Quicksand Phenomenon Explained

Quicksand is a hazardous condition that can occur in saturated, loose, cohesionless soils like sand or silt. It happens when there is an upward flow of water through the soil pores, and the pressure exerted by this upward flow becomes large enough to counteract the effective weight of the soil particles.

Conditions Leading to Quicksand

The critical condition for quicksand occurs when the effective stress in the soil becomes zero. Effective stress ($\sigma'$) is the stress transmitted through the soil particle skeleton, which is responsible for the soil's strength and stiffness. It is related to the total stress ($\sigma$) and pore water pressure ($u$) by the equation:

$$\sigma' = \sigma - u$$

When the upward seepage pressure ($u_{seepage}$) is equal to the total stress ($\sigma$) at a certain depth, the effective stress ($\sigma'$) becomes zero.

$$\sigma' = \sigma - u_{seepage} = 0$$

This condition is known as the quick condition or boiling condition.

Relationship with Shear Strength

For cohesionless soils (like sand), the shear strength ($\tau$) is primarily derived from the effective stress and the angle of internal friction ($\phi$). According to the Mohr-Coulomb failure criterion for cohesionless soils:

$$\tau = \sigma' \tan\phi$$

When the effective stress ($\sigma'$) becomes zero under quicksand conditions, the shear strength ($\tau$) of the soil also becomes zero.

$$\tau = 0 \times \tan\phi = 0$$

This means the soil loses its ability to support any load and behaves like a heavy fluid, making it difficult or impossible to stand on.

Soil Particle Movement in Quicksand

In quicksand, the upward flow of water exerts an upward drag force on the soil particles. When this upward drag force becomes equal to the submerged weight of the soil particles, the particles become suspended in the water. This gives the soil a fluidized appearance, and particles may move upwards or become unstable.

Examining the Options

Let's look at the given options in relation to quicksand:

• Option 1: Effective pressure becomes zero. This is a fundamental condition for quicksand. When effective stress is zero, the soil loses its strength.
• Option 3: Cohesionless soil loses all its shear strength. As explained above, when effective stress is zero in cohesionless soil, the shear strength becomes zero. This is a direct consequence of the quick condition.
• Option 4: soil particles have a tendency to move up in the direction of flow. This describes the fluidization of the soil due to upward water flow, which is characteristic of quicksand.
• Option 2: It is a type of sand wherein void ratio is 1. The void ratio ($e$) is the ratio of the volume of voids to the volume of soil solids ($e = V_v / V_s$). While quicksand occurs in loose sands which typically have high void ratios, a void ratio of exactly 1 is not a defining property of quicksand. Quicksand can occur in sands with various void ratios, provided the critical hydraulic gradient is reached for that specific soil's properties (including its void ratio and specific gravity). A void ratio of 1 is a specific value and not a necessary or universal characteristic of quicksand.

Based on this analysis, the statement that quicksand is a type of sand wherein void ratio is 1 is not necessarily true or directly related to the phenomenon of quicksand itself, unlike the other options which describe the conditions and consequences of the quick condition.