Soil liquefaction is a ground failure phenomenon where solid soil temporarily loses its strength and stiffness, causing it to behave like a viscous liquid when subjected to rapid loading, such as during an earthquake. This transformation occurs under specific subsurface conditions and is responsible for significant infrastructure damage during seismic events. The temporary loss of shear strength results from increased internal water pressure. Understanding the three primary requirements—soil composition, water saturation, and seismic trigger—is necessary to grasp this process.
Specific Soil Composition Requirements
The physical characteristics of the soil material are the first condition that must be met for liquefaction to occur. The most susceptible materials are loose, granular soils, primarily sand and silt. These soils are considered “cohesionless,” meaning their particles do not naturally bind together with significant attractive forces.
Liquefaction susceptibility is heightened in soils with a uniform grain size and a relatively high void ratio, or low density. This structure allows the soil grains to be easily rearranged and packed more tightly when shaken. This disruption attempts to compress the soil, initiating the liquefaction process.
Cohesive soils, such as most clays, are generally resistant to liquefaction because the electrical charges between their microscopic particles create strong inter-particle bonds. These bonds provide enough inherent shear strength to withstand seismic shaking. The soil must be one whose strength depends almost entirely on the friction and interlocking between grains.
The Presence of High Water Saturation
The second requirement involves the presence of water, which acts as the medium through which the soil loses its strength. The soil must be fully saturated, meaning the pore spaces between the soil particles are completely filled with water. This condition is frequently met in coastal areas, near rivers, or in regions with shallow groundwater.
In a saturated soil, the weight of the overlying soil and any structures is supported by both the contact forces between the soil grains (effective stress) and the pressure of the water in the pores (pore water pressure). When an earthquake strikes, the cyclic ground shaking causes the loose soil structure to momentarily try to compress. Because water is nearly incompressible and cannot drain instantly, this compression transfers the load from the soil skeleton to the pore water.
The rapid load transfer leads to a sudden and substantial increase in the pore water pressure. If this pressure builds up faster than the water can dissipate, the effective stress—the force holding the soil particles in contact—is reduced. Liquefaction is triggered when the pore water pressure rises to the point where it supports nearly all the overlying weight, reducing the soil’s effective stress to near zero.
Sufficient Seismic Shaking and Duration
The final requirement for liquefaction is the external trigger: an earthquake with sufficient ground motion. The seismic event must be powerful enough to generate the cyclic stresses needed to initiate the pore water pressure build-up. While extremely susceptible soils may liquefy in earthquakes as small as moment magnitude 4.5, for sites suitable for construction, a minimum magnitude of 5.0 is considered the threshold for liquefaction hazard assessment.
The intensity of the ground shaking must be high enough to cause the loose, saturated soil particles to lose their dense packing momentarily. This strong ground motion creates the repetitive stress cycles that repeatedly attempt to compress the soil. The shaking must also last for a sufficient duration for the pressure to reach a state that causes liquefaction.
A short, sharp shock, even if intense, may not provide enough time for the pore water pressure to accumulate to the critical level that reduces the effective stress to zero. Therefore, longer-duration earthquakes, typically those of higher magnitude, are more likely to cause widespread liquefaction because they sustain the cyclic loading long enough for the pore water pressure to fully mobilize and for the soil to lose its shear strength.