A phenomenon in which saturated, loose, unconsolidated soils abruptly lose shear strength and stiffness and behave like a fluid under transient or cyclic dynamic loading, commonly during earthquakes but also under other rapid loadings.

Principle

Under dynamic loading, rapid increases in pore water pressure reduce effective stress; if drainage is insufficient the soil skeleton cannot sustain contact forces, causing a collapse of structure and a dramatic drop in shear resistance (flow or cyclic mobility).

Demonstration

A layer of clean, loose saturated sand experiences repeated shear from ground shaking; pore pressures build to near overburden so that bearing capacity is lost, resulting in lateral spreading, large settlements and floatation of buried objects.

Misapplication

Using 'liquefaction' loosely to describe any temporary softening, large settlement from consolidation, or non-saturated strength loss; or attributing observed ground damage solely to liquefaction without distinguishing other processes (e.g., slope instability, amplification).

Consequence

When liquefaction occurs, foundations lose bearing, retaining structures and slopes can fail, and infrastructure suffers large displacements; proper recognition drives foundation design (improvement, densification, drainage) and seismic hazard planning.

Reversal

Densified, cemented, coarse-grained or well-drained soils resist pore-pressure build-up and maintain contact forces, behaving as solids even under strong dynamic loading; drained cyclic loading produces different, often smaller, responses.

Boundary

Applies to saturated, cohesionless or loose granular soils (sands, silty sands) subjected to transient or cyclic shear where pore pressures can rise faster than they can dissipate; excludes cemented, coarse gravels with sufficient drainage, non-saturated soils, and long-term consolidation processes.

Semantic Tension

Tension between 'liquefaction' as an instantaneous loss of shear strength and related concepts such as cyclic mobility, flow failure, or simple consolidation settlement; these processes share mechanisms but differ in conditions and outcomes.

Synthesis

Soil liquefaction is the dynamic pore-pressure–driven loss of intergranular contact strength in saturated loose soils under rapid loading, producing fluid-like behaviour, loss of bearing capacity and potential lateral flow and settlement.