To investigate the diffusion law of ultrafine cement slurry (UCS) with different water–cement ratios in tunnel second lining cracks during grouting, the grouting of ultrafine cement slurry with different water–cement ratios was carried out by experimental and theoretical analysis methods in this study. Through the collection and data analysis grouting experiment of the diffusion time history, the diffusion morphological characteristics based on different slurry viscosities were obtained, which were divided into three grouting diffusion patterns: circular diffusion zone, excessive diffusion zone, and elliptical diffusion zone. Furthermore, the spatiotemporal variation rules of the diffusion radius of ultrafine cement slurry with different water–cement ratios in tunnel secondary lining cracks were obtained as well. By analyzing the diffusion radius values under different water–cement ratios in each direction of x+, x−, y+, and y−, the critical water–cement ratios ξ were found to be 0.8, which affected the diffusion radius value in the vertical upward y+ direction. Meanwhile, when the grouting was completed, the maximum diffusion radius of the ultrafine cement slurry was obtained using different water–cement ratios in each direction. Moreover, the grouting diffusion equation of tunnel secondary lining cracks based on ultrafine cement slurry with different water–cement ratios is established. The research results can accurately predict the grouting diffusion pattern and diffusion radius in tunnel second lining cracks with different water–cement ratios of ultrafine cement slurry.

Strong seismic waves induced by drill-and-blast tunnel excavation threaten the structural integrity of adjacent existing tunnels; however, prevailing safety evaluation methods mostly simplify tunnel linings as homogeneous continua, failing to accurately characterize the meso-scale uncoordinated dynamic response between rebar and concrete under blast impact. To fill this research gap, a 1:1 full-scale separated three-dimensional finite element model of reinforced concrete composite linings was established using the LS-DYNA explicit dynamic numerical algorithm, which was verified by previous 1:25 scaled physical model tests. This study systematically quantifies the spatiotemporal evolution of lining dynamic responses under two core parameters—tunnel clear distance (10 m to 60 m) and single-delay detonating charge quantity (10.8 kg to 28.8 kg)—to validate the response differences between materials. It is abstracted that the structural failure is dominated by axial tensile stress, with the embedded rebar being significantly more sensitive to internal stress surges (reaching 3.5 times the peak stress of concrete), while the concrete is more sensitive to particle vibration velocity amplification, a mismatch that is particularly acute within a 30 m clear distance. This study highlights the severe interfacial stress gradient between rebar and concrete, providing an indirect but critical indicator for the potential risk of interface debonding under adjacent blasting, and offers a quantitative theoretical basis for extending safety assessments from macro-surface vibration control to refined meso-scale internal stress monitoring.