Q. What is liquefaction?
A. Liquefaction occurs in loose sand and sandy soils when it is saturated and subject to strong seismic shaking. In a dry sand, the shaking would normally joggle the sand particles into a denser, more compact arrangement, but with water filling the voids between the particles, the water becomes pressurised instead. This can result in the
- almost complete loss of friction between the particles and hence soil strength
- consequences of liquefaction including:
o Foundation bearing failure as the soils under shallow footings loses strength and it is no longer able to support the loads.
o Ground settlement, as the originally loose sand settles back into a somewhat denser state, sometimes as much as 0.3m or more.
o Ground movement as the surface unliquefied crust breaks and is jostled by the earthquake shaking (analogy is that of ice floes on the sea).
o Ejection of sand and water from the pressurised liquefied layer(s).
- lateral spread along any change of slope such as terrace risers and river banks, which may result in permanent ground displacement of 1.5 – 2m at the “free” face (ie the bank) and measurable displacements up to 300m back from the bank.
Once the groundwater pore pressures have returned to normal, the sand is essentially back to its pre-earthquake state. As such it can be built on again, in a similar way as before, except that it must be recognized that sands which liquefy in one earthquake have been shown to re-liquefy in a subsequent earthquake; it's not a one-off phenomena which can now be ignored .