QUICK SAND CONDITION IN COHESIONLESS SOILS

The shear strength of cohesionless soil depends upon the effective stress. The shear strength is give by

Where = effective stress.

= angle of shearing resistance.

The effective stress is reduced due to upward movement of flow of water. When the head causing upward flow is increased, a stage is reached when the effective stress is reduced to zero. The condition so developed is known as quick sand condition. Quick sand is not any special type of sand, but it is a condition of zero effective stress in cohesionless soils.

Effective stress is given by

Since

For effective stress to become zero,

Therefore,

Substituting the value of submerged unit weight in terms of void ratio,

Taking G=2.67, and e=0.67

Thus the effective stress becomes zero for the soil with above values of G and e, when the hydraulic gradient ‘i’ is unity, i.e. head causing the flow is equal to the length of the specimen.

If the critical gradient is exceeded, the soil moves upward, and the soil surface appears to be boiling. The quick condition is also known as boiling condition. During this stage, a violent and visible agitation of particles occurs. The discharge suddenly increases due to an increase in the coefficient of permeability occurred in the process. If a weight is placed on the surface of the soil, it sinks down. The soil behaves as a liquid having no shear strength.

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. However quick sand is highly viscous and movement in it would require a great effort and energy. A person may die by drowning (suffocation) if he gets tired and let his head fall into the quick sand in panic.

If a person is caught in quick sand conditions, he should keep his head high above the soil surface and move slowly towards the bank. He should try to catch some tree on the bank or try to pull himself out the quick sand.

The quick sand only occurs in cohesionless soils, as cohesion is always there in cohesive soils, which does not allow the shear strength to be zero.

When there is some surcharge on the cohesionless soils, the head required to cause quick sand increases.