Liquefaction Basics
What is liquefaction?
Liquefaction is the rapid loss of shear strength in saturated, cohesionless soils when subjected to a dynamic load, such as from an earthquake (Coduto, 1999). If the shear strength of the soil drops below the shear stress placed on the soil (e.g. from a building with foundations on the soil) this will lead to shear failure of the soil. This can lead to severe damage or even collapse of the structure. Figures 1 & 2 illustrate this phenomenon.
What is liquefaction?
Liquefaction is the rapid loss of shear strength in saturated, cohesionless soils when subjected to a dynamic load, such as from an earthquake (Coduto, 1999). If the shear strength of the soil drops below the shear stress placed on the soil (e.g. from a building with foundations on the soil) this will lead to shear failure of the soil. This can lead to severe damage or even collapse of the structure. Figures 1 & 2 illustrate this phenomenon.
Figure 1: Liquefaction induced damage in the 1964 Niigata, Japan Earthquake.
Figure 2: Rotation and settlement of a building due to soil liquefaction, Turkey, 2001.
How does liquefaction develop?
During the construction of civil engineering projects the load is applied slowly to the soil. In this situation the rate of drainage is greater than the rate of loading so little or no excess pore water pressures develop (Coduto, 1999).
However, during an earthquake the rate of loading is so fast that even cohesionless soils cannot drain fast enough. This results in an increase in pore water pressure which reduces the contact forces between the soil particles. It is the contact forces between particles that give a cohesionless soil its strength so a reduction in contact forces results in a reduction of the shear strength of the soil (Coduto, 1999).
Factors affecting the susceptibility of a soil to liquefaction
There are several factors that can affect the susceptibility of a soil to liquefaction:
1. The particles size distribution of the soil: poorly-graded soils are most susceptible to liquefaction.
2. Whether the soil is loose or dense: loose soils are much more susceptible to liquefaction.
3. Whether the soil is saturated or not: only saturated soils can undergo liquefaction.
How does liquefaction develop?
During the construction of civil engineering projects the load is applied slowly to the soil. In this situation the rate of drainage is greater than the rate of loading so little or no excess pore water pressures develop (Coduto, 1999).
However, during an earthquake the rate of loading is so fast that even cohesionless soils cannot drain fast enough. This results in an increase in pore water pressure which reduces the contact forces between the soil particles. It is the contact forces between particles that give a cohesionless soil its strength so a reduction in contact forces results in a reduction of the shear strength of the soil (Coduto, 1999).
Factors affecting the susceptibility of a soil to liquefaction
There are several factors that can affect the susceptibility of a soil to liquefaction:
1. The particles size distribution of the soil: poorly-graded soils are most susceptible to liquefaction.
2. Whether the soil is loose or dense: loose soils are much more susceptible to liquefaction.
3. Whether the soil is saturated or not: only saturated soils can undergo liquefaction.
References
Coduto, D. P., 1999. Geotechnical Engineering Principles and Practices. New Jersey: Prentice-Hall.
Madabhushi, S. P. G., 2007. Ground improvement methods for liquefaction. Proceedings of the Institution of Civil Engineers - Ground Improvement , pp. 195-206.
Figures coutesy of:
Figure 1: US geological survey
Figure 2: Madabhushi, 2007