Recent rapid urbanization has led to an increase in tunnel construction, escalating the prevalence of ground collapses. Ground collapses, characterized by large deformation and strain-softening, pose a significant challenge for classical numerical theories and simulation methods. Consequently, a numerical framework combining the material point method (MPM) and strain-softening Drucker–Prager plasticity is introduced in this study to more accurately describe the evolution process and failure mechanism of the subgrade during tunnel excavation. The proposed numerical framework was validated against an analytic solution employing a typical ‘dry bottom’ dam model with solid non-linearity and large deformation; some of the results are also compared with those of the SPH method and centrifugal modeling tests to verify the validity of the MPM method in this paper. The validated model was used in this study to conduct a comprehensive analysis of surface instability and ground collapse under varying soil conditions. This included factors such as strata thickness, cohesion, internal friction angle, and a quantitative description of the development of longitudinal subsidence of the surface. The aim was to clarify deformation responses, failure patterns, and excavation mechanisms, providing insights for underground tunneling practices.

Freeze–thaw cycles and the soil compaction coefficient ($\lambda \mathrm{c}$) have significant influence on the plastic strain for the foundation of underground structures in seasonal permafrost regions. Understanding the microstructural evolution of freeze–thawed soil is pivotal for assessing the long-term settlement of infrastructure foundation under repeated train loading. This study investigates the impacts of freeze–thaw cycles and $\lambda \mathrm{c}$ on the plastic strain and pore size distribution (PSD), as well as fractal characteristics, of soft clay via a set of cyclic triaxial tests and nuclear magnetic resonance (NMR) analyses. Fractal theory was adopted to analyze the heterogeneity of soil specimens. The results showed that an increase in $\lambda \mathrm{c}$ could efficiently alleviate the cumulative plastic strain. It also decreased the proportion of large pores and facilitated the generation of small and medium-sized pores. The analysis of the NMR test demonstrated that the freeze–thaw cycle led to the disruption of the soil’s microporous structure. Moreover, a higher value of $\lambda \mathrm{c}$ encouraged the formation of a more intricate and uniform pore structure. This, in turn, increased the fractal dimension, enhanced the structural heterogeneity, and thereby improved the soil’s structural complexity and its resistance to deformation. These findings underscore the significance of achieving optimal compaction levels to bolster soil stability under freeze–thaw conditions, provide valuable guidance for infrastructure design in permafrost regions, and help to ensure the durability and stability of transportation networks, such as railways and roads, over time.