Correct Answer - Option 2 : Becomes zero
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 condition:
Effective stress = γ'z - ps = 0
γ'z = ps = i × z × γw ,( where ps = seepage pressure = i.z.γw )
γ = i × γw
\(i = \frac{{\gamma '}}{{{\gamma _w}}} = \frac{{G - 1}}{{1 + e}}\) , (where \(\gamma ' = \frac{{G - 1}}{{1 + e}}.{\gamma _w}\))
\({i_c} = \frac{{G - 1}}{{1 + e}}\)
The hydraulic gradient at which quicksand condition occurs is termed as critical hydraulic gradient, piping gradient, floating gradient, or bursting gradient.
