Datum Beta
Start free

← Project Planning & Design

Sheet C-135
PPDPDD PA

Soilsinsight

Soil liquefaction: when shaken, saturated, granular soil acts like a liquid

One-line orientation

During an earthquake, loose saturated granular soil can lose shear strength—its resistance to sliding—and behave like a liquid. The key is recognizing the soil, water, and shaking conditions that create the risk.

Key points

  • Mechanism: seismic shaking (earthquake) raises pore-water pressure in saturated soil until the grains lose contact; the soil loses shear strength and behaves like a liquid, no longer supporting structures.
  • The three ingredients (all required):
    • Loose / uncompacted soil,
    • Saturated — below the water table,
    • Granular — sand or non-plastic silt.
    • Trigger = seismic shaking.
  • Most susceptible: loose, uncompacted granular soils (sands, non-plastic silts).
  • Clay-rich cohesive soils resist it: cohesive particles stay bound together, so they are much less susceptible than loose, saturated, cohesionless sands.

Liquefaction: shaken, saturated, loose sand acts like a liquid

SCHEMATIC

The same loose, saturated, granular soil before, during and after seismic shaking — watch the water between the grains.

View diagram Hide diagram 3 panels

3 panels side by side — swipe to compare

Soil liquefaction shown as a before, during and after section trio Three soil sections drawn at the same scale, sharing a grade line and a dashed water table, with the saturated ground below the table drawn as water and the sand grains drawn as opaque circles sitting in it, so the blue seen between the grains is the pore water. Before, at rest: the grains touch each other on an open lattice with only thin water between them, a cut pier on a cut spread footing bears level at grade, and an ink line traces the load path from the footing down through the chain of grain-to-grain contacts. During, shaken: a double-headed arrow across the structure shows cyclic shaking, arrows of pore-water pressure rise through the pore space, the same grains are pushed apart until none of them touch and water fills nearly half the mass, no load chain can be drawn, and the footing has begun to drop and rotate. After, settled: the pressure arrows now continue up through the ground line as water escapes upward, the grains have repacked into a dense lattice seated lower than it began, the contact chain has returned, and the pier and footing sit well below grade and rotated, with a dashed outline labelled was here marking their original position. Captions under the three sections read: grain contacts carry the load, pore water fills the voids; pore-water pressure rises, grains stop touching so there is no shear strength and no bearing; left settled and tilted, the ground drops a little while the building sinks far more. Below all three, four chips chained by plus signs give what liquefaction needs: loose, saturated, granular, shaking. Before — at restwater tablefootingDuring — shakencyclic shakingAfter — settledwas hereGrain contacts carry the loadpore water fills the voidsPore-water pressure risesgrains stop touching — no shear strength, no bearingLeft settled and tiltedground drops a little — the building sinks far moreLiquefaction needsall four together:loosesaturatedgranularshaking+++
  • grains of sand or non-plastic silt, in pore water
  • water table — soil below it is saturated
  • load path through touching grains
  • pore water pushed upward — it rises, then escapes

Clay (cohesive, low void ratio) resists · dry or dense sand does not liquefy.

Shear strength is what stops soil grains sliding past one another — it is what holds a footing up. Pore water carries the grains apart, so nothing is left to resist, and the soil behaves like a liquid until the water escapes.

Confusions / comparison

FactorLiquefiesResists liquefaction
Soil typeGranular: loose sand, non-plastic siltClay-rich cohesive soil; dense gravel
DensityLoose / uncompactedDense / well-compacted
MoistureSaturated (below water table)Dry / unsaturated
Particle behaviorCohesionless, high void ratioCohesive, low void ratio
TriggerSeismic shaking—

→ Soil types (this module): cohesive vs cohesionless soils · Soil properties (this module): shear strength is what liquefaction destroys · Water table (this module): saturation is the precondition for liquefaction.

Spotted an issue with this card? Tell us →

Related units

Geotech testingSoil propertiesSoil typesWater table