Quick sand condition

Quick sand condition is the state in which the effective stress of a cohesionless soil becomes zero because of upward seepage of water. The soil then has no shear strength and behaves like a liquid. It is not a type of sand; any cohesionless soil (sand, silt) can become "quick".

Explanation (upward flow)

Consider a soil layer of thickness $L$ with water flowing upward under a head difference $h$.

   h  ↑  water level in standpipe
  ----+----------------
      |   water
  ----+----------------  top of soil
      |   sand  L
  ----+----------------  base of soil
        ↑ ↑ ↑ upward flow

At the base of the soil layer:

• Total stress $\sigma = \gamma_{sat} L$
• Pore pressure $u = \gamma_w (L + h)$ (excess head $h$ added)
• Effective stress $\sigma' = \gamma_{sat}L - \gamma_w(L+h) = \gamma' L - \gamma_w h$

As $h$ increases, $\sigma'$ falls. When $\sigma' = 0$:

$$\gamma' L = \gamma_w h \quad \Rightarrow \quad i_c = \frac{h}{L} = \frac{\gamma'}{\gamma_w} = \frac{G-1}{1+e}$$

At this critical hydraulic gradient the seepage force just balances the submerged weight of the soil. The sand particles lose contact, boil up and the soil loses bearing capacity. For most sands $i_c \approx 1$.

Quick condition often causes failure of excavations, cofferdams and the downstream toe of hydraulic structures (piping).