Analysis of the crack development and mechanism of moderately expansive soil under different drying-wetting conditions

Expansive soil is classified as clayey soil that possesses a significant amount of hydrophilic minerals, including montmorillonite and illite. Under climatic conditions such as rainfall or drought, expansive soils demonstrate pronounced characteristics of swelling or shrinkage. The alteration in volume frequently leads to an uneven distribution of the stress field, resulting in crack formation when the tensile stresses surpass the tensile strength. The presence of cracks in the soil can deteriorate its mechanical characteristics, hence leading to a range of engineering and geological issues. In road and bridge engineering, the existence of cracks can decrease the soil bearing capacity, which may cause foundation settlement and superstructure inclination. In slope engineering, cracks development can lead to the rapid infiltration of rainwater, which will accelerate the formation of sliding surfaces while decreasing slope stability. Meanwhile, compacted expansive soils are widely distributed in the field of engineering. Due to their unique mineral structure, they are more sensitive to environmental changes compared to general clay. Therefore, the development of cracks under repeated drying-wetting cycles deserves special consideration. To investigate the development mechanism of cracks in expansive soil under the influence of drying-wetting cycles, this study focuses on moderately expansive soil found in the Su-Lian waterway project. Drying-wetting cycles were conducted at different temperatures, various drying methods, and drying-wetting ranges. Quantitative analysis of crack development in moderately expansive soil was performed using image processing techniques and scanning electron microscopy (SEM). The following conclusions were drawn: the development of cracks is significantly influenced by temperature. From natural air-dried to 50 ℃, the crack indicators gradually increase with the rising drying temperature. Under high-temperature conditions, the development pattern of cracks is characterized by initial growth followed by widening. Crack length tends to stabilize earlier, while crack average width continues to increase. A decreasing trend of 0.08 to 0.17 mm is observed after the moisture content drops below 15%. The cyclic drying-wetting process leads to a gradual deterioration of the internal structure of the soil, accumulating microscopic damage over time. Despite the initial influence of the body shrinkage effect on the crack average width in the early stages of drying-wetting cycles, a partial decreasing trend is observed as the drying-wetting range expands. However, with an increasing number of cycles, the range corresponding to the maximum crack indicators gradually shifts from 22%-33% to 9%-33%. Across various temperatures and drying-wetting ranges, the primary increase in crack indicators for the samples is concentrated in the first five cycles. Compared to the soil without drying-wetting cycles, the crack rate, length, and average width increase by 2.63% to 11.56%, 210.32 to 445.34 mm, and 0.39 to 0.83 mm, respectively. The unit-width fractal dimension, in comparison to the overall fractal dimension, more accurately reflects the impact of the drying-wetting range on the overall crack development. Under high-temperature and large drying-wetting range conditions, the geometric morphology and network distribution of cracks in the late stages of drying-wetting cycles exhibit increased complexity. Building upon unsaturated soil mechanics theories, the research analyzes the potential mechanisms of crack development in moderately expansive soils under different drying-wetting cycle conditions. The findings can serve as a valuable reference for studying shrinkage cracks in expansive soils under extreme weather conditions and for preventing geological disasters.