For a piping phenomenon to occur in soil, the most important condition to be satisfied is that 

Explanation:

• When flow takes place in an upward direction, seepage pressure (ps) also acts in an upward direction and effective stress is reduced.
• If seepage pressure equals the submerged weight of soil mass then effective stress reduces to zero. In such a case, cohesionless soil loses all its shear strength and has the tendency to flow along with water. This phenomenon is known as the quicksand condition.

For quicksand / Piping condition:

Effective stress = γ'z - Ps = 0

γ'z = Ps = i × z × γw

Where Ps = Seepage pressure = i z γw

γ = i × γw

i = \(\left ( \frac{\gamma '}{\gamma _{w}} \right )=\frac{G-1}{1+e}.\gamma _{w}\)

ic = \(\frac{G-1}{1+e}\)

Note:

The hydraulic gradient at which quicksand condition occurs is termed as the critical hydraulic gradient piping gradient, floating gradient, or bursting gradient.

For piping, the hydraulic gradient should be more than or equals to the critical hydraulic gradient.  ​

Important Points

• For most of the soils, the critical hydraulic gradient is close to unity. So when the hydraulic gradient is nearly unity piping will occur.