Quicksand condition is the floatation of particles of cohesionless soil, like fine gravel and sand, due to vertical upward seepage flow. As sand boiling occurs, the bearing capacity and shear strength of the cohesionless soil decrease and the agitations of soil particles become apparent.
Quicksand condition is not a type of soil but a flow condition that occurs in cohesionless soils. Practically, boiling condition may occur when excavations are made below the water table and water is pumped out from the excavation pit to keep the area free from water.
How Quicksand Condition Occurs?
Quicksand condition occurs when seepage pressure, which acts in the upward direction, overcomes the downward direction pressure due to weight of soil, and the sand grains are forced apart. The result is that the soil has no capability to support a load.
The soil that experiences quicksand condition would lose shear strength and bearing capacity. The shear strength of cohesionless soil depends on the effective stress. The shear strength is given by:
The effective stress is given by the following expression:
The terminologies of equation 2 are explained and illustrated in fig. 2. Plugging components of equation 2 results in the following expressions:
So, equation 5 may be expressed as follows:
Substituting the value of submerged unit weight in terms of void ratio:
Taking G=2.67, and e=0.67, the result of equation 9 is equal to one.
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 exceeds, 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.
Quicksand cannot 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 quicksand. However, quicksand 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 quicksand in panic.
If a person is caught in quicksand 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 quicksand.
When there is some surcharge on the cohesionless soils, the head required to cause quicksand increases.
Quicksand Condition at Construction Site
There are a number of construction sites which are susceptible to quicksand conditions:
- Excavations in granular materials behind cofferdams alongside rivers.
- Any place where artesian pressures exist i.e. where head of water is greater than the usual static water pressure.
- Behind embankments to protect floods.
How to Avoid Quicksand Condition?
It can be prevented by lowering the water table at the site before excavation or alternatively, by increasing the length of upward flow. Boiling condition is also common when a pervious sand stratum underlying a clay soil is in an artesian pressure condition.