What causes soil liquefaction?

Answer 1

Liquefaction is the name given to the process that converts a solid soil mass into a liquid.

Liquefaction occurs in cohesionless soils (typically those with a higher content of larger grains such as sand sized clasts) which have water in the pore spaces, and are poorly drained.

When the seismic waves from the earthquake pass through the soil, the vibrations cause the individual grains in the soil to move around and re-adjust their positions. This ultimately results in a decrease in volume of the soil mass as the grains pack more tightly together (a reduction in porosity).

The pore water which was originally in those spaces becomes compressed. Water is relatively incompressible and as such it pushes back against the soil grains (more correctly this is described as an increase in pore water pressure). The pore pressure becomes so high, that the soil grains become almost buoyant causing a significant drop in the shear strength of the soil to a very low value.

When this occurs the soil loses it's ability to support loads (technically described as a loss of bearing capacity) which can cause subsidence of building foundations.