In soils with high shrink-swell behavior, which clay mineral type is typically involved?

• A. 1:1 clays
• B. 2:1 clays
• C. 3:1 clays
• D. 4:1 clays

Answer: (B)

Shrink-swell in soils is driven by the ability of certain clays to expand when they hydrate. Two-to-one clays have a layered structure with two tetrahedral sheets sandwiching an octahedral sheet, creating interlayer spaces that readily take up water and swell. The layers carry permanent negative charges from isomorphous substitution, so exchangeable cations sit between layers and water can enter, causing the layers to separate and the soil to swell when wetted, then contract when dried. This expansive potential is most pronounced in clays of the smectite group, a classic example of 2:1 expandable clays.

One-to-one clays, like kaolinite, have little interlayer space and strong bonding between layers, so they don’t swell much. Terms like 3:1 or 4:1 aren’t the typical descriptors used for this shrink-swell behavior, and they don’t indicate the expansive potential seen in the 2:1 clays.