QUICKSAND

The shear strength of a cohesionless soil depends upon the effective stress. The shear strength is given by:

Where = effective stress
φ = angle of shearing resistance

Let us consider a soil specimen of length L subjected to an upward pressure shown in figure 1. Let us consider the stresses developed at section C – C.

Figure 1. Quicksand Condition

When water flows in an upward direction through soil, the effective pressure is given by:

where ps = seepage pressure

If the seepage pressure becomes equal to the submerged weight of the soil, the effective pressure of the soil reduces to zero. In such a case, a cohesionless soil loses all its shearing strength and the soil cannot support any load. In other words, the soil particles ted to be lifted up along with the flowing water. The soil is said to have become ‘quick’ or ‘alive’ and boiling will occur. The popular name of this phenomenon is quicksand. It may be emphasized that quicksand is not a type of sand but only a hydraulic condition occurring within a cohesionless soil when its effective pressure is reduced to zero due to upward seepage force. Thus during the quicksand condition:

The hydraulic gradient ic at which the quick condition occurs is called the critical hydraulic gradient. For loose deposits of sand or silt, if void ratio e is taken as 0.67 and G as 2.67, the critical hydraulic gradient works out to be unity.

Seepage forces affect sands more than clays because sands do not possess cohesion, while fine sands and silts have some inherent cohesion which holds the soil grains together even at the critical hydraulic gradient. In sands, the shear strength s is given by:

Hence, when

In clays, however,

The cohesion component of shear strength is independent of . Boiling does not occur in coarse sands and gravels either, because these soils are highly pervious; hence, according to Darcy’s law, large discharges are required to produce a critical gradient of unity and such flows rarely materialize in practice.

When a natural soil deposit becomes quick, it cannot support the weight of a man or an animal. But contrary to common belief, the soil does not suck the victims beneath its surface. As a matter of fact, quick sand 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 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 and try to pull himself out of the quick sand.