Effect of Liquefaction on the Built Environment
The liquefaction phenomenon by itself may not be particularly damaging or hazardous.
Only when liquefaction is accompanied by some form of ground displacement or ground failure is it destructive to the built environment.
For engineering purposes, it is not the occurrence of liquefaction that is of prime importance, but its severity or its capability to cause damage.
Adverse effects of liquefaction can take many forms. These include:
flow failures; lateral spreads; ground oscillation; loss of bearing strength; settlement; and increased lateral pressure on retaining walls.

Flow Failures
Flow failures are the most catastrophic ground failures caused by liquefaction.
These failures commonly displace large masses of soil laterally tens of meters and in a few instances, large masses of soil have traveled tens of kilometers down long slopes at velocities ranging up to tens of kilometers per hour.
Flows may be comprised of completely liquefied soil or blocks of intact material riding on a layer of liquefied soil.

Flows develop in loose saturated sands or silts on relatively steep slopes, usually greater than 3 degrees (Fig. 2).

Fig. 2 - Diagram of a flow failure caused by liquefaction and loss of strength of soils lying on a steep slope. [after EERI]

The strength loss creates instability and flow down the steep slope (Youd, 1992).