Subsequent tunnel construction method optimizations and basement reinforcement measures for double-arch tunnel without middle drift

ObjectiveTaking the double-arch section of a highway tunnel as the study object, this study focuses on the significant increase in stress on primary support and secondary lining of pilot-tunnel, as well as the lining cracking caused by subsequent tunnel construction method for double-arch tunnel without middle drift.MethodFirst, the field monitoring tests were carried out to determine the distribution characteristics of internal forces on primary support during construction of pilot-tunnel. Then, 3D numerical simulation was used to compare the variations in internal forces on primary support and secondary lining of pilot-tunnel caused by using different construction methods for the subsequent tunnel. The reasons for the excessive tensile stress in the secondary lining on the right side of inverted arch of pilot-tunnel were analyzed. Finally, the grouting reinforcement was applied to the central wall basement. This measure effectively improved the stress state of the secondary lining structure of pilot-tunnel.ResultWith the excavation of subsequent tunnel, the stress on primary support structure of pilot-tunnel increased significantly. The structure was also obviously in an unsymmetrical loading state. The maximum tensile stress, reaching 142.4 MPa, occurred on the outer side of steel frame at the crown. The maximum compressive stress, reaching 201.4 MPa, occurred on the outer side of steel frame at the right arch shoulder. The excavation of subsequent tunnel had a limited effect on the primary support of pilot-tunnel. However, it had a greater effect on the secondary lining of pilot-tunnel. When the subsequent tunnel was excavated by using CD method, the maximum tensile stress on the secondary lining of inverted arch of pilot-tunnel was significantly lower than that using the three-bench method. After excavation of subsequent tunnel, the plastic zone area in the upper part of central wall and the surrounding rock at basement increased sharply. As a result, the bearing capacity of pilot-tunnel basement decreased significantly. It led to the tensile stress concentration on the secondary lining on the right side of inverted arch. The reinforcement of basement surrounding rock can significantly improve the stress state of pilot-tunnel. It can also markedly reduce the maximum principal stress on the secondary lining of inverted arch. Therefore, the grouting reinforcement is recommended in a symmetrical zone on both sides of the central wall basement. The recommended zone is with 12-m-wide and 3-m-deep.ConclusionThis study optimized the construction method for subsequent tunnel. It significantly reduced its influence on the pilot-tunnel, as well as improved the stress characteristics of the secondary lining of pilot-tunnel by reinforcing the basement of central wall. The findings can provide a reference for the design and construction of similar double-arch tunnel projects.