Objective This study investigates the expansive characteristics of bund soil in purple-soil sloping farmlands under varying initial moisture contents and bulk densities, aiming to provide a scientific basis for optimizing bund management and protection measures.

Method The research focused on typical bund soil from purple-soil sloping farmlands in the Three Gorges Reservoir Area. A series of simulation experiments were conducted to examine the water absorption and expansion processes of bund soil under different initial moisture contents and bulk densities. The effects of these factors on expansion height, unloaded expansion rate, expansion rate, expansion change rate and other parameters were analyzed.

Results  ① The expansion time curve of bund soil can be divided into three stages: rapid linear expansion (I), slow nonlinear expansion (II), and stable expansion (III). The linear expansion rate varied from 0.33 % h⁻¹ to 30.09 % h⁻¹. Lower initial moisture content led to faster linear expansion rates, while higher initial bulk density resulted in slower linear expansion rates and longer durations. The range of variation in unloaded expansion rate and linear expansion rate increased with higher initial bulk density but decreased with higher initial moisture content. ② Under the same initial bulk density, the unloaded expansion rate and expansion rate were significantly negatively correlated with initial moisture content (P < 0.01). Conversely, under the same initial moisture content, the linear expansion rate was significantly negatively correlated with initial bulk density (P < 0.01). The duration of the linear expansion stage was significantly positively correlated with initial bulk density (P < 0.01). The unloaded expansion rate ranged from 1.25% to 16.15%. When initial moisture content was low (100 g kg⁻¹ ), the unloaded expansion rate increased with increasing initial bulk density. However, when initial moisture content exceeded 150 g kg⁻¹, the unloaded expansion rate initially increased (bulk density 1.2 - 1.4 g cm⁻³) and then decreased (bulk density 1.5 - 1.7 g cm⁻³) as initial bulk density increased. ③ The unloaded expansion rate and linear expansion rate were significantly affected by the interaction between initial moisture content and initial bulk density. As both factors increased simultaneously, the unloaded expansion rate and linear expansion rate generally showed a decreasing trend.

Conclusion  Appropriately increasing the bulk density of purple soil bunds and to enhancing moisture content through soil and water conservation measures can effectively control soil expansion potential.