of the blue column to
the right represents the level of
porewater pressure in the soil.
sand grains with high contact forces

The length of the arrows represent the size of the contact forces between individual soil grains. The contact forces are large when the porewater pressure is low.

Liquefaction occurs when the structure of a loose, saturated sand breaks down due to some rapidly applied loading. As the structure breaks down, the loosely-packed individual soil particles attempt to move into a denser configuration. In an earthquake, however, there is not enough time for the water in the pores of the soil to be squeezed out. Instead, the water is "trapped" and prevents the soil particles from moving closer together. This is accompanied by an increase in water pressure which reduces the contact forces between the individual soil particles, thereby softening and weakening the soil deposit.

Observe how small the contact forces are because of the high sand grains with low contact forces water pressure. In an extreme case, the porewater pressure may become so high that many of the soil particles lose contact with each other. In such cases, the soil will have very little strength, and will behave more like a liquid than a solid - hence, the name "liquefaction".
Watch the video , its about a simple experiment on liquefaction , it reminds me of quicksand .

what we could have been, 4:35 AM.