Study on Optimization of Construction Parameters and Schemes for Complex Connecting Tunnels of Extra-Long Highway Tunnels Based on Field Monitoring and Numerical Simulation

To study the optimization of construction parameters and schemes for complex connecting tunnels in extra-long highway tunnels in granite strata, the research team, relying on the construction project of the complex connecting tunnel between the Xiaolongmen Extra-long Highway Tunnel and the ultra-deep shaft, established an on-site monitoring scheme and a refined numerical simulation model. It systematically analyzed the impact of various construction parameters on the construction process of connecting tunnels and the main tunnel, and on this basis, optimized the construction scheme, improving construction efficiency. The research results show that (1) after the excavation of the connecting tunnel, the confining pressure at the top of the working face decreases rapidly, while the confining pressure on both sides increases rapidly; the extreme point of the confining pressure decrease is located at the central point at the top of the excavated working face. (2) For Class III surrounding rock excavated using the full-face blasting method, the maximum influence range of working face excavation on the stratum along the tunneling direction is approximately 4D (where D represents the excavation step). (3) The larger the excavation step of the connecting tunnel, the more obvious the stress concentration phenomenon at the central point of the working face arch crown, and the excavation step should be optimally controlled within the range of 2–3 m. (4) When explosives in the blast hole adopt decoupled charging, the ratio of borehole diameter to charge diameter can be increased to utilize the air gap to buffer the energy generated by the explosion.