Effects of the intensity and time of wetting-drying cycles on the pore distribution of granite red soil

Abstract: The wetting-drying cycle is a cycle of reciprocal action of soil experiencing multiple soil evaporation and wetting, which is a key environmental factor affecting the formation of changes in the pore structure of the soil. Rainfall is abundant in the granite regions of southern China, and geological disasters such as crumbling hills and landslides triggered by seasonal rainfall are widespread. To investigate the impact of wetting-drying cycle time as well as intensity on the pore distribution of granite red soil, three wetting-drying cycle intensities (10% water content to saturated, 20% water content to saturated, and 30% water content to saturated) and five wetting-drying cycle times (0, 2, 4, 7, 10 times) were set in this study. Moreover, the soil-water characteristic curves of soil samples were measured and the equivalent pore size percentage distribution was computed indirectly utilizing a 1-1500 type 15-bar pressure film meter. Simultaneously, the pore fractal dimension was introduced to quantify the effect of wet and dry cycling on the variation of soil pore structure. The findings indicated that the water content of the soil water characteristic curve in each suction section tended to decrease following wetting-drying cycles, the pore size distribution range of the soil gradually increased, and the water-holding characteristics gradually declined. The equivalent pore size calculation highlighted that the equivalent diameter of granite red soil was primarily concentrated in <0.2 μm and >57 μm which had a significant effect on the three types of equivalent pores of <0.2 μm, 3-15 μm and >57 μm. As the time of wetting-drying cycles increased, the content of large pores (>30 μm) increased rapidly while the content of inactive pores (<0.2 μm) decreased rapidly; the redistribution of soil pores occurred primarily during the first four times, after which the influence effect gradually decreased and stabilized. Various types of pores developed further, as the intensity of wetting-drying cycles increased, and inactive pores (<0.2 μm) as well as medium pores (0.2-30 μm) gradually developed into large pores (>30 μm). The findings of ANOVA pointed out that the intensity and time of the wetting-drying cycle had different effects on various types of pores, and the wetting-drying cycle intensity significantly affected large pores (>30 μm) with a contribution rate of 65.2%; the wetting-drying cycle time had a substantial effect on inactive pores (<0.2 μm) with a contribution rate of 91.9%. The pore fractal dimension D on the other hand was distributed in the range of 2.697-2.774, and the D value gradually decreased following wetting-drying cycles. Furthermore, the decrease rate of the D value was clearly stable following four high-intensity wetting-drying cycles, and it was negatively correlated with the wetting-daring cycle intensity (R2=0.868), indicating that the pore structure tended to develop in the direction of simple homogeneity. This study concluded that the wetting-drying cycle effect triggered by seasonal rainfall largely impacted the generation of large pores, enhanced the homogeneity and hydraulic conductivity of the soil, and intensified the risk of geotechnical instability and collapse, offering a theoretical basis for the reasonable protection as well as utilization of soil and water resources.