English：

To investigate the support effect of double⁃layer small conduit at the portal section of shallow⁃buried soft rock tunnel and determine the optimal support parameter configuration, based on the No. 2tunnel of Zijing Mountain, a numerical model was established using geological survey reports andconstruction organization design. The model considers four key factors: the overlap length of the smallpipes, the insertion angle combination of the small pipes, the grout thickness, and the arrangementrange. Various construction conditions were simulated, and the support effect of the double⁃layered smallpipes was compared from multiple perspectives, including the crown settlement, horizontal convergence,and bending moment of the advanced small pipes. The results were verified with field monitoring data.The findings indicate that the use of double⁃layer small conduit support significantly improves the controlof surrounding rock deformation during tunnel excavation, with the maximum crown settlement reduced byapproximately 39% and the maximum horizontal convergence reduced by approximately 8%. Compared tono overlap (with a settlement of about 6.735mm), the crown settlement is notably reduced with double⁃layer small conduit overlap. However, when the overlap length exceeds 1.0m, the reduction insettlement becomes less significant. The change in the insertion angles of the inner and outer layers of thedouble⁃layer small conduit has minimal effect on the crown settlement and horizontal convergence of thetunnel. The crown settlement and horizontal convergence decrease as the grouting thickness andarrangement range increase, and the optimal parameters are t = 1.5m and α = 120°. The measured data show good agreement with the numerical simulation results.