In the Unified Soil Classification System, the symbol CL refers to

• A. Low plastic clay with relatively high porosity and low permeability
• B. High plastic clay
• C. Silty clay with high permeability
• D. Organic clay

Answer: (A)

CL denotes a clay that has low plasticity. In the Unified Soil Classification System, the C stands for clay and the L indicates low plasticity, meaning the soil has a small plasticity index and does not become highly plastic as moisture changes. This type of clay is cohesive but relatively stiff and not very plastic.

Clays, in general, have very low permeability because of their tiny pore throats, so a low-plasticity clay tends to restrict fluid flow even more. Porosity can be comparatively high in clays (lots of void space relative to densely packed material), but that does not mean the soil will transmit water quickly—the permeability remains very low.

So, the description that best fits is a clay with low plasticity, distinguishing it from high-plasticity clays, silty clays with higher permeability (clays typically have low permeability), or organic clays (which are classified differently).