The mechanical properties of soft soil are crucial for the design and construction of underground space excavation; however, the current design and numerical analysis of underground spaces consider the loading mechanical parameters, ignoring the influence of the unloading stress path resulting in frequent construction accidents in practice. Here, soft soil in Shenzhen, China, is taken as the research subject, and a series of consolidated-undrained unloading tests are performed. First, K0 consolidation is conducted. Then, unloading tests are performed with different unloading ratios to simulate different unloading stress paths. The test results show that the soft soil deformation characteristics are closely related to the stress path and unloading ratio. Under different unloading ratios, soft soil will undergo compression deformation or rebound deformation. Under unloading conditions, the deviator stress-strain curve satisfies a hyperbolic function and can be normalized with the average consolidation confining pressure. With the increase in the unloading ratio, the initial tangent modulus first decreases and then increases, the cohesion decreases, and the internal friction angle does not change significantly. The loading mechanical parameters are not suitable for numerical calculation in unloading engineering. In this paper, more unloading paths are considered, such as UU1.0 and UU0.5. The results of the study provide a theoretical basis for the calculation of the numerical analysis of the soil body at different depths in rich soft soil pits.

In order to study the deformation and failure mechanism of the fault passage, this paper makes a series of research on the fault passage through theoretical analysis, field investigation and numerical simulation. Firstly, the mechanical characteristics of the fault structure and the deformation and failure characteristics of the surrounding rock passing through the fault are summarized. Then, the numerical analysis is carried out before and after the tunnel passing through the fault. The results show that the original support scheme has large deformation and failure in the surrounding rock of the fault section, and the deterioration and expansion of the plastic zone leads to the failure of the support. Finally, the comprehensive support scheme and principle of "bolt + anchor cable + metal mesh + grouting" is put forward, and the support for the broken tunnel passing through the fault is strengthened. The calculation results show that the support scheme can keep the tunnel passing through the fault in a stable deformation range, which is conducive to the long-term stability of the surrounding rock.