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.
sand grains in soil deposit
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.

sand grains with low contact forces      Observe how small the contact forces are because of the high 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".

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