Effect of Gravel on Soil Porosity and Water Infiltration of Lime Soil in Southwest Karst Area

[Objective] Gravel is an important factor affecting soil porosity and water infiltration. Therefore, it is of great significance to determine the specific soil porosity characteristics that are impacted by gravel-induced changes. [Methods] Taking lime soil with high gravel content in southwest karst area as the research object, CT scanning and one-dimensional soil column infiltration test were used to compare and analyze the soil pore characteristics and water infiltration characteristics under two gravel particle sizes (2.0~5.0 and 5.0~12.5 mm) and five gravel contents (0, 10%, 20%, 30%, 40%). The structural equation model was used to explore the coupling relationship between soil pore characteristics and infiltration characteristics. [Results] (1) The macroporosity, pore connectivity, pore shape factor, pore surface area and pore volume of gravel-bearing soil are higher than those of gravel-free soil. When the gravel content was 20% and 30%, there was significant difference between pore connectivity and pore shape factor (p < 0.05). (2) Gravel improved soil infiltration performance, with stable infiltration rates of gravel content from 10% to 40% being 4.02, 5.00, 2.88 and 5.14 times higher than those of gravel-free soils, and cumulative infiltration being 1.67, 1.76, 2.49 and 2.39 times higher, respectively, for particle sizes ranging from 2.0 to 5.0 mm. When the particle size was 5.0~12.5 mm, the stable infiltration rate was 2.20, 2.67, 4.78 and 2.78 times of that without gravel, and the cumulative infiltration amount was 1.42, 1.75, 2.46 and 2.02 times of that without gravel, respectively. (3) In the structural equation model, gravel content was positively correlated with pore connectivity and pore equivalent diameter. Pore connectivity had the greatest influence on the steady infiltration rate, and pore equivalent diameter had the greatest influence on the cumulative infiltration rate. [Conclusion] Gravel content promotes water infiltration mainly by increasing soil pore connectivity and pore equivalent diameter. The research results can provide scientific basis and theoretical reference for further understanding the hydrological process of gravel-bearing soil in southwest karst area.