Numerical Simulation and Construction Monitoring for Minimal-Angle Intersection of Main and Branch Tunnels in Water Conveyance Projects

In water conveyance tunnel construction, the layout of branch tunnels intersecting with the main tunnel at small angles—often deviating from design code recommendations—has long been a prevalent technical challenge. According to Article 4.6.8 of the Chinese industry specification for water conservancy (SL 303—2017), the intersection angle between the axes of a horizontal tunnel or branch adit and that of the main tunnel shall generally not be less than 45°. In practice, however, due to constraints such as complex geological conditions, limited construction space, and topographical restrictions, it is often difficult to meet this requirement, necessitating feasible construction schemes for small-angle intersections under non-standard conditions. Focusing on the Hejiang Branch Tunnel of the Gaobe Section in the Guangdong Water Resources Allocation Project around the Beibu Gulf, this study investigates the construction difficulties associated with a minimal-angle intersection where the included angle between the main tunnel and branch tunnel is less than 30°. Through systematic analysis of the geological characteristics of the intersection area, the mechanical response of surrounding rock during construction, and the influence of different construction sequences on tunnel structural stability, this research proposes a scientific process control strategy of "prioritizing the secondary lining of the branch tunnel over the excavation of the main tunnel." This approach breaks from the traditional sequence by reinforcing the branch tunnel first to provide stable structural support for subsequent main tunnel excavation. To verify the proposed strategy, a refined three-dimensional numerical model of the tunnel intersection area was established using Midas NX finite element software, fully simulating the actual construction process and the stress-strain behavior of surrounding rock. Comparative validation was then conducted by integrating real-time dynamic monitoring data obtained from on-site construction with typical engineering case experiences. The multi-faceted verification results demonstrate that the proposed construction scheme for minimal-angle intersection of main and branch tunnels exhibits good technical feasibility and engineering applicability, effectively reducing disturbance to the surrounding rock mass. Further research indicates that through comprehensive measures including in-depth geological assessment of the project area, accurate numerical simulation and optimization of construction schemes, real-time dynamic monitoring feedback, and reasonable optimization of support parameters based on monitoring results, the surrounding rock deformation and construction risks in the intersection zone can be effectively controlled. Finally, a comprehensive and systematic response strategy of "geological analysis—numerical simulation—dynamic monitoring" is proposed, forming a complete technical system from preliminary geological investigation and scheme design to on-site construction implementation and dynamic adjustment. This research provides practical solution ideas and technical references for similar tunnel engineering projects, and offers a valuable engineering example for the design and construction of tunnels with minimal-angle main-branch intersections under complex geological conditions.