Abstract: In tunnel construction through weak and homogeneous geological layers rich in water, such as the tertiary semi-diagenetic strata, horizontal high-pressure jet grouting piles are often used for advanced reinforcement within the tunnel. However, these horizontal jet grouting piles tend to fracture under significant loads due to their low tensile strength. Drawing on the Wangjiazhai Tunnel project of the Lincang to Qingshuihe Expressway in Yunnan, a new composite structure is put forward that involves inserting small-diameter steel pipes into the pile body. Through mechanical theoretical analysis, numerical simulations, and on-site monitoring, we conducted a comparative analysis of the mechanical mechanisms and reinforcement effects of raw jet grouting piles and composite structures on the surrounding rock. Furthermore, we optimized the layout and diameter of the steel pipes. The research indicates that under a water pressure of 300 kPa, the deflection of the composite structures decreases by 34.8%, and the maximum tensile stress decreases by 37.5% compared to the untreated piles, significantly improving the stress mode and enhancing the ultimate bearing capacity of the water pressure load to 700 kPa, while also reducing the settlement of the arch crown and stress on the surrounding rocks. Increasing the density of steel pipes arranged at the arch crown effectively reduces the tensile stress in the pile body, whereas reducing the diameter of steel pipes would lead to increased tensile stress in the pipes. For practical engineering applications, it is advisable to place 108 mm steel pipes at intervals of one pile at the arch crown, place 89 mm steel pipes at intervals of two to three piles at the arch shoulders, and steel pipes are not necessary for the sidewalls.

Key words: tertiary semi-diagenetic rock, horizontal high-pressure jet grouting pile, steel tube composite structure, force mechanism, ultimate water pressure load