Objective The lime treatment method is a commonly used approach in urban rail transit construction in coastal areas, aiming to enhance the bearing capacity and long-term stability of soft soil subgrades. However, lime-treated soft soil subgrades present problems such as difficulties in compaction and significant variations in mechanical properties. Therefore, it is necessary to investigate the influencing mechanisms of key factors such as lime content, moisture content, degree of compaction, and curing age on the mechanical properties of soft soil subgrades, and to explore the stress diffusion characteristics of lime-treated soft soil subgrades under rolling compaction and train loads.

Method Taking a coastal area urban rail transit construction project in southeastern China as a case study, a multi-dimensional investigation of lime-modified soft soil is carried out. Through orthogonal experiments, the effects of four key factors－lime content, moisture content, degree of compaction, and curing age－on the mechanical properties of soft soil subgrades are analyzed. Microstructural testing techniques, including compositional analysis, XRD (X-ray diffraction), XRF (X-ray fluorescence), and SEM (scanning electron microscopy), are employed to clarify the microstructural evolution mechanism of the soft soil subgrade and study the mechanism by which lime treatment affects its mechanical properties. Using earth pressure cells, the stress distribution during field rolling compaction is recorded, and the stress diffusion behavior of the modified soil under simulated train loading conditions is analyzed.

Result & Conclusion  After two passes of vibratory rolling compaction on the lime-treated soft soil subgrade, its microstructure is reconstructed, forming a high-modulus 'densified layer' at a depth of 10–25 cm in the subgrade soil. This results in a significant diffusion effect on the excitation force of the roller and the vertical load of trains. The effect increases the attenuation rate of the excitation force along the depth of the soil, leading to reduced stress in the subgrade soil below a depth of 25 cm, and consequently resulting in a low-degree compaction for the lower layer of the lime-treated soft-soil subgrade.