Given the complexity and diversity of rock formations, existing constitutive models struggle to accurately portray their mechanical properties, leading to substantial discrepancies between numerical simulation outcomes and reality. This inadequacy fails to meet the demands of numerical analysis in practical engineering. This study first analyzes the physical and mechanical properties of thin-layered carbonaceous phyllite. Subsequently, an improved Nishihara rheological constitutive model is established based on these analyses. Utilizing the secondary development function offered by FLAC3D, the proposed model is further developed. The program’s correctness and reliability are confirmed through a numerical simulation using the triaxial creep test from existing research. Finally, the established constitutive model is applied in the numerical simulation of an actual soft rock tunnel engineering, obtaining results compared to real monitoring data. The results demonstrate that the improved Nishihara model is more effective at describing the creep deformation characteristics of soft rock. Moreover, the findings from this study can serve as a theoretical reference for predicting deformation in soft rock tunnel engineering.

Double-layer primary support is proposed to control the deformation of surrounding rock in tunnels within weak geological conditions, where engineering challenges such as large deformations, tunnel faces, and arch collapse are encountered. This approach is based on the principle of combined resistance and release. A combined approach of numerical modeling and on-site surveillance was utilized to analyze the displacement and stress state of the tunnel support structure at different construction stages of primary support for the second layer, using Xiejiapo Tunnel as an engineering case. The findings indicate that the implementation of two-layer primary support can mitigate the progression of large deformations effectively in weak surrounding rock; the sooner the primary support for the second layer is applied, the better the deformation control, and the later the application takes place, the more effectively the tension in the surrounding rock is diminished, whereby the self-supporting capacity of surrounding rock comes into its own. The force of the shotcrete is reduced. Considering the structural deformation and stress state, as well as combination of resistance and release, it is best to implement the primary support for the second layer 10 feet behind the primary support for the first layer.

This study presents a fuzzy analytic hierarchy process (FAHP) by integrating analytic hierarchy process (AHP) and fuzzy cognition to evaluate the construction risks of tunnel portals. Wuguanyi Tunnel is taken as the research objective to validate the performance of the proposed method. The result shows that the proposed decision making method can effectively identify risk factors and determine the risk level during the construction of tunnel portals. Finally, the corresponding control measures during the construction of the Wuguanyi Tunnel portal are proposed according to the risk assessment results.