Liquefaction, as it relates to earthquakes, occurs when the strength and stiffness of a soil is reduced by the strong shaking caused by seismic events. It more commonly occurs in soils where the space between the individual soil particles is completely filled with water (known as saturated soils), where particles are loose and granular (such as silty sands from the late Holocene period) and where there is poor drainage. The individual particles, locked into place with each other, give the soil its strength. The strong shaking caused by seismic event increases the water pressure within the soil, pushing the particles away from each other, which unlocks them and reduces the overall strength of the soil. This allows the soil to flow like a fluid.
The effects of liquefaction are commonly seen in low-lying areas near bodies of water such as rivers, lakes, bays, and oceans. These effects include major sliding and slumping of soil towards the body of water.
The most recent effects of liquefaction have been seen at the Alaska (1964), Niigata, Japan (1964), Loma Prieta (1989) and Kobe, Japan (1995) events. In the case Niigata, entire buildings overturned.
Mitigation of liquefaction can be accomplished through site selection, construction of liquefaction resistant structures, and soil improvement.
Site selection is simply avoiding areas where there are soils susceptible to liquefaction. Knowledge of the site can come through historical information (areas where liquefaction has occurred in the past are likely to occur again), geological criteria (how and when was the soil created), and, the composition of the soil (soils that have particles of the same size are more susceptible than soils with a wide range of particle sizes). Various governmental agencies have developed maps indicating areas where there is the potential for liquefaction during an earthquake.
Construction of liquefaction resistant structures can include:
- Berms, dikes and other edge containment structures. This resists lateral spreading of the soil during and earthquake.
- Piles installed by driving or vibration.
- Piers installed by drilling or excavation.
- Grade beams.
- Reinforced mat foundations.
- Post-tension mat foundations.
Soil improvement methods include:
- Excavation and/or compaction
- Excavation and replacement of liquefiable soils
- Excavation and re-compaction
- Ground densification (compaction without excavation)
- Compaction by vibratory probes
- Heavy tamping
- Compaction piles
- Compaction grouting
- Soil drainage
Some of the construction and soil improvement techniques are done in conjunction with each other.
Risk Logic can provide help in the selection of new locations to help identify areas subject to liquefaction.