The time-dependent behavior of power-law fluid has a significant influence on the grouting effects of reinforcing loose gravel soil. In this paper, based on basic rheological equations and the time-dependent behavior of rheological parameters (consistency coefficient and rheological index), rheological equations and penetration equations of time-dependent power-law fluid are proposed. Its penetration grouting diffusion mechanism for reinforcing loose gravel soil was then theoretically induced. A set of indoor experimental devices for simulating penetration grouting was designed to simulate the penetration grouting of power-law fluid with different time-dependent behaviors for reinforcing loose gravel soil. Then, relying on the COMSOL Multiphysics platform and Darcy’s law, three-dimensional numerical calculation programs for this mechanism were obtained using secondary-development programming technology. Thus, the numerical simulations of the penetration grouting process of power-law fluid with different time-dependent behaviors for reinforcing loose gravel soil were carried out. This theoretical mechanism was validated by comparing results from theoretical analyses, indoor experiments, and numerical simulations. Research results show that the three-dimensional numerical calculation programs can successfully simulate the penetration diffusion patterns of a time-dependent power-law fluid in loose gravel soil. The theoretical calculation values and numerical simulation values of the diffusion radius obtained from this mechanism are closer to indoor experimental values than those obtained from the penetration grouting diffusion theory of power-law fluid without considering time-dependent behavior. This mechanism can better reflect the penetration grouting diffusion laws of a power-law fluid in loose gravel soil than the theory, which can provide theoretical support and guidance for practical grouting construction.

The newly constructed Luofengshan Tunnel obliquely underpasses the operating Hanyicun railway tunnel, requiring an integrated assessment of deformation response and service safety. This study combined field monitoring, ultrasonic shear-wave reflection imaging, and three-dimensional numerical simulation to evaluate track-bed settlement, lining deformation, floor integrity, lining stress, and structural safety. The complete third-party monitoring record showed that vertical deformation dominated the tunnel response. The final maximum cumulative values were −17.44 mm for manual lining settlement, −14.91 mm for automated lining settlement, and −11.30 mm for track-bed settlement; the corresponding maximum absolute values of horizontal displacement and convergence were 1.82 mm and 1.40 mm, respectively. Same-period comparisons showed that deformation increments decreased after breakthrough, and post-lining flood-season monitoring showed monthly rates below 1 mm/month. Ultrasonic imaging detected no evident voids or collapse within 2.0–2.2 m below the floor. Numerical simulation showed that incorporating measured initial deformation increased the maximum compressive stress from 4.95 MPa to 6.26 MPa, whereas subsequent underpass excavation and train loading increased it slightly to 6.28 MPa and 6.33 MPa, respectively. The final minimum safety factor was 3.18, indicating that the underpass impact remained controllable.