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Soil curling is a common phenomenon in nature due to the rearrangement of soil particles caused by moisture loss. The occurrence of curling in soils significantly affects soil performance in various disciplines. Despite its importance, most existing studies describe the soil curling process within the context of soil desiccation cracking, where boundary conditions facilitating soil curling are not well controlled, or often use the final stage of the desiccation process to infer the soil curling behaviour. Consequently, the underlying soil curling mechanism, the state of the curled soil, and the influencing factors (i.e., clay type, drying temperature, initial water content, and sand content) are not fully understood. In this study, soil curling tests were conducted to study the above-mentioned issues in different types of soils under well-controlled boundary and environmental conditions. It was found that natural clays consisting of higher portions of smectite underwent both upward curling (concave-up) and downward curling (convex-up), while artificial clay experienced only concave-up curling. Concave-up curling initiated when the samples were almost in the saturated condition, while convex-up curling started when the water content of samples was close to their plastic limits. The influencing factors had a profound effect on the moisture evaporation and thus on the soil curling state and its lift-off height. Finally, a conceptual model isproposed to explain the soil curling mechanism and factors influencing soil curling.

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