In tunnel operation ventilation systems, the arrangement of jet fans plays a decisive role in ensuring ventilation efficiency. Curved tunnels, due to their unique radius of curvature, exhibit significant differences in fan installation parameters and jet flow field distribution compared to traditional straight-line tunnels. In order to investigate the distinct characteristics, this research utilized computational fluid dynamics (CFD) simulation methods to analyze the ventilation performance of both an innovative, adjustable jet fan system and conventional jet fans within the context of curved tunnel configurations. The findings reveal that by adjusting the horizontal deflection angle of the novel jet fan, the flow field distribution can be effectively optimized, and the jet effect can be enhanced, thereby improving ventilation efficiency. Compared to traditional jet fans, the novel fan demonstrates a significantly greater capability in flow field optimization, especially when its horizontal deflection angle is adjusted, showing a trend where the jet effect initially increases and then decreases, with the longitudinal impact range being adjustable within a range of 5 m to 25 m.

With the accelerated development of urban underground spaces, prefabricated tunnel linings have become a research focus due to their advantages in construction efficiency and cost effectiveness. However, issues such as stress concentration at joints and insufficient overall stability hinder their broader application. This study investigates a cut-and-cover prefabricated tunnel project in the Chongqing High-Tech Zone through scale model tests and numerical simulations to systematically compare the mechanical behaviors of cast-in situ linings and three-segment prefabricated linings under surrounding rock loads. The experimental results show that the ultimate bearing capacity of the prefabricated lining is 15.3% lower than that of the cast-in situ lining, with asymmetric failure modes and cracks concentrated near joint regions. Numerical simulations further reveal the influence of joint stiffness on structural performance: when the joint stiffness is 30 MN·m/rad, the bending moment of the segmented lining decreases by 37.7% compared to the cast-in situ lining, while displacement increments remain controllable. By optimising joint pre-tightening forces and stiffness parameters, prefabricated linings can achieve stability comparable to cast-in situ structures while retaining construction efficiency. This research provides theoretical and technical references for the design and construction of open-cut prefabricated tunnel linings.

To enhance the economic and safety aspects of tunnel structural design, this study optimizes the mix proportion of steel fiber-reinforced concrete (SFRC). It investigates the stress characteristics and support parameters of SFRC secondary lining structures via small-scale model tests and finite element analysis. The research focuses on the cracking process, deformation, and stress characteristics of SFRC linings under various loads. Compared with conventional reinforced concrete tunnels, SFRC tunnels show a significant increase in lining stiffness and load capacity, with a 20% reduction in reinforcement yielding load. When the damage factor is 0.43, the addition of steel fibers increases compressive stress by 22.18%. Using ABAQUS, simulations of SFRC linings with thicknesses ranging from 400 mm to 600 mm and reinforcement ratios of 0% to 0.28% were conducted. The results indicate that a 450 mm thick SFRC lining matches the mechanical performance of a 600 mm thick conventional reinforced concrete lining. Notably, an SFRC lining with a 0.20% circumferential reinforcement ratio equals a conventional lining with a 0.28% reinforcement ratio in overall mechanical performance.

To investigate the mechanical properties of steel fiber-reinforced concrete under freeze-thaw cycles and the accuracy of its finite element simulation, a constitutive model and its functional expressions for steel fiber-reinforced concrete under tension and compression before and after freeze-thaw cycles were developed. This was based on the stress-strain curve characteristics obtained from experiments, combined with the Hognestad model, the Guo Zhenhai model, and the tensile-compressive model. Finite element simulations were conducted using ABAQUS to model the evolution of the mechanical properties of the lining structure during freeze-thaw processes, revealing the damage characteristics and failure modes of the lining mechanical properties induced by freeze-thaw cycles. The results indicated that after experiencing freeze-thaw cycles, the peak strength of the specimens decreased from 43.3 GPa to 35.3 GPa. Validation through scaled model tests confirmed that the established constitutive model and the corresponding finite element method accurately reflect the cumulative process of freeze-thaw damage, with the numerical simulation results showing good agreement with the experimental data. This study verifies the feasibility of accurately simulating the structural performance of steel fiber-reinforced concrete by developing a freeze-thaw constitutive model, thereby providing a theoretical basis and analytical method for the design and durability assessment of tunnel linings in cold regions.

Tunnel lining models were cast at a 1:20 scale using four different materials: plain concrete (PC), steel fiber-reinforced concrete (SFRC), reinforced concrete (RC), and rebar-reinforced steel fiber-reinforced concrete (R/SFRC). Loading tests were performed on these models before and after freeze–thaw cycles to investigate the failure modes, analyze the mechanical behavior, and determine the optimal reinforcement scheme in this study. The results indicated that freeze–thaw cycling reduced the load-bearing capacity of tunnel linings by 12% to 28% compared to non-freeze–thaw linings. Adding steel fibers significantly enhanced the ductility of the lining models. The mechanical performance of linings with an optimal steel fiber content surpassed that of models with either increased rebar alone or steel fibers alone. In this study, an optimal combination of a 0.36% rebar ratio and a 1.5% steel fiber volume fraction effectively improved the tensile performance of the lining while reducing rebar consumption, without compromising the inherent mechanical performance of the tunnel structure.

In the rapid process of urban development, prefabricated segmental lining structures have become one of the primary lining forms for super-large section tu