Quick Sand Condition in cohesionless soils
Quick Sand Condition
Quicksand condition is a condition of flow in which vertical upward seepage flow causes floating condition of particles in cohesionless soils such as sands and fine gravels due to loss of shear strength in between particles. Quicksand is not a type of sand.
Here the sand boiling occurs, and the bearing capacity and the shear strength of this type of soil decreases, and the agitations of the soil particles are then apparent.
Quicksand condition is actually not a type of soil rather this is a flow condition of the soil that occurs in cohesion less soils.
ic = (G-1)/(1+e)
where ic = critical hydraulic gradient
G = Specific gravity
e = void ratio
- The range of void ratio(e) between which quick sand condition occurs in cohesionless granular soil deposits is 0.6 to 0.7.
- For Sandy soils G is lies between 2.6 to 2.7.
At quicksand condition, soil particles are removed with water, which creates piping below the hydraulic structure which is called piping failure.
To prevent the piping failure or quicksand condition, the hydraulic gradient permitted should be less than the critical hydraulic gradient.
- The factor of safety against quicksand condition of piping failure is defined as
FOS=Critical hydraulic gradient/Hydraulic gradient
FOS = ic/i
Improvement in bearing capacity of Soils
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- By providing drainage to improve the bearing capacity of certain soils. Drains are laid in trenches just at the footing base.The sub-soil water thus collected is drained out through a system of pipe drains provided outside the external walls of the building.
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- By confining the soil in an enclosed area with the help of sheet piles.This method is used with advantage in shallow foundations in sandy soils.
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- By increasing the depth of the footing.This method is restricted to sites where the subsoil water level is much below and deep excavations do not increase the cost of foundations.
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- By blending granular material, like sand, gravel or crushed stone into the natural soil by ramming.The layer of soil thus formed is much stronger and improved.
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- By driving sand piles-in this holes are made in the soft soil with the help of wooden piles or other means and then sand is filled in the holes and rammed. They are called sand piles.
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[…] Sand-containing gravels usually have a friction angle of 34° to 48°. Loose to dense sand has a friction angle of 30° to 45°. Silts have a friction angle of 26° to 35°, and clay has a friction angle of about 20°. […]