During a seismic event, loose cohesionless soils (sands) tend to densify into a tighter configuration. When these soils are located above the water table, earthquake shaking causes these sands to settle into a tighter packing (not unlike potato chips settling during shipment).
When these cohesionless deposits exist below the groundwater table, these sands are saturated (the void space between the sand grains is filled with water). Because water is essentially incompressible, any densification of saturated cohesionless soils due to earthquake shaking results in an increase in pore water pressure in the soil skeleton.
The strength of a cohesionless (sandy) soil is obtained from the grain-to-grain contact of individual particles. During a seismic event, the increase in pore water pressure reduces the pressure between these contact points.
Liquefaction is triggered when the earthquake-generated pore pressure equals the effective stress (grain-to-grain contact strength) of the soil. During liquefaction, cohesionless soils lose grain-to-grain contact and behave as a fluid. This phenomenon has catastrophic effects on above ground and below ground structures.
- Liquefaction can result in bearing capacity and lateral spread failures for bridges and embankments.
- Liquefaction results in intolerable settlements and, in some cases, a total loss of bearing capacity for buildings.
- Subsurface structures constructed below the water table rely on the weight and strength of overlying soils to “hold” them down. In earthquake prone areas, liquefaction of near-surface sands causes these air-filled tunnels and utilities to “float” to the ground surface.