The properties of completely weathered argillaceous siltstone are between soil and rock. Due to its long-term settlement under traffic cyclic load is smaller than that of soft soil, it is often regarded as rock material and thus has less research on cumulative deformation. However, after investigation, it is found that the highly weathered soft rock has a great influence on the deformation-sensitive structures such as subway shield tunnel. Hence, it is necessary to conduct a more in-depth study on the mechanical properties of completely weathered argillaceous siltstone. By analyzing the undrained static shear strength of the rock sample under different test conditions (consolidation mode, water infiltration, and different confining pressure), and drawing the critical state line, the structural characteristics of the rock sample are understood. The results show that: The failure deviatoric stress and peak pore water pressure of rock sample under K₀ consolidation are higher than those under isotropic consolidation, indicating that the consolidation mode has a significant effect on the stress-strain and pore water pressure of the completely weathered argillaceous siltstone. Under the same test conditions, the failure deviatoric stress and peak pore water pressure of the samples increases linearly with the increase of the consolidation pressure. After water infiltration treatment, the effective stress of the rock sample decreases, the shear strength reduces, and the degree of damage intensifies.

In order to study the constitutive relationship of fully weathered argillaceous siltstone under the influence of seepage, based on the fully weathered argillaceous siltstone under the Nanchang subway tunnel, according to the triaxial undrained shear test, the mechanical response mechanism under static load is revealed. At the same time, with the support of the data of the test results, combined with the existing elastic-plastic constitutive model, the initial crack damage variable and the seepage sub-damage variable are introduced to construct the applicable constitutive model, and the secondary development is carried out through the program. Finally, the correctness of the self-developed elastic-plastic damage model of fully weathered argillaceous siltstone is verified by comparison. The findings indicate: (1) By considering the coupling damage variable relationship under the combined action of initial damage and seepage secondary damage, the constitutive relationship suitable for the rock sample is established. (2) The corresponding constitutive integration algorithm is deduced, and the secondary development of the constitutive model is completed through the UMAT interface program. (3) Based on the secondary developed constitutive model, the fully weathered argillaceous siltstone considering damage is numerically simulated by finite element software. Through numerical verification, it is found that the model can better describe the stress-strain relationship under different confining pressures and different stress levels, and to a certain extent, it can also reflect its strain accumulation.