Seismic liquefaction represents one of the most destructive hazards associated with major earthquakes. It occurs when saturated, loose granular soil temporarily loses its strength and stiffness due to intense ground shaking, causing it to behave like a dense liquid. This sudden transformation allows the ground to flow or deform, leading to catastrophic damage to structures built on top of or within the affected soil layer. This process is a direct consequence of the rapid pressure changes that occur beneath the surface when the earth moves violently.
The Subsurface Process
The stability of soil begins with the concept of effective stress, which is the force transmitted through the points of contact between individual soil grains. In a stable, saturated soil deposit, the weight of the overlying soil is carried by this grain-to-grain contact. This contact friction provides the soil its shear strength, allowing it to support weight and resist lateral forces.
When seismic waves travel through the saturated soil, the rapid, cyclic shaking causes the loose soil particles to attempt to rearrange into a denser configuration. Because the soil is fully saturated, there is no time for the water to drain out of the pore spaces. This attempt at volume reduction transfers the load from the solid soil particles to the incompressible water filling the voids.
The result is a rapid buildup of pore water pressure, which pushes the soil grains apart. As this pressure increases, the effective stress between the particles drops sharply, ultimately approaching zero. When the pore water pressure nears the total overburden stress, the soil particles become suspended, momentarily losing their contact friction and their ability to resist shear forces.
At this point, the soil has effectively liquefied, behaving more like a dense fluid than a solid. The temporary loss of shear strength means the soil cannot support the weight of foundations or maintain its lateral stability. Once the earthquake shaking subsides, the excess pore water pressure begins to dissipate, and the soil particles settle into a denser state, gradually regaining their strength.
Prerequisites for Liquefaction
The occurrence of liquefaction is not universal during an earthquake; it requires a specific combination of three geological and seismic conditions. The first requirement relates to the composition of the subsurface material, which must be non-cohesive and granular. Loose, poorly compacted sands and silts are the most susceptible materials, particularly those deposited recently in riverbeds or coastal areas.
Soils with a high clay content are resistant to this process because the microscopic clay particles adhere to one another, preventing the complete loss of effective stress even under cyclic loading. The second condition is the presence of water, as the soil layer must be fully or nearly fully saturated, which means a high groundwater table is present. Without the pore spaces being completely filled with water, the volume reduction from shaking would simply compress the air, preventing the rapid buildup of pressure.
The final prerequisite is the intensity and duration of the seismic input itself. The ground shaking must be strong enough to initiate the rearrangement of the soil particles and must last long enough for the pore water pressure to build to a necessary level. Strong shaking associated with earthquakes of magnitude 6.5 or greater often provides the necessary energy and duration to trigger widespread liquefaction.
Visible Ground Deformation
Once the subsurface soil has liquefied, the effects become immediately visible on the ground surface, often leading to severe structural damage. One of the most common signs is the formation of sand boils, sometimes called sand volcanoes. These occur when the highly pressurized water forces its way upward through cracks in the overlying crust, carrying fluidized sand with it and depositing cone-shaped mounds of sediment on the surface.
A more destructive effect is lateral spreading, which involves the horizontal movement of large blocks of unliquefied soil across the liquefied layer underneath. This movement is driven by gravity on gentle slopes or toward a free face, such as a riverbank or an excavation. Horizontal displacements can range from a few inches to many feet, causing massive destruction to infrastructure, including the buckling of pipelines, the cracking of roads, and the collapse of bridge supports.
Another significant consequence is ground settlement, which occurs as the excess pore water pressure dissipates after the shaking stops, allowing the soil to compact into a denser state. This volume reduction causes the ground surface to subside, often unevenly, leading to buildings sinking into the ground or tilting dramatically. The 1964 Niigata earthquake in Japan, for example, saw multi-story apartment buildings tilt over while remaining largely intact, demonstrating the complete loss of foundation support caused by the liquefied soil.