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Stiff clays are ______________ permeable.
Highly
Least
Partially
None of the mentioned
Least
Stiff clays are classified as having extremely low permeability because of their very small particle size and high degree of surface area. Their pore spaces are interconnected but extremely narrow, which significantly hinders the flow of water, making them nearly impermeable in practical geotechnical applications.
Stiff clays are classified as having extremely low permeability because of their very small particle size and high degree of surface area. Their pore spaces are interconnected but extremely narrow, which significantly hinders the flow of water, making them nearly impermeable in practical geotechnical applications.
k=ivтАЛ тАФ Coefficient of permeability where v is velocity and i is hydraulic gradient
k=CтЛЕD102тАЛ тАФ Hazen's empirical formula for permeability
Permeability in soils is governed by Darcy's Law, v=ki, where k is the coefficient of permeability. In stiff clays, the clay mineral structure (such as montmorillonite or illite) creates a very high specific surface area, leading to strong adsorption of water molecules. This effectively reduces the 'effective porosity' or the interconnected voids available for fluid movement, resulting in a coefficient of permeability (k) typically in the range of 10тИТ7 to 10тИТ10 cm/s.
Clays consist of plate-like particles that orient themselves to minimize flow paths.
The adsorbed water layer (diffuse double layer) around clay particles reduces the effective size of pore channels.
Stiff clays show lower permeability compared to soft clays due to higher density and reduced void ratio (e).
Excellent for constructing impervious cores in earth dams.
Acts as an effective barrier for landfill liners to prevent leachate migration.
High susceptibility to volume change (shrink/swell behavior).
Consolidation settlement occurs very slowly due to low permeability.
Core material for zoned earth dams.
Foundation base for containment lagoons.
The coefficient of permeability k is inversely proportional to the viscosity of the pore fluid.
Option B (Least) is correct because as soil texture becomes finer (from gravel to clay), the permeability decreases exponentially.
B is correct тАФ Stiff clays have the lowest coefficient of permeability among standard soil types due to their microscopic void spaces and strong water-adsorption properties.
Always remember that permeability decreases as grain size decreases; if a question compares clay, silt, and sand, clay will always be the least permeable.