Effect of dry–wet cycles on the mechanical properties and microscopic characteristics of fine breccia soil from karst areas

The quality of the railway subgrade is directly related to the fill soil structure, which, in turn, is determined by the local physical and chemical environment. A karst environment, with its frequent rainfall, promotes the dissolution of soluble rocks and underground transportation of solutes, altering the soil structure and performance. To investigate these alterations, we analyzed the properties of underground soil from highly developed karst areas. Fine breccia soil from karst regions was tested to assess its macroscopic mechanical properties and microstructural features, for differing initial water contents and compaction levels. Samples were subjected to simulated rainfall conditions through dry–wet cycles, and then underwent triaxial shear and electron microscopy tests. From these data, a micro-to-macro correlation model and a normalization model were developed. The findings suggest that the resistance of fine angular breccia soil to degradation during dry–wet cycles can be enhanced through high-pressure compaction and by maintaining a moisture content close to 15.6%. Increasing the degree of compaction improves the particle size distribution and the density of the soil skeleton. This is advantageous for minimizing soil particle erosion, thereby ensuring the strong performance of railway subgrades in karst areas with frequent rainfall.