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Which of the following soil is practically impermeable?
gravel
sand
clay
Coarse sand
clay
Clay is practically impermeable because its particles are extremely small (typically <0.002┬аmm), resulting in very small void spaces that restrict the flow of water ┬╖ Due to the high specific surface area, water is held tightly around particles by adsorption forces, creating a very low coefficient of permeability (k).
Clay is practically impermeable because its particles are extremely small (typically <0.002┬аmm), resulting in very small void spaces that restrict the flow of water ┬╖ Due to the high specific surface area, water is held tightly around particles by adsorption forces, creating a very low coefficient of permeability (k).
v=ki тАФ Darcy's Law, where v is discharge velocity, k is permeability coefficient, and i is hydraulic gradient.
k=iAQтАЛ тАФ Calculation of permeability based on flow rate Q and cross-sectional area A.
The permeability of soil is governed by Darcy's Law: v=ki. In clay, the pore channels are so narrow that the viscous drag forces of water molecules against the particle surfaces dominate over the gravitational flow ┬╖ This leads to a permeability coefficient often as low as 10тИТ7┬аto┬а10тИТ9┬аcm/s, effectively making it an aquiclude or an impermeable layer in engineering applications.
Clay has high plasticity and small particle size compared to gravel and sand.
The permeability of soil depends on void ratio (e) and particle size (D10тАЛ); per Hazen's formula kтИЭD102тАЛ.
Impermeability makes clay ideal for liners in landfills and dams.
Excellent for water-tight cores in earthen dams.
High mineral surface area allows for cation exchange capacity.
High susceptibility to volume change (swelling/shrinkage).
Extremely slow rate of consolidation under load.
Core of earth dams
Landfill liners
Grouting for seepage control
Gravel and sand are considered highly permeable soils, often classified as pervious.
Clay is defined by particle sizes typically smaller than $0.002 \text{ mm}$ (clay fraction) in soil classification systems like the ISSS.
C is correct тАФ Clay particles are so fine and their interconnected voids so small that water movement is negligible, classifying it as practically impermeable.
Always remember that permeability k is proportional to the square of particle size (D102тАЛ); thus, the transition from sand to clay causes a drastic drop in k by several orders of magnitude.