Seepage pressure the pressure exerted by water on soil through which it percolates is known as seepage pressure $(P_s)$ it is given by:

$P_s = h * \eta_w$

$ P_s=\frac{h}{L} * L * \eta_w$

$P_s = iL\eta_w$

Quicksand condition: The seepage is responsible for the phenomenon of quicksand this condition occurs when the flow takes place in an upward direction.

When the flow takes place in an upward direction the effective pressure get reduced since the seepage pressure also acts in an upward direction when the. Seepage pressure becomes exactly equal to the submerged weight of the soil through which the flow is taking place the effective pressure becomes zero, in this case, the soil with fewer Cohesion losses all of its shear strength and soil particles move up in the direction of flow.

This lifting of soil particle is known as quick sand boiling condition or quicksand condition during this condition the effective pressure reduces to zero.

$P_e = L\gamma^/ - P_s = 0$

$P_s = L\gamma^/$

$iL\gamma_w = L\gamma^/$

$i\gamma_w = \gamma^/$

$i = \frac{\gamma^/}{\gamma_w}$

$\gamma^/ = \frac{(G-1) \gamma_w}{1+e}$

The hydraulic gradient of the quick sand condition is known as critical hydraulic gradient $(i_c)$ does quick sand condition is the particular flow condition which occurs when effective pressure reduces to zero doing upward flow.