Which statement best describes soils that are predominantly 2:1 clay minerals (including mixed mineralogy clays) in terms of clay mineralogy?

• A. Suitable
• B. Unsuitable
• C. Provisionally suitable
• D. Not classified

Answer: (B)

Soils with predominantly 2:1 clay minerals have layered silicate structures that swell strongly when wet and shrink when dry. This shrink-swell behavior destabilizes soil structure, reduces macroporosity, and often leads to low or highly variable infiltration. In septic system design, you want stable, permeable soil that allows effluent to infiltrate predictably; expansive 2:1 clays disrupt that, creating perched water and unreliable drainage. Even mixed mineralogy clays that include these 2:1 minerals can exhibit the same problematic behavior, so they’re not suitable for absorption fields. That’s why this type of soil is considered unsuitable.