Quick sand conditions occur in cohesionless soils like sand and fine gravel when upward seepage flow reduces the effective pressure in the soil to zero. This causes the soil grains to lose their shear strength and bearing capacity, violently agitating as the soil behaves like a liquid. It occurs when the hydraulic gradient reaches a critical value that equalizes the upward seepage pressure and downward pressure of the submerged soil weight. Cohesive soils and gravel soils do not experience this condition because clays retain some shear strength even at zero effective pressure, while gravel soils require higher seepage pressures to exceed their self-weight.

2. SUBJECT TEACHER;
 Respected Sir,
Umair Masood Memon

3. Khalil ur Rehman dawar
L-14CE-55

4. Topic to be present;
QuickSand
condtions in soil

5. Quick sand condition or boiling
Quick sand condition is a condition of flow, not a type of
soil, in which a vertical upward seepage flow causes
floating condition of a particle in cohesion less soil such
as Sand and fine gravel .

6.  In such cases, cohesion less soils lose all of their shear
strength and bearing capacity and a visible agitation of soil
grains is observed. This phenomenon is known as boiling
or a quick sand condition

7. When occur such condition ..?
 When seepage flow takes place in an upward direction, the
seepage pressure also acts in the upwards direction and the
effective pressure is reduced.
 There is also a pressure in downward direction due to
submerged weight of soil.
 If seepage pressure becomes equal to the pressure due to
submerged weight of soil, the effective pressure is reduced
to zero.

8. The water surface in container B is kept above that of A
by h units. This arrangement permits water to flow
upwards through the sample in container A.
The total piezometric or the pore water head at the
bottom of the sample is given by (z1+z2+h)
Therefore, the pore water pressure uc at the bottom of
the sample is

9. The total pressure head at the bottom of the sample is
Thus the effective pressure at the bottom of sample is, there
fore

10. The general equation for effective pressure at any
depth Z is given as:
This indicates that there is a decrease in the effective
pressure due to upward flow of water
At any depth z: is the pressure of the submerged
soil acting downward and is the seepage
pressure acting upward.
The effective pressure becomes zero when

11. It indicates that the effective pressure reduces to
zero when the hydraulic gradient attains a
maximum value which is equal to the ratio of the
submerged unit weight of soil and the unit weight
of water.
This gradient is known as the critical hydraulic
gradient ic.

12.  In such cases, cohesion less soils lose all of their shear
strength and bearing capacity and a violent agitation of soil
grains is observed. This phenomenon is known as boiling
or a quick sand condition.

13. Quick sand can not support the weight of man or
animal and it behaves like a liquid with a unit weight
about twice that of water. A person can easily float in it
with about one third of his body out of quick sand

14. Why quick sand condition not
occurs in cohesive soil and in
gravel soil …?

15. In clays the shear strength of soil does not get
reduced to zero even when the effective stress
becomes zero, due to its cohesion. Hence,
quicksand condition does not occur in clays

16. As the pressure of the escaping water exceeds
the weight of the soil and the sand grains are
forced apart. The result is that the sandu soil
has no capability to support the load.
BUT
Pebbles or gravelly soils cannot have because it
has to exceed the self weight than those of
escaping water weights.