However, if the loading is rapidly applied and large enough, or is repeated many times (e.g., earthquake shaking, storm wave loading) such that the water does not flow out before the next cycle of load is applied, the water pressures may build to the extent that it exceeds the force ( contact stresses) between the grains of soil that keep them in contact. In response to soil compressing, the pore water pressure increases and the water attempts to flow out from the soil to zones of low pressure (usually upward towards the ground surface). If the soil is saturated by water, a condition that often exists when the soil is below the water table or sea level, then water fills the gaps between soil grains ('pore spaces').

Dense sands, by contrast, tend to expand in volume or ' dilate'. This is because a loose sand has a tendency to compress when a load is applied. The phenomenon is most often observed in saturated, loose (low density or uncompacted), sandy soils. the initial movement of some part of the material might result in accumulating pressure, first on one point, and then on another, successively, as the early points of concentration were liquefied. If the pressure of the water in the pores is great enough to carry all the load, it will have the effect of holding the particles apart and of producing a condition that is practically equivalent to that of quicksand. He described the mechanism of flow liquefaction of the embankment dam as: In soil mechanics, the term 'liquefied' was first used by Allen Hazen in reference to the 1918 failure of the Calaveras Dam in California.

Soil liquefaction occurs when a cohesionless saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress such as shaking during an earthquake or other sudden change in stress condition, in which material that is ordinarily a solid behaves like a liquid. The 2011 earthquake caused a layer of water and fine sand to collect on the surface of this street.

Soil material that is ordinarily a solid behaving like a liquid Some effects of soil liquefaction after the 1964 Niigata earthquake Soil liquefaction allowed this sewer manhole to float upward and breach the pavement during the 2004 Chūetsu earthquake Soil liquefaction in Christchurch.

Quicksand where does it exist