Terzaghi’s principle of effective stress

When flow takes place in an upward direction, the seepage pressure also acts in the upward direction and effective stress is reduced. If the seepage pressure becomes equal to the pressure due to the submerged weight of the soil, the effective stress is reduced to zero.

In such a case, cohesionless soil losses all its shear strength and the soil particles have a tendency to move up in the direction of flow. This phenomenon of the lifting of soil particles is called quicksand or quick condition. It is also known as boiling sand. It occurs in the case of granular materials like clay, silt and sand.