Geotechnical Engineering

DHARM

SEEPAGE AND FLOW NETS 193

sand becomes ‘‘Quick’’ or ‘‘Alive’’ at this gradient; this is only a conditions and not a type of
sand.

According to Darcy’s law, the velocity at which water flows varies as the permeability,
in order to maintain a specified hydraulic gradient such as unity. This explains the fact that
quicksand conditions occur more commonly in fine sands with low permeability. In case of
gravels with high permeability, much higher velocity of flow will be required to cause the
‘‘quicksand condition’’.

Quicksand conditions are likely to occur in nature in a number of instances; however,
the widespread belief that animals and man could be sucked into the quicksand is a myth,
since the unit weight of the saturated sand is nearly double that of water. However, quicksand
conditions present constructional difficulties. When the exit gradient for a hydraulic structure
like a dam assumes the critical value, boiling occurs. This may lead to the phenomenon of
progressive backward erosion in the form of a pipe or closed channel underneath the structure
and ultimately failure of the structure. This is called, ‘‘piping’’. The ratio of the critical gradi-
ent to the actual exit gradient is called the ‘‘factor of safety against piping’’.

In summary, we may note:

1. ‘‘Quick’’ refers to a condition and not to a material.


2. Two factors are required for a soil to become quick: Strength must be proportional to
   effective stress and the effective stress must be equal to zero.


3. The upward gradient needed to cause a quick condition in a cohesionless soil is equal
   to γ′/γw and is approximately equal to unity. This leads to boiling, piping and ultimate failure of
   the structure.


4. The amount of flow required to maintain quick condition increases as the permeabil-
   ity of the soil increases.

6.10 Seepage Forces

Quicksand conditions are caused by seepage forces. These forces have importance in many
situations, even when there is no quick condition. Seepage forces are present in clays through
which flow occurs, but cohesion prevents the occurrence of boiling.

Referring to Fig. 6.26, the head h is expended in forcing water through the pores of the
soil. This head is dissipated in viscous friction, a drag being exerted in the direction of motion.

The effective weight of the submerged mass is the submerged weight γ′. LA or ()

()

G

e

−

+

1

1

γw. LA. An upward force h. γw. A is dissipated, or transferred by viscous friction into an
upward frictional drag on the particles. When quick condition is incipent, these forces are
equal:

hγw. A =

()

()

G

e

−

+

1

1. γw. L. A ...(Eq. 6.32)

which again leads to equation 6.31. The only difference between Eqs. 6.30 and 6.32 is that both
side of Eq. 6.30 include a force γwLA.